ADEC Module-1 PDF

JNNCE Srinivasa Murthy MK LECTURE NOTES Analog and Digital Electronic Circuits MODULE-1 Srinivasa Murthy MK Asst. Professor...

JNNCE Srinivasa Murthy MK LECTURE NOTES Analog and Digital Electronic Circuits MODULE-1 Srinivasa Murthy MK Asst. Professor Department of Robotics and Artificial Intelligence Dept. of Robo cs & Ar ﬁcial Intelligence 1 JNNCE Srinivasa Murthy MK Contents Rec ﬁers................................................................................................................................................. 3 Half Wave Rec ﬁer Theory................................................................................................................. 3 Full Wave Rec ﬁers with centre trap.................................................................................................. 5 Full Wave Rec ﬁers with Wheatstone Bridge..................................................................................... 6 The Clippers............................................................................................................................................. 6 Posi ve Series Clippers....................................................................................................................... 7 Nega ve Series Clipper: Clipping the Downward Swing..................................................................... 8 Biased Series Clippers: Tailoring the Clip with DC Bias....................................................................... 9 Clamper circuits.................................................................................................................................... 10 Posi ve clamper................................................................................................................................ 10 Nega ve clamper circuit................................................................................................................... 11 DC Load Line analysis of transistor........................................................................................................ 12 Transistor Biasing.................................................................................................................................. 15 Base Resistor Method (Fixed Biased Method).................................................................................. 16 Collector to Base Bias (Feedback Resistor Method)......................................................................... 17 Self-Biased Circuit (Voltage Divider Circuit)...................................................................................... 19 Bias Stability.......................................................................................................................................... 21 Bias Stability and Need for Bias Stability........................................................................................... 22 Transistor as switch............................................................................................................................... 22 Power Ampliﬁers................................................................................................................................... 23 Class A Power Ampliﬁers................................................................................................................... 24 Class B Ampliﬁers.............................................................................................................................. 25 Class B Push-Pull Ampliﬁer................................................................................................................ 26 Feedback Ampliﬁers.............................................................................................................................. 27 Introduc on to Feedback Ampliﬁers................................................................................................ 28 Principle of Feedback Ampliﬁer........................................................................................................ 28 Types of Ampliﬁers............................................................................................................................ 29 RC Coupled Ampliﬁer............................................................................................................................ 30 Oscillators.............................................................................................................................................. 32 Prac cal Oscillator Circuit................................................................................................................. 33 The Barkhausen Criterion................................................................................................................. 33 Frequency Stability of an Oscillator.................................................................................................. 34 Audio Oscillator..................................................................................................................................... 35 Radio Oscillator..................................................................................................................................... 35 JFET-Junc on Field Eﬀec ve Transistor................................................................................................. 36 Dept. of Robo cs & Ar ﬁcial Intelligence 2 JNNCE Srinivasa Murthy MK Metal-Oxide-Semiconductor Field-Eﬀect Transistor (MOSFET)............................................................ 39 VTU Syllabus -Module-1 -ADEC Junction diode for HW and FW rectification, Clippers and Clamping circuits, Transistor biasing, Dc load line analysis, Different biasing circuits, stability factors(without derivation), Transistor switching networks. Concept of Amplifiers : RC Coupled Amplifier (Analysis), Feedback Amplifiers: Different types of feedback amplifers (Analysis),Power Amplifiers: Concept of Power Amplifiers , Class A and Class B , Push-pull power amplifier, Oscillators: Concept, Audio and Radio Frequency Oscillators, JFET and MOSFET - Working Principle and Biasing. Rectifiers A rectifier is a device that converts alternating current (AC) to direct current (DC). It is done by using a diode or a group of diodes. Half wave rectifiers use one diode, while a full wave rectifier uses multiple diodes. The working of a half wave rectifier takes advantage of the fact that diodes only allow current to flow in one direction. Half Wave Rectifier Theory The diagram below illustrates the basic principle of a half-wave rectifier. When a standard AC waveform is passed through a half-wave rectifier, only half of the AC waveform remains. Half- wave rectifiers only allow one half-cycle (positive or negative half-cycle) of the AC voltage through and will block the other half-cycle on the DC side, as seen below. Half-wave rectifier circuit consists of 3 main parts: 1. A transformer 2. A resistive load 3. A diode First, a high AC voltage is applied to the to the primary side of the step-down transformer and we will get a low voltage at the secondary winding which will be applied to the diode. Working: Dept. of Robo cs & Ar ﬁcial Intelligence 3 JNNCE Srinivasa Murthy MK 1. Positive Half Cycle: During the positive half cycle of the AC input voltage, the diode is forward-biased, allowing current to flow through it and the load resistor. This positive current charges any connected capacitor and contributes to the pulsating DC output voltage. 2. Negative Half Cycle: During the negative half cycle of the AC input voltage, the diode is reverse-biased, acting like an open circuit and blocking any current flow. The output voltage drops to zero during this period. Output Waveform: The output waveform of a half-wave rectifier is a pulsating DC voltage that resembles the positive half cycle of the original AC input waveform. However, it contains significant ripples due to the missing negative half cycle. Let Vi = Vp sin (ωt) be the input voltage to the rectifiers where Vp is the peak input voltage. While the diode is conduction, let id b ethe current flowing through the circuit and Vd be the voltage across the diode. For half wave rectifier, we have 𝐼 sin(𝜔𝑡) 𝑓𝑜𝑟 0 < 𝜔𝑡 < 𝜋 𝐼 = 0 𝑓𝑜𝑟 𝜋 < 𝜔𝑡 < 2𝜋 Applications: Half-wave rectifiers are simple and cost-effective solutions for basic DC power conversion in low-power applications like: 1. Battery chargers for small devices 2. LED lighting circuits 3. Simple power supplies for electronic circuits. Dept. of Robo cs & Ar ﬁcial Intelligence 4 JNNCE Srinivasa Murthy MK Full Wave Rectifiers with centre trap Components: 1. Center-Tapped Transformer: A transformer with a centre tap on its secondary winding. 2. Diodes:Two diodes connected in a specific configuration. Working: 1. Transformer Action: a. The AC input is provided to the center-tapped secondary winding of the transformer. b. The center tap serves as a reference point (ground or zero potential). 2. Positive Half-Cycle: a. During the positive half-cycle of the AC input, the upper end of the secondary winding becomes positive. b. The diode connected to this end becomes forward-biased, allowing current to flow through it. 3. Negative Half-Cycle: a. During the negative half-cycle, the lower end of the secondary winding becomes positive. b. Now, the other diode becomes forward-biased, allowing current to flow through it. 4. Output Current: a. As a result, current flows through the load during both half-cycles of the AC input. b. This creates a pulsating DC output across the load. 5. Center Tap as Reference: o The center tap ensures that the potential at the center is always at zero volts, providing a reference point for the rectification process. Advantages: 1. The full-wave rectifier with a center-tapped transformer provides a more efficient use of the transformer as both halves of the secondary winding are utilized. 2. It produces a smoother DC output compared to a half-wave rectifier. Disadvantages: 1. The center-tapped transformer is bulkier and more expensive than a simple transformer. Dept. of Robo cs & Ar ﬁcial Intelligence 5 JNNCE Srinivasa Murthy MK Full Wave Rectifiers with Wheatstone Bridge The working & operation of a full wave bridge rectifier is pretty simple. In the circuit diagram, 4 diodes are arranged in the form of a bridge. The transformer secondary is connected to two diametrically opposite points of the bridge at points A & C. The load resistance RL is connected to bridge through points B and D. During the first half cycle During the first half cycle of the input voltage, the upper end of the transformer secondary winding is positive with respect to the lower end. Thus during the first half cycle diodes D1 and D3 are forward biased and current flows through arm AB, enters the load resistance RL, and returns back flowing through arm DC. During this half of each input cycle, the diodes D2 and D4 are reverse biased and current is not allowed to flow in arms AD and BC. During the second half cycle During the second half cycle of the input voltage, the lower end of the transformer secondary winding is positive with respect to the upper end. Thus diodes D2 and D4 become forward biased and current flows through arm CB, enters the load resistance RL, and returns back to the source flowing through arm DA. The flow of current has been shown by dotted arrows in the figure. Thus the direction of flow of current through the load resistance RL remains the same during both half cycles of the input supply voltage. The Clippers Clippers can be categorized into various types based on several factors, including their connection to the input signal, the polarity they clip, and the presence of biasing: 1. Based on Connection: i. Series Clippers: The diode is connected in series with the input signal and the load resistor. They can clip either the positive or negative portion of the waveform depending on the biasing and polarity. a. Positive Series Clipper: Clips the positive portion of the input waveform above a certain threshold. b. Negative Series Clipper: Clips the negative portion of the input waveform below a certain threshold. c. Biased Series Clipper: Uses a DC bias voltage to adjust the clipping levels independently for both positive and negative portions. Dept. of Robo cs & Ar ﬁcial Intelligence 6 JNNCE Srinivasa Murthy MK ii. Shunt Clippers: The diode is connected in parallel with the input signal and the load resistor. They shunt one polarity of the waveform to ground, effectively clipping it. a. Positive Shunt Clipper: Shunts the positive portion of the input waveform above a certain threshold to ground. b. Negative Shunt Clipper: Shunts the negative portion of the input waveform below a certain threshold to ground. c. Biased Shunt Clipper: Uses a DC bias voltage to adjust the clipping levels independently for both positive and negative portions. 2. Based on Polarity Clipped: i. Unilateral Clippers: Clip only one polarity of the input waveform, either positive or negative. ii. Bilateral Clippers: Clip both the positive and negative portions of the input waveform at different levels, resulting in a symmetrical output waveform. 3. Based on Biasing: i. Unbiased Clippers: Rely solely on the diode characteristics and polarity of the input signal for clipping. ii. Biased Clippers: Utilize a DC bias voltage to adjust the clipping levels and control the clipping behavior independently for both polarities. Positive Series Clippers A positive series clipper is a specific type of electronic circuit that modifies an input signal by limiting its positive amplitude at a predetermined level. In simpler terms, it "clips" off the top, positive portions of the input waveform that exceed a certain threshold, resulting in a modified output waveform with flattened peaks. Construction: 1. Diode: The core component is a single diode connected in series with the input signal and the load resistor. The arrowhead of the diode symbol should point towards the input. 2. Load Resistor: This resistor determines the output current and limits the current flow during the clipping process. Dept. of Robo cs & Ar ﬁcial Intelligence 7 JNNCE Srinivasa Murthy MK 3. Optional Bias Voltage: In some cases, a DC bias voltage might be applied to adjust the clipping threshold level. Working Principle: 1. Positive Half Cycle: During the positive half cycle of the input signal, the voltage at the point where the diode meets the input exceeds the voltage at the point where it meets the load resistor. This forward biases the diode, allowing current to flow through it and the load resistor. The output voltage follows the input voltage until it reaches the clipping threshold determined by the diode's characteristics and the DC bias voltage (if present). 2. Clipping: Once the input voltage exceeds the clipping threshold, the diode remains forward-biased, but the excess voltage gets dropped across the diode instead of the load resistor. This effectively clips off the positive peak of the input waveform, resulting in a flat output voltage at the clipping level. 3. Negative Half Cycle: During the negative half cycle of the input signal, the diode is reverse-biased, acting like an open circuit and blocking any current flow. The output voltage drops to zero during this period. Output Waveform: The output waveform of a positive series clipper shows a flattened version of the original input waveform with its positive peaks clipped off at the predetermined level. The negative half cycle remains unchanged. Negative Series Clipper: Clipping the Downward Swing A negative series clipper is a specific type of electronic circuit that modifies an input signal by limiting its negative amplitude at a predetermined level. In simpler terms, it "clips" off the bottom, negative portions of the input waveform that fall below a certain threshold, resulting in a modified output waveform with flattened dips. Construction: 1. Diode: The core component is a single diode connected in series with the input signal and the load resistor. The arrowhead of the diode symbol should point away from the input. Dept. of Robo cs & Ar ﬁcial Intelligence 8 JNNCE Srinivasa Murthy MK 2. Load Resistor: This resistor determines the output current and limits the current flow during the clipping process. 3. Optional Bias Voltage: In some cases, a DC bias voltage might be applied to adjust the clipping threshold level. Working Principle: Positive Half Cycle: During the positive half cycle of the input signal, the voltage at the point where the diode meets the input is lower than the voltage at the point where it meets the load resistor. This reverse biases the diode, acting like an open circuit and blocking any current flow. The output voltage drops to zero during this period. Clipping: Once the input voltage falls below the clipping threshold determined by the diode's characteristics and the DC bias voltage (if present), the diode becomes forward-biased, allowing current to flow through it and the load resistor. However, the excess voltage below the threshold gets dropped across the diode instead of the load resistor, effectively clipping off the negative peak of the input waveform. Negative Half Cycle: During the negative half cycle of the input signal, the diode remains forward-biased, conducting current and allowing the output voltage to follow the input voltage until it reaches the clipping level. The output waveform shows a flattened version of the negative half cycle below the threshold. Output Waveform: The output waveform of a negative series clipper shows a modified version of the original input waveform with its negative peaks clipped off at the predetermined level. The positive half cycle remains unchanged. Biased Series Clippers: Tailoring the Clip with DC Bias A biased series clipper is a special type of electronic circuit that builds upon the basic concept of a series clipper by introducing a DC bias voltage to adjust the clipping levels for both the positive and negative portions of the input signal independently. This allows for greater flexibility and control over the waveform modification compared to traditional series clippers. Construction: 1. Diode: Similar to a standard series clipper, it uses a single diode connected in series with the input signal and the load resistor. The arrowhead of the diode symbol should point towards the input. 2. Load Resistor: This resistor determines the output current and controls the clipping process. 3. DC Bias Voltage Source: This additional element introduces a controllable DC voltage, allowing independent adjustments of the clipping thresholds. Working Principle: 1. Positive Half Cycle: During the positive half cycle of the input signal, the DC bias voltage adds to the input voltage. If the combined voltage exceeds the diode's forward- biasing threshold, the diode conducts current and the output voltage follows the input Dept. of Robo cs & Ar ﬁcial Intelligence 9 JNNCE Srinivasa Murthy MK until it reaches the clipping level determined by the DC bias. At this point, the remaining voltage drops across the diode, clipping the positive peak. 2. Negative Half Cycle: Conversely, during the negative half cycle, the DC bias voltage subtracts from the input voltage. If the combined voltage falls below the diode's reverse- biasing threshold, the diode acts as an open circuit and the output voltage drops to zero. This clips the negative portion of the waveform at the level defined by the DC bias. 3. Adjustable Clipping Levels: By varying the DC bias voltage, you can independently shift the clipping thresholds for both positive and negative halves of the input signal. This gives you control over the clipping levels and the overall shape of the output waveform. Output Waveform: The output waveform of a biased series clipper shows a modified version of the original input waveform with its positive and negative peaks clipped at predetermined levels defined by the DC bias voltage. The clipping levels can be adjusted independently, resulting in various output shapes depending on the desired effect. Clamper circuits A clamper is an electronic circuit that changes the DC level of a signal to the desired level without changing the shape of the applied signal. In other words, the clamper circuit moves the whole signal up or down to set either the positive peak or negative peak of the signal at the desired level. Types of clampers Clamper circuits are of three types: 1. Positive clampers 2. Negative clampers 3. Biased clampers Positive clamper positive clamper is an electronic circuit that shifts the positive portions of an input signal upwards to a certain defined level. Essentially, it "fixes" the positive peaks above a specific threshold, resulting in a modified output waveform with a consistently boosted positive half cycle. Dept. of Robo cs & Ar ﬁcial Intelligence 10 JNNCE Srinivasa Murthy MK Construction: 1. Diode: The core component is a single diode connected in parallel with the input signal and the load resistor. The arrowhead of the diode symbol should point away from the input. 2. Load Resistor: This resistor determines the output current and influences the charging and discharging of the capacitor. 3. Capacitor: This component stores charge and helps maintain the clamped level during the negative half cycle of the input signal. Working Principle: 1. Positive Half Cycle: During the positive half cycle of the input signal, the voltage across the diode is lower than the input voltage. This reverse biases the diode, essentially acting like an open circuit and blocking any current flow through the diode branch. The capacitor charges through the load resistor, reaching a voltage level roughly equal to the peak of the input signal. 2. Negative Half Cycle: When the input signal falls below the clamped level (capacitor voltage) during the negative half cycle, the diode becomes forward-biased, allowing current to flow through it. This discharges the capacitor partially, maintaining the clamped level at the output despite the lower input voltage. The discharge rate depends on the load resistor and capacitor values. 3. Output Waveform: The output waveform of a positive clamper shows a modified version of the input signal with its positive peaks clamped above the capacitor voltage, which acts as the reference level. The negative half cycle may be slightly lower and exhibit some ripple depending on the capacitor discharge rate. Negative clamper circuit A negative clamper is an electronic circuit that "fixes" the negative portions of an input signal downwards to a certain defined level. It essentially shifts the negative peaks below a specific threshold, resulting in a modified output waveform with a consistently clamped negative half cycle. Construction: 1. Diode: The core component is a single diode connected in parallel with the input signal and the load resistor. The arrowhead of the diode symbol should point towards the input. Dept. of Robo cs & Ar ﬁcial Intelligence 11 JNNCE Srinivasa Murthy MK 2. Load Resistor: This resistor determines the output current and influences the charging and discharging of the capacitor. 3. Capacitor: This component stores charge and helps maintain the clamped level during the positive half cycle of the input signal. Working Principle: 1. Negative Half Cycle: During the negative half cycle of the input signal, the voltage across the diode is higher than the input voltage. This forward biases the diode, allowing current to flow through it and the load resistor. The capacitor charges through this path, reaching a voltage level roughly equal to the peak of the negative input signal (but with opposite polarity). 2. Positive Half Cycle: When the input signal rises above the clamped level (capacitor voltage) during the positive half cycle, the diode becomes reverse-biased, essentially acting like an open circuit and blocking any current flow through the diode branch. The capacitor discharges partially through the load resistor, maintaining the clamped level at the output despite the higher input voltage. The discharge rate depends on the load resistor and capacitor values. 3. Output Waveform: The output waveform of a negative clamper shows a modified version of the input signal with its negative peaks clamped below the capacitor voltage, which acts as the reference level. The positive half cycle may be slightly higher and exhibit some ripple depending on the capacitor discharge rate. DC Load Line analysis of transistor When the output characteristics of a transistor are considered, the curve looks as below for different input values. Dept. of Robo cs & Ar ﬁcial Intelligence 12 JNNCE Srinivasa Murthy MK In the above figure, the output characteristics are drawn between collector current IC and collector voltage VCE for different values of base current IB. These are considered here for different input values to obtain different output curves. Operating point When a value for the maximum possible collector current is considered, that point will be present on the Y-axis, which is nothing but the saturation point. As well, when a value for the maximum possible collector emitter voltage is considered, that point will be present on the X- axis, which is the cutoff point. When a line is drawn joining these two points, such a line can be called as Load line. This is called so as it symbolizes the output at the load. This line, when drawn over the output characteristic curve, makes contact at a point called as Operating point. This operating point is also called as quiescent point or simply Q-point. There can be many such intersecting points, but the Q-point is selected in such a way that irrespective of AC signal swing, the transistor remains in active region. This can be better understood through the figure below. The load line has to be drawn in order to obtain the Q-point. A transistor acts as a good amplifier when it is in active region and when it is made to operate at Q-point, faithful amplification is achieved. DC Load line When the transistor is given the bias and no signal is applied at its input, the load line drawn at such condition, can be understood as DC condition. Here there will be no amplification as the signal is absent. The circuit will be as shown below. Dept. of Robo cs & Ar ﬁcial Intelligence 13 JNNCE Srinivasa Murthy MK The value of collector emitter voltage at any given time will be 𝑉 == 𝑉 −𝐼 𝑅 As VCC and RC are fixed values, the above one is a first degree equation and hence will be a straight line on the output characteristics. This line is called as D.C. Load line. The figure below shows the DC load line. To obtain the load line, the two end points of the straight line are to be determined. Let those two points be A and B. Dept. of Robo cs & Ar ﬁcial Intelligence 14 JNNCE Srinivasa Murthy MK To obtain A (When 𝑉 = 0) When collector emitter voltage VCE = 0, the collector current is maximum and is equal to VCC/RC. This gives the maximum value of VCE. This is shown as: Applying KVL in output circuit 𝑉 =𝑉 +𝐼 𝑅 𝑉 =𝑉 −𝐼 𝑅 When VCE = 0 𝑉 =0=𝑉 −𝐼 𝑅 𝑉 𝐼 = 𝑅 This gives the point A (OA = VCC/RC) on collector current axis, shown in the above figure. To obtain B A (When 𝐼 = 0) When the collector current IC = 0, then collector emitter voltage is maximum and will be equal to the VCC. This gives the maximum value of IC. This is shown as 𝑉 =𝑉 −𝐼 𝑅 𝑉 =𝑉 (As IC = 0) This gives the point B, which means (OB = VCC) on the collector emitter voltage axis shown in the above figure. Hence we got both the saturation and cutoff point determined and learnt that the load line is a straight line. So, a DC load line can be drawn. Quiescent point or simply Q-point is got by the intersection of IB with the DC load line. Transistor Biasing Transistor biasing is the process of applying DC voltage to the transistor's base-emitter junction, to establish the desired operating point of the transistor in the active region so that it can amplify the input AC signal. The DC voltage applied to the base-emitter junction is known as the bias voltage, and the purpose of biasing is to keep the transistor operating in the active region of operation, where it amplifies the signal faithfully without producing distortions. Methods of transistor biasing include: 1. Fixed Biasing: In this method, the base of the transistor is connected to the voltage source through a resistor. The other end of the resistor is connected to the base of the transistor, while the emitter is connected to the ground. This type of biasing provides a constant base-emitter voltage for the transistor. However, this method is sensitive to temperature changes and produces significant distortion. Dept. of Robo cs & Ar ﬁcial Intelligence 15 JNNCE Srinivasa Murthy MK 2. Collector-to-Base Biasing: Here, the collector-base junction is reverse-biased through a resistor, and a bias voltage is applied to the base-emitter junction through another resistor. This method has stable biasing, and is commonly used in audio amplifiers. 3. Voltage Divider Biasing: This method uses a voltage divider network consisting of two resistors connected in series between the power supply and the ground. The tap of the two resistors is connected to the base of the transistor, with the transistor emitter grounded. This method provides consistent DC bias voltage and is less sensitive to temperature changes. It is commonly used in amplifier circuits. 4. Emitter Biasing: Here, a resistor is connected between the emitter and ground, controlling the emitter current. This method provides a stable bias current, and a constant voltage from the emitter to ground, which avoids thermal runaway of the transistor.There are other biasing methods like self-biasing, collector feedback biasing and more, each with unique features and applications. Proper selection of biasing method ensures stable and reliable operation of the transistor circuit. Other names of biasing 1. Base Resistor Method (Fixed Biased Method) 2. Collector to Base Bias (Feedback Resistor Method) 3. Self-Biased Circuit (Voltage Divider Circuit) 4. Biasing with Emitter Resister (Emitter Biasing) Base Resistor Method (Fixed Biased Method) A common emitter transistor amplifier circuit with a fixed bias resistor RB is shown in figure. In this circuit only one DC supply Vcc is used to forward bias the emitter-base junction and reverse bias the collector emitter junction. A high resistance RB (of several hundred k Ω) is connected between +ve terminal of supply Vcc & the base. Vcc is connected to collector through Rc. The Capacitors C1 and C2 block DC voltage signals (if any) in the input & output stage respectively and hence helps in shifting the Q Point. Applying KVL in the input circuit 𝑉 −𝑅 𝐼 −𝑉 =0 Dept. of Robo cs & Ar ﬁcial Intelligence 16 JNNCE Srinivasa Murthy MK 𝑉 −𝑉 𝐼 = −−−−−−−−−−−−−−−−−−−−−−−−−−−1 𝑅 Usually VBE (= 0.7V for Si ) is very small as compared to VCC. 𝑉 𝐼 = −−−−−−−−−−−−−−−−−−−−−−−−−−−−−2 𝑅 Both Vcc an RB are constatnt for a given circuit, so from equation 2, IB is fixed and due to this reason the circuit is called fixed bias circuit and method is called fixed bias method. Now the collector current , 𝐼 = 𝛽𝐼 − − − − − − − − − − − − − − − − − − − − − − − − − − − − − 3 𝐼 =𝛽 −−−−−−−−−−−−−−−−−−−−−−−−−−−−−4 Using Kirchoff voltage law in the output circuit we get: 𝑉 −𝐼 𝑅 −𝑉 =0 𝑉 =𝑉 − 𝐼 𝑅 − − − − − − − − − − − − − − − − − − − − − − − − − −5 Using equation 4 and 5 we get 𝑉 𝑉 =𝑉 −𝛽 𝑅 − − − − − − − − − − − − − − − − − − − − − − − −6 𝑅 𝑅 𝑉 =𝑉 1−𝛽 − − − − − − − − − − − − − − − − − − − − − − − −7 𝑅 The value of the base current IB , collector current IC and the collector emitter voltage VCE at Q point can be determined from equation 2, 4 and 6 Collector to Base Bias (Feedback Resistor Method) Applying KVL in the input circuit 𝑉 − 𝑅 (𝐼 + 𝐼 ) − 𝐼 𝑅 − 𝑉 =0 𝑉 −𝑅 𝐼 −𝑉 = 𝐼 (𝑅 − 𝑅 ) 𝑉 −𝑅 𝐼 −𝑉 𝐼 = − − − − − − − − − − − − − − − − − − − − − − − −1 (𝑅 − 𝑅 ) Applying Kirchoff voltage law in the output circuit we get, 𝑉 − 𝑅 (𝐼 + 𝐼 ) − 𝑉 =0 𝑎𝑠 𝐼 ≪ 𝐼 𝑉 =𝑅 𝐼 +𝑉 Dept. of Robo cs & Ar ﬁcial Intelligence 17 JNNCE Srinivasa Murthy MK 𝑉 =𝑉 −𝑅 𝐼 −−−−−−−−−−−−−−−−−−−−−−−−−−−2 Using 1 and 2 we get 𝑉 −𝑉 𝐼 = − − − − − − − − − − − − − − − − − − − − − − − − − − − −3 (𝑅 + 𝑅 ) Now, 𝐼 = 𝛽𝐼 − − − − − − − − − − − − − − − − − − − − − − − − − − − −4 𝑉 −𝑉 𝐼 =𝛽 −−−−−−−−−−−−−−−−−−−−−−−−−−−5 (𝑅 + 𝑅 ) The value of IB , VCE and IC at Q-point can be determined from the equation 3 , 2 and 4. Biasing with Emitter Resister The collector to base bias circuit is same as base bias circuit except that the base resistor RB is returned to collector, rather than to VCC supply as shown in the figure below. This circuit helps in improving the stability considerably. If the value of IC increases, the voltage across RL increases and hence the VCE also increases. This in turn reduces the base current IB. This action somewhat compensates the original increase. Applying KVL to the input circuit, 𝑉 −𝐼 𝑅 −𝑉 −𝐼 𝑅 =0 We know 𝐼 = 𝐼 + 𝐼 𝑉 −𝐼 𝑅 −𝑉 − (𝐼 + 𝐼 )𝑅 = 0 𝑉 −𝐼 𝑅 −𝑉 −𝐼 𝑅 −𝐼 𝑅 =0 Dept. of Robo cs & Ar ﬁcial Intelligence 18 JNNCE Srinivasa Murthy MK We know, 𝐼 = 𝛽𝐼 𝑉 −𝐼 𝑅 −𝑉 − 𝐼 𝑅 − 𝛽𝐼 𝑅 = 0 𝑉 −𝑉 = 𝐼 [𝑅 + 𝑅 + 𝛽𝑅 ] 𝑉 −𝑉 𝐼 = −−−−−−−−−−−−−−−−−−−−−−−−−1 𝑅 + 𝑅 (𝛽 + 1) 𝑉 −𝑉 𝐼 = 𝛽𝐼 = 𝛽 −−−−−−−−−−−−−−−−−−−−−2 𝑅 + 𝑅 (𝛽 + 1) 𝐴𝑠 𝑉 ≫ 𝑉 , 𝑉 − 𝑉 ≈ 𝑉 𝑎𝑛𝑑 𝛽 ≫ 1 𝑠𝑜 𝑡ℎ𝑒 𝑎𝑏𝑜𝑣𝑒 𝑏𝑒𝑐𝑜𝑚𝑒𝑠 𝑉 𝐼 =𝛽 𝑅 + 𝛽𝑅 𝑉 𝐼 = −−−−−−−−−−−−−−−−−−−−−−−−−−−−−3 𝑅 +𝑅 𝛽 By applying KVL to output circuit 𝑉 =𝐼 𝑅 +𝑉 +𝐼 𝑅 𝑉 =𝐼 𝑅 +𝑉 +𝐼 𝑅 𝑎𝑠 𝐼 ≈ 𝐼 𝑉 =𝐼 𝑅 +𝑉 +𝐼 𝑅 𝑉 = 𝐼 (𝑅 + 𝑅 ) + 𝑉 𝑉 = 𝑉 −𝐼 (𝑅 + 𝑅 ) − − − − − − − − − − − − − − − − − − − − − − − 4 The values of IB, IC and VCE at point Q can be determined by equation 1, 3 and 4. Self-Biased Circuit (Voltage Divider Circuit) Among all the methods of providing biasing and stabilization, the voltage divider bias method is the most prominent one. Here, two resistors R1 and R2 are employed, which are connected to VCC and provide biasing. The resistor RE employed in the emitter provides stabilization. The name voltage divider comes from the voltage divider formed by R1 and R2. The voltage drop across R2 forward biases the base-emitter junction. This causes the base current and hence collector current flow in the zero signal conditions. The figure below shows the circuit of voltage divider bias method. Dept. of Robo cs & Ar ﬁcial Intelligence 19 JNNCE Srinivasa Murthy MK According to voltage divider theorem, Thevenin Theory theorem the voltage V2 across R2 is given by 𝑅 𝑉 = 𝑉 −−−−−−−−−−−−−−−−−−−−−−−−−−−−−1 𝑅 +𝑅 Applying KVL in the input circuit, 𝑉 +𝐼 𝑅 −𝑉 =0 𝑉 =𝑉 +𝐼 𝑅 𝑉 −𝑉 𝐼 = −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−2 𝑅 We know 𝐼 = 𝐼 + 𝐼 and 𝐼 ≈ 𝐼 as IB is very small Now equation 2 becomes 𝑉 −𝑉 ≈𝐼 −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−3 𝑅 Now V2 >> VBE 𝑉 ≈ 𝐼 − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − −4 𝑅 The collector current at Q-point does not depend upon transistor parameters β and Q-point is stabilised against transistor replacement Applying KVL to output circuit we get, 𝑉 =𝐼 𝑅 +𝑉 +𝐼 𝑅 𝑉 = 𝑉 −𝐼 𝑅 −𝐼 𝑅 We know 𝐼 = 𝐼 + 𝐼 and 𝐼 ≈ 𝐼 as IB is very small Dept. of Robo cs & Ar ﬁcial Intelligence 20 JNNCE Srinivasa Murthy MK 𝑉 = 𝑉 − 𝐼 {𝑅 + 𝑅 } Bias Stability Transistor stability is an important consideration in amplifier design, as unstable circuits can oscillate and produce unwanted signals and noise. There are several factors that impact the stability of a transistor amplifier. 5. Biasing: Incorrect biasing of the transistor can cause instability in the amplifier. Over-biasing or under-biasing the transistor can result in thermal runaway or saturation, respectively. 6. Gain: High gain can make the amplifier more prone to oscillations, especially at high frequencies. The gain can be reduced by using negative feedback. 7. Feedback: Positive feedback can cause oscillations at a certain frequency, while negative feedback can improve stability by reducing gain at higher frequencies. 8. Unmatched components: Unequal resistances, capacitances, and inductances can cause instability in the amplifier. 9. Parasitic Capacitances: Parasitic capacitances like collector to base and collector to emitter capacitances can resonate with the external components and decrease the amplifier's stability. 10. Temperature: Changes in temperature can impact transistor operation and affect stability. Careful design and thermal management can mitigate this effect. 11. Frequency: Some circuits can become unstable at certain frequencies due to resonances or phase shifts. Overall, transistor stability can be improved through careful design, selection of appropriate components, proper biasing, and the use of negative feedback. Simulation tools can also be used to analyze and optimize designs for stability. Temperature stability of the transistor Temperature stability is an essential factor to consider while designing transistor circuits. Temperature affects the conductivity of the semiconductor materials used in the transistor, which affects the transistor's current and voltage characteristics, leading to the amplification of undesired signals. Temperature stability is expected to prevent the change in the transistor's characteristics like gain, emitter current, base current, noise figure, and output impedance with the temperature. There are a few ways to increase transistor stability based on temperature: 1. Thermal Management: Good thermal management is essential to increase the stability of transistors, as overheating can lead to changes in the transistor's characteristics. Using heat sinks, fans, and other cooling techniques can help manage heat and ensure stable operation. 2. Choice of transistor: When selecting a transistor, it is essential to choose one with a low TC (temperature coefficient) of important parameters like hfe, Vbe, and the saturation current, as they provide more stability against the variations in temperature. Dept. of Robo cs & Ar ﬁcial Intelligence 21 JNNCE Srinivasa Murthy MK 3. Biasing Circuit: The circuit should be designed to compensate for the changes in transistor's characteristics with temperature. A feedback circuit that regulates the collector current or a network that compensates for the base and emitter resistances can provide temperature stability. 4. External Components: The use of temperature compensating resistors, capacitors, and other passive components can improve transistor stability. Bias Stability and Need for Bias Stability The value of collector current IC and VCC under no signal condition corresponds to on a to your point on a load line which is called as Q-point of transistor. The process of making Q- point independent of the variation in temperature and transistor parameters is called as BIOS stabilization. Factors responsible for shifting the Q point shifting the cube point are: 1. Temperature dependence of IC: i. It consists of useful component and undesirable component that is leakage current ICBO. ii. It is found that ICBO doubles for every 10 degree rise in temperature in both silicon and germanium transistor thus ICC increases with increase in temperature. 2. Thermal runaway i. IC produces heat in the transistor and due to this temperature of collection junction increases. This is intern increases the leakage current ICBO and hence IC. ii. It further increases the temperature of the collector junction and hence IC further increases. This cumulative process may increase the value of IC to such a value that transistor may burn out. This self destruction of transistor is called as thermal runaway. 3. Variation in transistor parameter i. In spite of huge advancement in semiconductor technology the parameter of 2 transistor of same type are not always the same. ii. Example the value of β for same type of two transistor range from 1 to 3. When a transistor is replaced by same type then β will change and hence the Q- point will shift this new Q-point may not be suitable one. Transistor as switch A bipolar junction transistor (BJT) can be used as a switching circuit. The basic construction and working of transistor switching is as follows: Construction: The switching circuit consists of a transistor, a load, and a voltage source. The load could be a relay, motor, or any other element that can be turned on and off using a voltage signal. The voltage source is connected to the base of the transistor through a resistor. Dept. of Robo cs & Ar ﬁcial Intelligence 22 JNNCE Srinivasa Murthy MK Equivalent Circuit Working: When a voltage signal is applied to the base of the transistor, the transistor starts conducting current from the collector to the emitter, and the load current starts flowing. The amount of current that flows through the load depends on the base voltage. In the absence of a voltage signal at the base, the transistor remains in the non-conducting state, and the load current is zero. In practical applications, the switching circuit has two operating modes: 1. Cut-off mode: In this mode, the base-emitter junction is reverse-biased, and the transistor is non-conducting. No load current flows through the circuit. 2. Saturation mode: In this mode, the base-emitter junction is forward-biased and the transistor is conducting. The maximum amount of load current flows through the transistor. The switching speed of the circuit depends on the response time of the transistor. Therefore, switching circuits use transistors with fast response times. Overall, the transistor switching circuit allows the control of the load current through the use of a voltage signal. It is useful in a variety of applications that involve controlling the flow of current, including motor control, relay control, and lighting control Power Amplifiers Power amplifiers are electronic devices that increase the amplitude of a signal to a level suitable for driving the load, such as speakers or antennas.  In audio systems, power amplifiers are crucial components for driving speakers and providing the necessary power to reproduce sound at desired levels. Dept. of Robo cs & Ar ﬁcial Intelligence 23 JNNCE Srinivasa Murthy MK  In radio frequency (RF) and communication systems, power amplifiers are used to boost signals before transmission. Classification Based on Frequencies Power amplifiers are divided into two categories, based on the frequencies they handle. They are as follows. 1. Audio Power Amplifiers − The audio power amplifiers raise the power level of signals that have audio frequency range (20 Hz to 20 KHz). They are also known as Small signal power amplifiers. 2. Radio Power Amplifiers − Radio Power Amplifiers or tuned power amplifiers raise the power level of signals that have radio frequency range (3 KHz to 300 GHz). They are also known as large signal power amplifiers. Classification Based on Mode of Operation On the basis of the mode of operation, i.e., the portion of the input cycle during which collector current flows, the power amplifiers may be classified as follows. 1. Class A: Operates in the linear region of the output transistors, prioritizing sound quality with high fidelity and low distortion but at the cost of lower efficiency and higher heat generation. 2. Class B: Utilizes two transistors, each conducting during half of the signal cycle, achieving higher efficiency but potentially introducing crossover distortion where the transistors switch. 3. Class AB: Blends Class A and Class B principles, operating part of the cycle in Class A for smooth sound and part in Class B for improved efficiency. A common balance between performance and efficiency. 4. Class D: Employs pulse-width modulation (PWM) to switch the output transistors rapidly, offering high efficiency and compact size but potentially exhibiting switching noise artifacts. Class A Power Amplifiers A Class A power amplifier is one in which the output current flows for the entire cycle of the AC input supply. Hence the complete signal present at the input is amplified at the output. The following figure shows the circuit diagram for Class A Power amplifier. Dept. of Robo cs & Ar ﬁcial Intelligence 24 JNNCE Srinivasa Murthy MK From the above figure, it can be observed that the transformer is present at the collector as a load. The use of transformer permits the impedance matching, resulting in the transference of maximum power to the load e.g. loud speaker. 1. For this class, position of the Q point is approximately at the midpoint of the load line. 2. For all values of input signal, the transistor remains in the active region and never enters into cut-off or saturation region. 3. The collector current flows for 360° (full cycle) of the input signal. In other words, the angle of the collector current flow is 360° i.e. one full cycle. 4. The current and voltage waveforms for a class A operation are shown with the help of output characteristics and the load line. 5. For full input cycle, a full output cycle is obtained. The signal is faithfully reproduced, at the output, without any distortion. This is important feature of a class A operation. Class B Amplifiers The power amplifier is said to be class B amplifier if the Q point and the input signal are selected, such that the output signal is obtained only for one half cycle for a input cycle. The figure below shows the input and output waveforms during class B operation. Dept. of Robo cs & Ar ﬁcial Intelligence 25 JNNCE Srinivasa Murthy MK 1. For this operation, the Q point is shifted on X-axis i.e. transistor is biased to cut-off. 2. Due to the selection of Q point on the X-axis, the transistor remains, in the a region, only for positive half cycle of the input signal. Hence this half cyc reproduced at the output. 3. But in a negative half cycle of the input signal, the transistor enters into a cu region and no signal is produced at the output. 4. The collector current flows only for 180° (half cycle) of the input signal. In words, the angle of the collector current flow is 180° i.e. one half cycle. 5. The current and voltage waveforms for a class B operation are shown in Fig. 9.5.2. As only a half cycle is obtained at the output, for full input cycle, the output sig distorted in this mode of operation. Class B Push-Pull Amplifier A push-pull class B amplifier is a type of power amplifier that uses two transistors, one pushing current through the load while the other pulls current. In a class B amplifier, each transistor conducts only half of the input signal cycle, resulting in high efficiency but with distortion in the output waveform. A push-pull class B amplifier typically includes a complementary pair of NPN and PNP transistors to push and pull current through the load alternately, thus Dept. of Robo cs & Ar ﬁcial Intelligence 26 JNNCE Srinivasa Murthy MK accomplishing Class B operation with reduced distortion. Here's how a push-pull class B amplifier works, step by step: 1. The input signal is connected to the base of the first transistor. 2. When the input signal is positive, the first transistor starts to conduct, allowing current to flow through the load. 3. When the input signal goes negative, the first transistor stops conducting, and the second transistor starts conducting, allowing current to flow through the load in the opposite direction. 4. When the input signal goes back to positive again, the process repeats, with the first transistor conducting once more. 5. The output signal of the amplifier is the sum of the two currents flowing through the load, which is now twice the power level of a single-ended class B amplifier. Push-pull class B amplifiers have high efficiency and reduced distortion compared to class A and class B single-ended amplifiers. They are commonly used in audio applications where high power output is required, such as in home theater systems or professional sound equipment. One disadvantage of the push-pull class B amplifier is that it requires a complex circuit design and two output devices, which makes the amplifier more expensive and complex to build than single-ended class A or B amplifiers. But overall, a push-pull class B amplifier provides an excellent balance between power. Feedback Amplifiers Feedback amplifiers are electronic circuits that utilize feedback to modify the gain, bandwidth, or other characteristics of an amplifier. The feedback reduces or eliminates distortion or other unwanted effects in the output. There are several types of feedback amplifiers, as follows: 1. Voltage-series feedback amplifier: In this type of amplifier, the feedback signal is taken from the output voltage and sent to the input through a series resistance. It provides good stability and wide frequency response. 2. Voltage-shunt feedback amplifier: In this type of amplifier, the feedback signal is taken from the output voltage and sent to the input through a shunt resistance. It provides high input impedance and relatively low output impedance. 3. Current-series feedback amplifier: In this type of amplifier, the feedback signal is taken from the output current and sent to the input through a series resistance. It provides high gain and good bandwidth. 4. Current-shunt feedback amplifier: In this type of amplifier, the feedback signal is taken from the output current and sent to the input through a shunt resistance. It provides good linearity and low output impedance. 5. Transconductance amplifier: In this type of amplifier, the feedback signal is taken from the output current and sent to the input through a voltage-controlled current source. It provides high linearity and low noise. Dept. of Robo cs & Ar ﬁcial Intelligence 27 JNNCE Srinivasa Murthy MK Overall, feedback amplifiers help improve the performance of electronic circuits, making them more reliable and efficient. Introduction to Feedback Amplifiers An amplifier circuit boosts the strength of a signal. However, as it amplifies, it not only increases the useful information in the signal but also enhances any unwanted noise that might be present. The noise is introduced because amplifiers are sensitive to temperature changes and stray electric or magnetic fields. Consequently, high-gain amplifiers often produce both signal and undesirable noise in their output, which is not ideal. Using negative feedback, where a portion of the output is injected in the opposite phase to the input signal, can significantly decrease the noise in amplifier circuits. Principle of Feedback Amplifier A feedback amplifier generally consists of two parts. They are the amplifier and the feedback circuit. The feedback circuit usually consists of resistors. The concept of feedback amplifier can be understood from the following figure. From the above figure, the gain of the amplifier is represented as A. the gain of the amplifier is the ratio of output voltage Vo to the input voltage Vi. the feedback network extracts a voltage Vf = β ×Vo from the output Vo of the amplifier. This voltage is added for positive feedback and subtracted for negative feedback, from the signal voltage Vs. Now, for positive feedback 𝑉 = 𝑉 + 𝑉 = 𝑉 + 𝛽𝑉 𝛽 for negative feedback 𝑉 = 𝑉 − 𝑉 = 𝑉 − 𝛽𝑉 𝛽 β is called the feedback ration or feedback fraction. β is defined as ratio of feedback voltage to the output voltage. β can be negative if it is negative feedback and positive if it is a positive feedback. ±𝑉 feedback voltage 𝛽= = 𝑉 output voltage The Gain A of the amplifier is defined as the ratio of output voltage to the ration of input voltage. Dept. of Robo cs & Ar ﬁcial Intelligence 28 JNNCE Srinivasa Murthy MK 𝑉 output voltage 𝐴= = 𝑉 input voltage 𝑉 𝑉 𝑉 𝐴= = = 𝑉 𝑉 −𝑉 (𝑉 − 𝛽𝑉 ) Rearranging the above 𝐴𝑉 = 𝑉 (1 + 𝐴𝛽) 𝑉 𝐴 = 𝑉 1 + 𝐴𝛽 Let Af be the overall gain (gain with the feedback) of the amplifier. This is defined as the ratio of output voltage Vo to the applied signal voltage Vs, i.e., 𝐹𝑖𝑛𝑎𝑙 𝑂𝑢𝑡𝑝𝑢𝑡 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑉 𝐴 𝑂𝑣𝑒𝑟𝑎𝑙𝑙 𝑔𝑎𝑖𝑛, 𝐴 = = = 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝐼𝑛𝑝𝑢𝑡 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑉 1 + 𝐴𝛽 The above derivation is for negative feedback. Now for positive feedback 𝐹𝑖𝑛𝑎𝑙 𝑂𝑢𝑡𝑝𝑢𝑡 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑉 𝐴 𝑂𝑣𝑒𝑟𝑎𝑙𝑙 𝑔𝑎𝑖𝑛, 𝐴 = = = 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝐼𝑛𝑝𝑢𝑡 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑉 1 − 𝐴𝛽 Types of Amplifiers There are two main types of feedback amplifiers, categorized by the effect the feedback signal has on the input signal: Positive Feedback Amplifiers: ii. Also called regenerative or direct feedback amplifiers. iii. The feedback signal is in phase with the input signal, meaning it adds to and amplifies it. iv. This increases the gain of the amplifier significantly. v. However, it also leads to: vi. Distortion: The amplified signal becomes easily distorted due to non-linearities in the circuit. vii. Instability: The gain can become highly sensitive to changes in component values and operating conditions, leading to unpredictable behavior. viii. Applications: Oscillators, regenerative receivers, filters with sharp cutoffs (where some distortion is acceptable). Negative Feedback Amplifiers: i. Also called degenerative feedback amplifiers. ii. The feedback signal is out of phase with the input signal, meaning it subtracts from it. iii. This reduces the gain of the amplifier, but offers several advantages: iv. Reduced distortion: The feedback signal cancels out non-linearities in the circuit, resulting in a cleaner output signal. Dept. of Robo cs & Ar ﬁcial Intelligence 29 JNNCE Srinivasa Murthy MK v. Increased stability: The gain is less sensitive to changes in components and operating conditions, making the amplifier more predictable. vi. Improved frequency response: The feedback can be used to flatten the frequency response of the amplifier, making it more suitable for applications like audio amplification. vii. Applications: Most modern amplifiers, signal conditioners, and control systems. Besides these two main types, feedback amplifiers can also be classified based on the type of signal (voltage or current) and the way it is fed back (series or shunt): Voltage-series feedback amplifier: In this type of amplifier, the feedback signal is taken from the output voltage and sent to the input through a series resistance. It provides good stability and wide frequency response. Voltage-shunt feedback amplifier: In this type of amplifier, the feedback signal is taken from the output voltage and sent to the input through a shunt resistance. It provides high input impedance and relatively low output impedance. Current-series feedback amplifier: In this type of amplifier, the feedback signal is taken from the output current and sent to the input through a series resistance. It provides high gain and good bandwidth. Current-shunt feedback amplifier: In this type of amplifier, the feedback signal is taken from the output current and sent to the input through a shunt resistance. It provides good linearity and low output impedance. 5. Transconductance amplifier: In this type of amplifier, the feedback signal is taken from the output current and sent to the input through a voltage-controlled current source. It provides high linearity and low noise. Choosing the right type of feedback amplifier depends on the desired gain, bandwidth, distortion level, and stability requirements of the application. RC Coupled Amplifier RC coupled amplifiers are electronic circuits that amplify audio signals using resistors and capacitors to couple different stages of amplification. The basic construction and working of RC coupled amplifiers are as follows: Construction: RC coupled amplifiers consist of two or more stages of amplification that are connected through capacitive coupling as shown in the circuit diagram. Each stage of amplification Dept. of Robo cs & Ar ﬁcial Intelligence 30 JNNCE Srinivasa Murthy MK consists of a transistor, resistors, and capacitors. 1. Transistors (Q1 & Q2): RC amplifiers typically use bipolar junction transistors (BJTs) in the common emitter (CE) configuration. 2. Biasing Network: Resistors form a voltage divider network to properly bias the transistor and establish a stable operating point. 3. Input Capacitor (Cin): This blocks DC components of the input signal and allows only AC signals to pass through. 4. Coupling Capacitor (Cc): This couples the amplified output of one stage to the input of the next stage, blocking DC components again. 5. By-pass Capacitor (Ce): This shunts the emitter resistor, providing a low-impedance path for AC signals and further stabilizing the amplifier. 6. Load Resistor (Rl): This provides the appropriate load for the transistor and determines the current and voltage gain of the stage. 7. Power Supply: Typically a DC power supply of 12 V is provides the necessary voltage and current for the amplifier operation. Working: 1. Input Signal: An AC input signal is applied to the base of the transistor through the input capacitor (Cin). 2. Amplification: The transistor amplifies the AC signal, resulting in a larger AC voltage across the collector load resistor (Rl). 3. Coupling: The amplified AC signal is then coupled to the next stage via the coupling capacitor (Cc). This capacitor blocks any DC component from previous stages. 4. Stage Cascade: This process of amplification and coupling can be cascaded through multiple stages, further increasing the overall gain of the amplifier. 5. Frequency Response: The values of the resistors and capacitors affect the amplifier's frequency response. Larger capacitors pass lower frequencies while smaller ones pass Dept. of Robo cs & Ar ﬁcial Intelligence 31 JNNCE Srinivasa Murthy MK higher frequencies. By carefully choosing these values, we can control the amplifier's bandwidth and roll-off characteristics. 6. Output: The final amplified AC signal is available at the output of the last stage. Advantages of RC amplifiers: 1. Simple and cost-effective design. 2. Good for AC signal amplification. 3. Can be cascaded for higher gain. 4. Relatively stable due to inherent capacitive coupling. Disadvantages of RC amplifiers: 1. Lower gain compared to transformer-coupled amplifiers. 2. Limited DC response due to capacitor coupling. 3. Frequency response limitations based on chosen resistor and capacitor values. Oscillators Def:Oscillators are electronic circuits that generate an output signal without the necessity of an input signal. Oscillators produces a periodic waveform on its output with only the DC supply voltage as an input.that is, it produces a periodic waveform on its output with only the DC supply voltage as an input. Different types of oscillators produce various types of outputs including sine waves, square waves, triangular waves, and sawtooth waves. The basic structure of a sinusoidal oscillator consists of an amplifier and a frequency-selective network connected in a positive feedback loop as shown in figure 1. Dept. of Robo cs & Ar ﬁcial Intelligence 32 JNNCE Srinivasa Murthy MK Practical Oscillator Circuit A Practical Oscillator circuit consists of a tank circuit, a transistor amplifier, and a feedback circuit. The following circuit diagram shows the arrangement of a practical oscillator. 1. Tank Circuit − The tank circuit consists of an inductance L connected in parallel with capacitor C. The values of these two components determine the frequency of the oscillator circuit and hence this is called as Frequency determining circuit. 2. Transistor Amplifier − The output of the tank circuit is connected to the amplifier circuit so that the oscillations produced by the tank circuit are amplified here. Hence the output of these oscillations are increased by the amplifier. 3. Feedback Circuit − The function of feedback circuit is to transfer a part of the output energy to LC circuit in proper phase. This feedback is positive in oscillators while negative in amplifiers. The Barkhausen Criterion A feedback amplifier generally consists of two parts. They are the amplifier and the feedback circuit. The feedback circuit usually consists of resistors. From the figure 1, the gain of the amplifier is represented as A. The gain of the amplifier is the ratio of output voltage Vo to the input voltage Vi. The feedback network extracts a voltage Vf = β Vo from the output Vo of the amplifier. The quantity β = Vf/Vo is called as feedback ratio or feedback fraction. This voltage is added for positive feedback and subtracted for negative feedback, from the signal voltage Vin. Dept. of Robo cs & Ar ﬁcial Intelligence 33 JNNCE Srinivasa Murthy MK 𝑉 = 𝑉 − −𝑉 = 𝑉 + 𝑉 − − − − − − − − − − − − − − − − − − − −1 𝑉 =𝐴×𝑉 𝑉 = 𝛽 × 𝑉 = 𝐴𝛽𝑉 − − − − − − − − − − − − − − − − − − − − − − − −2 From 1 and 2 we have 𝑉 𝐴𝑉 𝐴𝑉 𝐴𝑉 𝐴 = = = = = 𝐴 −−−−−−−−−3 𝑉 𝑉 −𝑉 𝑉 − 𝐴𝛽𝑉 𝑉 (1 − 𝐴𝛽) 1 − 𝐴𝛽 Af is call the overall gain of the oscillator 𝑜𝑢𝑡𝑝𝑢𝑡 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑉 𝐴 𝐴 = = = 𝑖𝑛𝑝𝑢𝑡 𝑠𝑖𝑔𝑛𝑎𝑙 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑉 1 − 𝐴𝛽 Where Aβ is the feedback factor or the loop gain However, Vin=0 and Vo ≠ 0 implies that: If Aβ = 1, then Af = ∞. Thus the gain becomes infinity, i.e., there is output without any input. In another words, the amplifier works as an Oscillator. 𝐴𝛽 = 1 − − − − − − − − − − − − − − − − − − − − − − − − − − − −4 Expressed in polar form 𝐴𝛽 = 1∠0 𝑜𝑟 360 − − − − − − − − − − − − − − − − − − − − − − − 5 Equation 4 & 5 gives two requirements for oscillation: The magnitude of the loop gain Aβ must be at least 1, and The total phase shift of the loop gain Aβ must be equal to 0° or 360°. The condition Aβ = 1 is called as Barkhausen Criterion of oscillations. This is a very important factor to be always kept in mind, in the concept of Oscillators In the figure , if the amplifier causes a phase shift of 180°, the feedback circuit must provide an additional phase shift of 180° so that the total phase shift around the loop is 360°. The type of waveform generated by an oscillator depends on the components in the circuit hence may be sinusoidal, square or triangular. In addition, the frequency of oscillation is determined by the components in the feedback circuit. Frequency Stability of an Oscillator The frequency stability of an oscillator is a measure of its ability to maintain a constant frequency, over a long time interval. When operated over a longer period of time, the oscillator frequency may have a drift from the previously set value either by increasing or by decreasing. The change in oscillator frequency may arise due to the following factors − 1. Operating point of the active device such as BJT or FET used should lie in the linear region of the amplifier. Its deviation will affect the oscillator frequency. Dept. of Robo cs & Ar ﬁcial Intelligence 34 JNNCE Srinivasa Murthy MK 2. The temperature dependency of the performance of circuit components affect the oscillator frequency. 3. The changes in d.c. supply voltage applied to the active device, shift the oscillator frequency. This can be avoided if a regulated power supply is used. 4. A change in output load may cause a change in the Q-factor of the tank circuit, thereby causing a change in oscillator output frequency. 5. The presence of inter element capacitances and stray capacitances affect the oscillator output frequency and thus frequency stability. Audio Oscillator An audio oscillator is an electronic oscillator circuit that is specifically designed to produce an audio frequency range signal. Unlike the waveform generators or general-purpose oscillators, which can generate signals of various frequency ranges, an audio oscillator is designed to generate only audio signals (20Hz to 20kHz) that can be heard by humans. Audio oscillators are mainly used in the testing and calibration of audio equipment like speakers, amplifiers, and other audio devices. Here are some common types of audio oscillators: 1. RC phase-shift oscillator - This type of oscillator generates a sine wave output and is based on the RC phase-shift network. It is commonly used in audio applications and provides a stable, low distortion output. 2. Wien bridge oscillator - This type of oscillator generates a low-distortion sinusoidal wave output that can be fine-tuned to different frequencies. It is commonly used in audio applications where a stable, low distortion waveform is required. 3. Hartley oscillator - This oscillator uses a tapped coil and a capacitor to generate a high quality sine wave output. It is commonly used in audio applications where a stable waveform with low distortion is required. 4. Clapp oscillator - This type of oscillator is similar to the Colpitts oscillator, but it includes an additional parallel resonant circuit. The Clapp oscillator is used in audio applications and provides a stable, low distortion waveform. Overall, audio oscillators are essential in the production, testing, and calibration of audio equipment. There are many different types of audio oscillators that provide different waveform outputs and characteristics, making them versatile tools for audio applications. Radio Oscillator A radio oscillator is a type of electronic oscillator circuit that produces an alternating current (AC) output at a specific frequency, typically within the range of radio frequencies (30Hz to 300 GHz). These oscillators are used in radio transmitters, receivers, and other related equipment. The frequency stability and accuracy of the oscillator are critical in radio communications where signals require to be on a specific frequency to communicate effectively. 1. LC oscillator - An LC oscillator is made up of an inductor and capacitor resonant circuit and produces a sinusoidal waveform. It is commonly used in radio transmitters and receivers. Dept. of Robo cs & Ar ﬁcial Intelligence 35 JNNCE Srinivasa Murthy MK 2. Crystal oscillator - A crystal oscillator uses a quartz crystal to create a precise frequency reference and produces a very stable oscillation signal suitable for use in digital circuits. In summary, Radio oscillators are essential in radio communication and have played a significant role in the development of modern communication technology. The selection of the type of oscillator depends on specific requirements such as frequency range required, stability and accuracy, and the application of the oscillator circuit. JFET-Junction Field Effective Transistor A Junction Field Effect Transistor (JFET) is a type of transistor that relies on the electric field effect for its operation. JFETs are three-terminal semiconductor devices that can be used as voltage-controlled resistors. They are commonly used in electronic circuits for various applications, such as amplifiers, switches, and voltage-controlled resistors. Construction: A JFET typically consists of a bar of semiconductor material, which can be either n-type or p- type. There are two main types of JFETs based on their channel doping: N-channel JFET and P-channel JFET. N-Channel JFET: 1. The semiconductor bar is predominantly made of n-type material. 2. The central region of the bar forms the channel through which current flows. 3. Two p-type regions, known as the "gate," are located on either side of the channel. P-Channel JFET: 1. The semiconductor bar is predominantly made of p-type material. 2. The central region of the bar forms the channel. 3. Two n-type regions, known as the "gate," are located on either side of the channel. Both types of JFETs have three terminals: the source (S), the drain (D), and the gate (G). Dept. of Robo cs & Ar ﬁcial Intelligence 36 JNNCE Srinivasa Murthy MK Working Principle: The operation of a JFET is based on the voltage applied to the gate terminal. Here's a general overview of how an N-channel JFET works: The operation of a JFET is based on the voltage applied to the gate terminal. 1. Zero Gate-to-Source Voltage (Vgs): When no external voltage is applied between the gate (G) and the source (S), the JFET is in its normal "off" state. 2. Applying a Negative Vgs (Reverse-Bias): When a negative voltage is applied to the gate with respect to the source (Vgs < 0), it creates an electric field that repels the majority charge carriers (electrons in the case of an N-channel JFET) from the channel. This increases the resistance of the channel, restricting the flow of current between the source and drain. 3. Increasing Vgs (More Reverse-Bias): As the negative voltage applied to the gate increases, the electric field becomes stronger, further restricting the flow of current through the channel. The JFET operates as a voltage-controlled resistor, and the resistance between the source and drain increases with an increasing negative Vgs. This increase in current is linear up to a certain point A, known as Knee Voltage. 4. Cut-off Region: There is a critical voltage, often referred to as the pinch-off voltage (Vp), beyond which the channel is completely pinched off, and the current between the source and drain becomes very small. In this region, the JFET is said to be in the "cut- off" state or region. JFETs operate primarily in the depletion mode, where the application of a reverse-bias voltage to the gate depletes the channel of charge carriers, reducing the current flow. The opposite behavior occurs for P-channel JFETs, where a positive Vgs is applied to increase the channel current. Dept. of Robo cs & Ar ﬁcial Intelligence 37 JNNCE Srinivasa Murthy MK The JFET has different stages of operation depending on the input voltages, and the characteristics of JFET in different regions are explained below. It operates in ohmic, saturation, cut-off, and break-down regions. 1. Ohmic Region: If the voltage across Gate and Source is zero, then the depletion region of the channel is tiny, and in this region, it acts as a voltage-controlled resistor. 2. Pinched-off Region: It is the cut-off region. JFET enters this region when the gate voltage is negative; then, the channel closes; hence no current flows through the channel. 3. Saturation or Active Region: In this region, the channel acts as a good conductor, which is controlled by the gate voltage. 4. Breakdown Region: If the drain to source voltage is very high, then the channel of the JFET breaks down, and uncontrolled maximum current passes through the device. Applications of JFET 1. FET is used as a switch, JFET amplifier, and buffer. 2. JFET is used as a chopper. 3. JFET is used in oscillatory circuits because of its low frequency, used in cascade amplifiers in RF amplifiers. 4. Used in digital circuits because of their small size. 5. Used in communication equipment because of their low modulation distortion. 6. Used as voltage-controlled resistors in operational amplifiers. Dept. of Robo cs & Ar ﬁcial Intelligence 38 JNNCE Srinivasa Murthy MK Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) A Metal Oxide Semiconductor Field Effect Transistor (MOSFET) has four terminals − Source (S), Gate (G), Drain (D), Body (B). It is a semiconductor device which is used for switching and amplification applications in electronic circuits. In general, the body terminal is connected with the source thus forming a three terminal device just like an FET. The MOSFET is a voltage-controlled device. Since its operation depends upon the flow of majority carriers only, hence MOSFET is a unipolar device. Classification of MOSFET MOSFETs are of two classes: Enhancement mode and depletion mode. Each class is available as n-channel or p-channel; hence overall they tally up to four types of MOSFETs. Depletion Mode When there is no voltage across the gate terminal, the channel shows maximum conductance. When the voltage across the gate terminal is either positive or negative, then the channel conductivity decreases. Enhancement Mode When there is no voltage across the gate terminal, then the device does not conduct. When there is the maximum voltage across the gate terminal, then the device shows enhanced conductivity. Dept. of Robo cs & Ar ﬁcial Intelligence 39 JNNCE Srinivasa Murthy MK Construction: A MOSFET consists of three main regions: the source (S), the drain (D), and the gate (G). The MOSFET can be classified into two main types based on the type of semiconductor material used in its construction:  N-Channel MOSFET (NMOS): 1. The semiconductor material is predominantly of n-type. 2. The source and drain terminals are doped with n-type material. 3. The gate is separated from the semiconductor material by a thin insulating layer (oxide), typically made of silicon dioxide (SiO₂).  P-Channel MOSFET (PMOS): 1. The semiconductor material is predominantly of p-type. 2. The source and drain terminals are doped with p-type material. 3. The gate is separated from the semiconductor material by a thin insulating layer, usually silicon dioxide. Dept. of Robo cs & Ar ﬁcial Intelligence 40 JNNCE Srinivasa Murthy MK Working Principle: The operation of a MOSFET is based on the modulation of the conductive channel between the source and drain terminals by an electric field applied to the gate terminal. Here's a general overview of how an N-channel MOSFET works: 1. Zero Gate-to-Source Voltage (Vgs): When no voltage is applied between the gate (G) and source (S) terminals (Vgs = 0), the MOSFET is in its default state, known as the "off" state. In this state, there is no direct electrical connection between the source and drain terminals. 2. Applying a Positive Vgs (Threshold Voltage): When a positive voltage is applied to the gate with respect to the source (Vgs > threshold voltage, Vth), an electric field is created, which attracts electrons from the n-type semiconductor material near the surface, forming an electron-rich region called the "inversion layer" or "channel" between the source and drain. 3. Increasing Vgs (Enhancement Mode): As the positive voltage on the gate increases, the width of the inversion layer increases, allowing more current to flow between the source and drain terminals. The MOSFET operates in the "enhancement mode," meaning that the application of a voltage enhances the conductivity of the channel. 4. Cut-off Region: The MOSFET operates as a voltage-controlled switch. When the gate- to-source voltage is below the threshold voltage, the channel is depleted, and the MOSFET is in the "off" state, limiting the current flow. Operating Regions of MOSFET A MOSFET is seen to exhibit three operating regions. Here, we will discuss those regions. 1. Cut-Off Region: The cut-off region is a region in which there will be no conduction and as a result, the MOSFET will be OFF. In this condition, MOSFET behaves like an open switch. Dept. of Robo cs & Ar ﬁcial Intelligence 41 JNNCE Srinivasa Murthy MK 2. Ohmic Region: The ohmic region is a region where the current (IDS)increases with an increase in the value of VDS. When MOSFETs are made to operate in this region, they are used as amplifiers. 3. Saturation Region: In the saturation region, the MOSFETs have their IDS constant in spite of an increase in VDS and occurs once VDS exceeds the value of pinch-off voltage VP. Under this condition, the device will act like a closed switch through which a saturated value of IDS flows. As a result, this operating region is chosen whenever MOSFETs are required to perform switching operations. Question Bank for first test from module-1 1. Explain the working of halfwave and full wave rectifiers with proper circuits and output waveforms. 2. Describe the principles of clippers and clamper. Explain any two circuit. 3. What is transistor biasing? List the methods of transistor biasing. 4. Make an circuit analysis on voltage divider transistor biasing to find collector current, emitter current and collector emitter voltage. 5. Explain the thermal stability of transistor biasing. 6. What are feed back amplifiers? Explain any two types. 7. What are power amplifiers? Explain the circuit of PUSH PULL amplifiers circuits. 8. Describe the characteristics of MOSFET with a neat diagram. 9. Draw circuit and do DC load line for the transistor with Rc=2.5kΩ, VCC = 12.5 V. 10. Draw DC load line and Q-point on the output characteristics of CE configuration (Si material is used). Given: RB = 12.5 kΩ , RC =1kΩ, VBB= 2.7 V, VCC= 6V. Dept. of Robo cs & Ar ﬁcial Intelligence 42

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