BE Unit - 4 Material (English) PDF

School of Diploma Studies Faculty of Engineering and Technology Computer Engineering Department Semester - 1 Basic Electronics (24DIEE102) Unit – 4 Oscillators and Measuring Instruments Mr. Punit C. Trivedi...

School of Diploma Studies Faculty of Engineering and Technology Computer Engineering Department Semester - 1 Basic Electronics (24DIEE102) Unit – 4 Oscillators and Measuring Instruments Mr. Punit C. Trivedi Lecturer (DCE) Yogidham Gurukul, Kalawad Road, Rajkot - 360005 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4 Q - 1 What is feedback in amplifier?  Feedback:  “The process of feeding some part of the output of any device back into the input is called feedback”. Fig. 4.1 Feedback in amplifier  From the figure, some of the output signal of the amplifier is fed into the input signal. This is called feedback. Feedback causes the amplifier's signal to change.  Here, A gain amplifier is given an input voltage of Vi which results in an output voltage of Vo.  Now here the output voltage Vo is fed to the input of feedback network with 𝛽 gain  Consequently the output voltage Vf = βVo is obtained which is given in the input.  Here the value of β gain is less than 1.  Feedback causes the amplifier input voltage to change from Vi to VI.  VI= Vi + Vf  There are two types of feedback o Positive Feedback o Negative Feedback Q - 2 Explain Positive feedback in details.  Positive feedback:  The feedback in which the feedback energy i.e., either voltage or current is in phase with the input signal and thus aids it is called as Positive feedback.  Both the input signal and feedback signal introduces a phase shift of 180o thus making a 360o resultant phase shift around the loop, to be finally in phase with the input signal.  Though the positive feedback increases the gain of the amplifier, it has the disadvantages such as o Increasing distortion o Instability Page 1 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4  It is because of these disadvantages the positive feedback is not recommended for the amplifiers. If the positive feedback is sufficiently large, it leads to oscillations, by which oscillator circuits are formed.  Positive feedback aslo known as “Regenerative feedback”. Fig. 4.2 Positive feedback in Amplifier  System gain with positive feedback: 𝑉𝑜  Without feedback voltage gain: 𝐴 = ……………………(1) 𝑉𝑖 𝑉𝑜  With Positive feedback: 𝐴𝑓 = ……………………(2) 𝑉𝑠  Vs and Vf both in Phase: Vi = Vs + Vf Vi = Vs + βVo (Vf= βVo) Vs = Vi - βVo 𝑉𝑜  Put value of Vs in equation(1): 𝐴𝑓 = 𝑉𝑖 – 𝛽𝑉𝑜 𝑉𝑜 𝑉𝑖 𝐴𝑓 = 𝛽𝑉𝑜 𝑉𝑖 – 𝑉𝑖 𝑉𝑜/𝑉𝑖 𝐴𝑓 = 𝑉𝑖 – 𝛽𝑉𝑜/𝑉𝑖 𝐴 𝐴𝑓 = 1 – 𝐴𝛽 𝐴 𝐴= 1 – 𝐴𝛽 Page 2 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4  Advantages of positive feedback: o Input voltage/current increases. o Increases in frequency.  Disadvantages of positive feedback: o The system has no control over the gain and output. As a result, the quality of the output decreases. Q - 3 Explain negative feedback in details.  Negative feedback:  The feedback in which the feedback energy i.e., either voltage or current is out of phase with the input and thus opposes it, is called as negative feedback.  In negative feedback, the amplifier introduces a phase shift of 180o into the circuit while the feedback network is so designed that it produces no phase shift or zero phase shift. Thus the resultant feedback voltage Vf is 180o out of phase with the input signal Vi.  Though the gain of negative feedback amplifier is reduced, there are many advantages of negative feedback such as o Stability of gain is improved o Reduction in distortion o Reduction in noise o Increase in input impedance o Decrease in output impedance o Increase in the range of uniform application  It is because of these advantages negative feedback is frequently employed in amplifiers. Fig. 4.3 Negative Feedback in Amplifier Page 3 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4  System gain with negative feedback: 𝑉𝑜  Without feedback voltage gain: 𝐴 = ……………………(1) 𝑉𝑖 𝑉𝑜  With Positive feedback: 𝐴𝑓 = ……………………(2) 𝑉𝑠  Vs and Vf not in Phase: Vi = Vs - Vf Vi = Vs - βVo (Vf= βVo) Vs = Vi + βVo 𝑉𝑜  Put value of Vs in equation(2): 𝐴𝑓 = 𝑉𝑠 𝑉𝑜 𝐴𝑓 = 𝑉𝑖 + 𝛽𝑉𝑜 𝑉𝑜/𝑉𝑖 𝐴𝑓 = 𝑉𝑖 + 𝛽𝑉𝑜/𝑉𝑖 𝐴 𝐴𝑓 = 1 + 𝐴𝛽 𝐴 𝐴= 1 + 𝐴𝛽  Advantages of negative feedback: o Controls the gain of the output. o Bandwidth increases. o Amplitude and frequency distortion are reduced. o The quality of the output noise increases. o Harmonics and distortion are reduced  Disadvantages of negative feedback: o The gain decreases o The value of input current decreases. Q - 4 What is oscillator?  Oscillator:  “The oscillator is an electronic device capable of producing electronic oscillations the form of signal waves, popularly sine waves, and square waves”. Oscillations refer to the forward and backward movement. It also converts one form of a power supply to another.  Types of oscillator: o Hartely Oscillator o Colpitt’s Oscillator Page 4 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4 Q - 5 Explain Hartley oscillator with proper diagram. Fig. 4.4 Hartley Oscillator  Construction:  The Hartley oscillator is designed for generation of sinusoidal oscillations in the R.F range (20 KHz - 30 MHz).  It is very popular and used in radio receivers as a local oscillator.  The circuit diagram of Hartley oscillator (parallel or shunt-fed) using BJT is shown in Figure.  It consists of an R-C coupled amplifier using an n-p-n transistor in CE configuration.  R1 and R2 are two resistors which form a voltage divider bias to the transistor.  A resistor RE is connected in the circuit which stabilizes the circuit against temperature variations.  A capacitor CE is connected in parallel with RE, acts as a bypass capacitor and provides a low reactive path to the amplified ac signal.  The coupling capacitor CC blocks dc and provides an ac path from the collector to the tank circuit.  The L-C feedback network (tank circuit) consists of two inductors L1, and L2 (in series) which are placed across a common capacitor C1 and the centre of the two inductors is tapped as shown in fig.  The feedback network (L1, L2 and C1) determines the frequency of oscillation of the oscillator.  Working:  When the collector supply is given, a transient current is produced in the oscillatory or tank circuit. The oscillatory current in the tank circuit produces a.c. voltage across L1.  The auto-transformer made by the inductive coupling of L1 and L2 helps in determining the frequency and establishes the feedback. As the CE configured transistor provides 180o phase shift, another 180o phase shift is provided by the transformer, which makes 360o phase shift between the input and output voltages.  This makes the feedback positive which is essential for the condition of oscillations. When the loop gain |βA| of the amplifier is greater than one, oscillations are sustained in the circuit.  Frequency Page 5 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4  The equation for frequency of Hartley oscillator is given as 1  F= 2𝜋√𝐿𝐶  Here, L is the total cumulatively coupled inductance; L1 and L2 represent inductances of 1st and 2nd coils; and M represents mutual inductance.  Mutual inductance is calculated when two windings are considered.  Advantages: o Frequency can be varied by employing either a variable capacitor or a variable inductor. o Less number of components is sufficient. o The amplitude of the output remains constant over a fixed frequency range.  Disadvantages: o It cannot be a low frequency oscillator. o Harmonic distortions are present.  Applications: o It is used to produce a sine wave of desired frequency. o Mostly used as a local oscillator in radio receivers. o It is also used as R.F. Oscillator. Q - 6 Draw and explain the working of Colpitt’s oscillator. Fig. 4.5 Colpitt’s Oscillator Page 6 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4  Construction:  Colpitt’s oscillator is generally used in RF applications and the typical operating range is 20 KHz to300MHz.  In Colpitt’s oscillator, the capacitive voltage divider setup in the tank circuit works as the feedback source and this arrangement gives better frequency stability when compared to the Hartley oscillator which uses an inductive voltage divider setup for feedback.  The circuit diagram of a Colpitt’s oscillator using transistor is shown in the figure as shown.  In the circuit diagram resistors R1 and R2 gives a voltage divider biasing to the transistor  Resistor R4 limits the collector current of the transistor. C in is the input DC decoupling capacitor while Cout is the output decoupling capacitor.  Re is the emitter resistor and it’s meant for thermal stability. Ce is the emitter by-pass capacitor.  Job of the emitter by-pass capacitor is to by-pass the amplified AC signals from dropping across Re.  The emitter by-pass capacitor is not there, the amplified AC signal will drop across Re and it will alter the DC biasing conditions of the transistor and the result will be reduced gain.  Capacitors C1, C2 and inductor L1 forms the tank circuit. Feedback to the base of transistor is taken from the junction of Capacitor C2 and inductor L1 in the tank circuit.  Working:  When the collector supply is given, a transient current is produced in the tank circuit. The oscillatory current in the tank circuit produces a.c. voltage across C1 which are applied to the base emitter junction and appear in the amplified form in the collector circuit and supply losses to the tank circuit.  If Cin is at positive potential with respect to terminal Re at any instant, then Cout will be at negative potential with respect to Re at that instant because Re is grounded. Therefore, Cin and Cout are out of phase by 180o.  As the CE configured transistor provides 180o phase shift, it makes 360o phase shift between the input and output voltages. Hence, feedback is properly phased to produce continuous Undampped oscillations. When the loop gain |βA| of the amplifier is greater than one, oscillations are sustained in the circuit.  The equation for frequency of Colpitt’s oscillator is given as 1  F= 2𝜋√𝐿𝐶  Advantages: o Colpitt’s oscillator can generate sinusoidal signals of very high frequencies. o The frequency stability is high. o Frequency can be varied by using both the variable capacitors. o Less number of components is sufficient. o The amplitude of the output remains constant over a fixed frequency range. Page 7 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4 o The Colpitt’s oscillator is designed to eliminate the disadvantages of Hartley oscillator and is known to have no specific disadvantages. Hence there are many applications of a Colpitt’s oscillator.  Applications: o Colpitt’s oscillator can be used as High frequency sine wave generator. o Mostly used as a local oscillator in radio receivers. o It is also used in Mobile applications. Q - 7 Different electrical quantity and instrument for measuring them.  Voltage:  By using voltmeter, we can measure voltage and potential difference.  There are two types: A.C. and D.C.  It is available in wide range and it always connected in parallel. Fig. 5.1 Voltmeter  Current:  By using ammeter, we can measure current.  There are two types: A.C. and D.C.  It always connected in parallel and available in wide range Fig. 5.2 Ammeter Page 8 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4  Power:  By using wattmeter, we can measure power.  There are two coils. One is current coil which is connected in series. Second is pressure coil which is connected in parallel like voltmeter. Fig. 5.3 Wattmeter  Energy: Electrical energy is measured by energy meter. Fig. 5.4 Energy meter  Frequency:  Electrical energy is measured by energy meter.  Frequency of A.C. supply is measured by frequency meter.  Power factor :  Power factor of load is measured by power factor meter. Page 9 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4 Q - 8 What is Analog and digital display?  Analog display:  In analog type, there are two main parts. one part is moving and other part is stationary.  The part which move is called moving part.  It is mounted on spindle.  The spindle is pivoted on the jewel bearings.  A pointer is attached to the spindle which moves over the graduated scale.  Digital display:  There is no moving part. The quantity measured is converted into analog to digital and it is display on the panel.  Types of digital display (1) LED Display(2) LCD Display Fig. 5.6 Analog and Digital Display Q - 9 Explain the Digital display (LED and LCD).  LED display:  Light Emitting Diode is known as LED. LCD and LED both are same technology, but main difference is in BLU.  It is one type of junction diode. It emits light when it is forward biased.  The color of light depends upon the material of the diode.  Seven segments are arranged and by exciting the particular segment, several numerals can be displayed. Page 10 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4  The parts of LED are similar to LCD. The backlight unit is the most important part in which the bulbs are arranged in rows. CCFL used in LCD consume a lot of energy and have to be exported in a certain manner as they destroy. Also, CCFL has a short lifespan and cannot improve its contrast ratio much.  LED displays have rows of light emitting diodes in BLU instead of CCFL. These LEDs consume very little energy. Fig. 5.7 Seven segment of LED Diode Fig. 5.8 Light Emitting Diode  Advantages of LED: o Better contrast ratio than LCD. This gives excellent output quality. o Much better "viewing angle" than LCD. o The use of LED in BLU saves a lot of energy. ' o Better colors, combinations and brightness result in exceptional visual quality  Disadvantages of LED: o Disturbances in high pressure areas. o More prone to breakage than CRT. o Certain parts of the screen cannot be viewed properly as certain pixels decline in the long run. Page 11 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4  Applications of LED: o Television o Computer monitor o Mobile phone display o Digital camera display o I-pod, Tab display.  LCD display:  In LCD type display, liquid crystals are used.  These crystals are kept between two transparent glass plates.  When electric field is not applied, the crystals are arranged in definite pattern. But when certain part is excited, the crystals are in that part is disturbed.  Liquid crystal display is very popular these days. The most important issue of this technology is the backlight. LCD panels are of the trans missive type and do not produce light themselves. Thus a back light is required for the operation of LCD. This backlight panel is known as BLU.  LCD Panel: LCD panel is a very energy consuming display system. LCD panels are electronically modulated optical devices. LCD panel consists of several layers, which are above on liquid crystals. These liquid crystals are arranged in rows and are perpendicular to the BLU.  BLU (Backlighting Unit): BLU is the light source illuminating the LCD panel. Which are several bulbs arranged in a row. This bulb is called CCFL (Cathode Compact Fluorescent Lamp) is a common household CFL technology. This consumes a lot of energy and has to follow certain rules to destroy it.  Housing Panel: The housing panel aligns the LCD panel and the BLU panel in parallel. CCFL rows are arranged in this panel.  Apart from this there are many parts like reflector, brightness controller and diffuser. Fig. 5.9 Liquid Crystal Display (LCD) Page 12 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4  Advantages of LCD: o Lighter in weight with respect to screen size o It is very compact, thin, and light CRT displays.  Disadvantages of LCD: o Gives off good heat while working o A failure of the BLU causes the LCD display to stop working. o VIEWING ANGLE is limited so you have to take care of the exact angle to watch the scene.  Applications of LCD: o Television o Computer monitor o Digital camera and projector o In many digital display devices. Q - 10 Explain Analog and Digital multimeter.  Analog multimeter:  An analog multimeter is a permanent magnet moving coil (PMMC) meter type measuring instrument. It works on the principle of d’ Arsonval galvanometer.  The analog multimeter has an analog display that uses the deflection of a pointer on the scale to indicate the level of measurement being made.  The pointer deflects from its initial position increasingly as the measuring quantity increases.  The function can be selected by the function switch.  D.C. voltage is given through the voltage divider which is a chain of resistors.  Voltage is display on the scale with the help of pointer.  The PMMC type meter is respond to d.c. voltage only.  Different shunts are used to measure current in different ranges.  To measure resistance, a constant known current is passed through the resistance to be measured. The voltage drop across the resistance is proportional to the resistance.  This voltage drop is measured and the reading is indicated on the scale which shows resistance in ohms. Page 13 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4 Fig.5.21 Analog Multimeter  Advantages of Analog Multimeter o It gives the continuous reading, thus a sudden change in signal can be detected which is not possible with digital multimeter. o Analog multimeter are very cheap. o All measurement can be made using a single meter only.  Disadvantages of Analog Multimeter o They are bulky and larger sized. o Multiple scales, these can cause confusion. o Low input resistance. o Less accurate than a digital multimeter.  Digital multimeter:  A digital multimeter (DMM) is a measuring instrument used to measure various electrical quantities. The standard measurements that are performed by a DMM are current, voltage and resistance. Apart from these, a digital multimeter can also measure temperature, frequency, capacitance, continuity, transistor gains etc.  Function switch: With the help of this switch, different functions like d.c. voltage (DCV), a.c. voltage (ACV), current (DCA/ACV), resistance can be selected.  D.C. Voltage divider: A chain of resisters is used to measure d.c. Voltages of different ranges. Basic range of DVM is 2V.  Rectifier: An output d.c. voltage is proportional to the rms value of the a.c. Voltage. Resistance chain for the voltage divider is the same for the voltage divider is the same for both the d.c. And a.c. Voltage measurement.  Current sensing resistor: Current to be measured is passed through one of the standard resistor voltage drop across this is measured which is proportional to the current. Page 14 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4  Digital volt meter (DVM): d.c. voltage is measured by this voltmeter. Its basic range is of 2V.This is in form of chip.  Display (LED, LCD): The display is either of LCD type and LED type. The quantity measured by the DMM is display on this. Fig.5.22 Digital Multimeter  Advantages of Digital Multimeter o Cheap and easily available. o Auto range according to requirements. o Smaller size and light in weight. o Auto off. Auto polarity, etc. Q - 11 Explain C.R.O. (Cathode Ray Oscilloscope).  Full form of CRO is cathode ray oscilloscope.  CRO is an electronic instrument which converts electrical signal into visual signal.  The heart of CRO is CRT (cathode ray tube).  In this high velocity sharp beam of electron produced.  This beam is passed through the vertical and horizontal deflecting plates.  The beam then strikes the fluorescent screen and we can see the waveform of the applied to the CRO input. Page 15 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4 Fig. 5.23 Cathode Ray Oscilloscope Fig. 5.24 Block Diagram of Cathode Ray Oscilloscope  Working principle of CRO:  The above block diagram is the basic block diagram of the procedure of CRO.  The input signal is fed to amplifier-1. If the input signal comes from an electrical circuit, it is given directly. If it comes from a physical condition other than an electrical circuit, it is passed through the transducer and fed to amplifier-1.  Amplifier-1 is a vertical type amplifier that amplifies the vertical amplifier input wave, and feeds it to the vertical deflection coil of the cathode ray tube. A vertical deflection coil controls the electron beam up and down.  Cathode ray oscilloscope front panel controls: o ON/OFF Switch: It switches on/off the mains power supply of CRO. It has one LED indicator given on the front panel. o Intensity Control: By this switch the brightness of the given signal is controlled. o Focus Control: By this the sharpness of the given signal is controlled. Page 16 Subject Name and Code: Basic Electronics (24DIEE102) Unit – 4 o Y shift: with the help of this control beam and hence the waveform can be shifted in vertical direction. o X shift: with the help of this control beam and hence the waveform can be shifted in horizontal direction. o VOLTS/DIV Control: This control calibrates the 'volts' for one cut. A cut is 1 centimeter, if set to 2 V, the calibration is calculated to be 2 volts /div. With this control, the complete waveform can be studied on the screen by changing it according to convenience. o Time/DIV: This control changes the speed of the wave. This time interval can range from 0.2 to 0.5 microseconds per centimeter to 0.2 seconds per centimeter. With this control the waveform can also be controlled as per the convenience of the display. o Mono/Dual: With the help of this control, we can see single waveform in mono mode else in dual mode we have two waveforms for comparison. o CH1/CH2: With the help of this control, we can observe signals connected to CH1 and/or CH o Trigger Control: Trigger control is used to study the waves by stopping them at a specific place or time on the screen. Waves can be controlled in many ways by trigger control.  Application of CRO o Measurement of electrical quantity o Measurement of unknown voltage(a) D.C.voltage (b) A.C.voltage o Measurement of electric current o Measurement of unknown frequency(a) By measuring the time period T of one wave(b) By comparing with known frequency Page 17