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Unit-3 Low Power Amplifier 1 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Performance parameter of...

Unit-3 Low Power Amplifier 1 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Performance parameter of Amplifier Voltage Gain (Av) :The ratio of output voltage to input voltage of a BJT amplifier is known as Voltage Gain. Current Gain (Ai): The ratio of output current to input current of a BJT amplifier is known as Current Gain. 2 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Performance parameter of Amplifier Power Gain (Ap): The ratio of output power to input power of a BJT amplifier is known as Voltage Gain. OR Ap=Av ×Ai Input Resistance (Ri) :The resistance directly looking into the base of a BJT amplifier is known as input resistance. Ri = (Vb / Ib) 3 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Performance parameter of Amplifier Output Resistance (Ro) :The resistance looking into the collector of a BJT from the load is known as Output Resistance. Ro RC Bandwidth of Amplifiers: The range of frequency over which the voltage gain of an amplifier remains constant is known as Bandwidth of an amplifier. OR The difference between higher frequency and lower frequency is known as Bandwidth of an amplifier. BW= FH-FL 4 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Performance parameter of Amplifier Frequency Response: The characteristic curve plotted between voltage gain and frequency of an input AC signal applied to a BJT amplifier is known as frequency response of an amplifier. Figure 1.1 Frequency response 5 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Single Stage Common Emitter Amplifier Figure 1.2 Circuit diagram of single stage Figure 1.3 Input Output CE amplifier Waveforms When only one transistor with associated circuitry is used for amplifying a weak signal, the circuit is known as single-stage amplifier. 6 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Construction of Single Stage Common Emitter Amplifier Biasing circuit : The resistances R1, R2 and RE form the biasing and stabilization circuit. Input capacitance Cc1 : This is used to couple the signal to the base of the transistor. If this is not used, the signal source resistance will come across R2 and thus change the bias. The capacitor Cc1 allows only a.c. signal to flow. Emitter bypass capacitor CE : This is connected in parallel with RE to provide a low reactance path to the amplified a.c. signal. If it is not used, then amplified a.c. signal flowing through RE will cause a voltage drop across it, thereby shifting the output voltage. 7 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Coupling capacitor Cc2 : This is used to couple the amplified signal to the output device. This capacitor Cc2 allows only a.c. signal to flow. When a weak input signal is given to the base of the transistor a small amount of base current flows. Due to the transistor action, a larger current flows in the collector of the transistor. (As the collector current is β times of the base current which means IC = βIB). Now, as the collector current increases, the voltage drop across the resistor RC also increases, which is collected as the output. The output of single stage CE amplifier is 180° out of phase with the input single. 8 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Frequency Response of Single Stage CE Amplifier Figure 1.4 Frequency Response of Single Stage CE Amplifier 9 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Frequency Response of Single Stage CE Amplifier 10 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Advantages: It has wide frequency response and large bandwidth. It is most convenient and least expensive amplifier. It provides high audio fidelity. It has low amplitude distortion. It provides low frequency distortion. 11 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Disadvantages: It has a tendency to become noisy with age especially in moist climate. The voltage gain reduces at low as well as high frequencies. It provides poor impedance matching and hence it cannot be used as a final stage of an amplifier. 12 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Applications: It is used in the initial stages of public address (PA) amplifier systems. It is used in stereo amplifier. It is widely used as a voltages amplifier. It is used in tape recorder, CD players, VCRs, DVD players, etc. It is used in radio and television receivers. 13 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Multistage Amplifier In practical applications, the output of a single state amplifier is usually insufficient, though it is a voltage or power amplifier. Hence they are replaced by Multi-stage transistor amplifiers. In Multi-stage amplifiers, the output of first stage is coupled to the input of next stage using a coupling device. This process of joining two amplifier stages using a coupling device can be called as Cascading. Figure 1.5 Multistage Amplifier 14 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Purpose of coupling device To transfer the AC from the output of one stage to the input of next stage. To block the DC to pass from the output of one stage to the input of next stage, which means to isolate the DC conditions. 15 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Multistage Amplifier Gain The overall gain is the product of voltage gain of individual stages. Where AV = Overall gain, AV1 = Voltage gain of 1st stage, and AV2 = Voltage gain of 2nd stage. If there are n number of stages, the product of voltage gains of those n stages will be the overall gain of that multistage amplifier circuit. Overall voltage gain in decibels(dB) is the sum of the decibel gains of the individual stages. Gv = Gv1+Gv2 Where, Gv =Overall voltage gain in dB, Gv1 = Voltage gain in dB of 1st stage, Gv2 = Voltage gain in dB of 2nd stage 16 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Multistage Amplifier Gain 17 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Types of Coupling Resistance-Capacitance Coupling Transformer Coupling Direct Coupling 18 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Two-stage RC Coupled Amplifier The two stage amplifier circuit has two transistors, connected in CE configuration and a common power supply VCC is used. The potential divider network R1 and R2 and the resistor R E form the biasing and stabilization network. The emitter by-pass capacitor CE offers a low reactance path to the signal. The resistor RC is used as a load impedance. The input capacitor Cc1 present at the initial stage of the amplifier couples AC signal to the base of the transistor. The capacitor CC2 is the coupling capacitor that connects two stages and prevents DC interference between the stages and controls the shift of operating point. 19 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Two-stage RC Coupled Amplifier Figure 1.6 Circuit diagram two stage RC coupled amplifier 20 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Operation of Two-stage RC Coupled Amplifier When an AC input signal is applied to the base of first transistor, it gets amplified and appears at the collector load Rc which is then passed through the coupling capacitor Cc2 to the next stage. This becomes the input of the next stage, whose amplified output again appears across its collector load. Thus the signal is amplified in stage by stage action. The total gain is less than the product of the gains of individual stages. This is because when a second stage is made to follow the first stage, the effective load resistance of the first stage is reduced due to the shunting effect of the input resistance of the second stage. Hence, in a multistage amplifier, only the gain of the last stage remains unchanged. As we consider a two stage amplifier here, the output phase is same as input. Because the phase reversal is done two times by the two stage CE configured amplifier circuit. 21 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Frequency Response of Two-stage RC Coupled Amplifier Figure 1.7 Frequency Response of two stage RC coupled amplifier Frequency response curve is a graph that indicates the relationship between voltage gain and function of frequency. 22 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Frequency Response of Two-stage RC Coupled Amplifier The frequency rolls off or decreases for the frequencies below 50Hz and for the frequencies above 20 KHz. whereas the voltage gain for the range of frequencies between 50Hz and 20 KHz is constant. The capacitive reactance is inversely proportional to the frequency. 23 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Frequency Response of Two-stage RC Coupled Amplifier At Low frequencies (i.e. below 50 Hz): The capacitive reactance is inversely proportional to the frequency. At low frequencies, the reactance is quite high. The reactance of input capacitor Cc1 and the coupling capacitor CC2 are so high that only small part of the input signal is allowed. The reactance of the emitter by pass capacitor CE is also very high during low frequencies. Hence it cannot shunt the emitter resistance effectively. With all these factors, the voltage gain rolls off at low frequencies. 24 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Frequency Response of Two-stage RC Coupled Amplifier At High frequencies (i.e. above 20 KHz) Again considering the same point, we know that the capacitive reactance is low at high frequencies. So, a capacitor behaves as a short circuit, at high frequencies. As a result of this, the loading effect of the next stage increases, which reduces the voltage gain. Along with this, as the capacitance of emitter diode decreases, it increases the base current of the transistor due to which the current gain (β) reduces. Hence the voltage gain rolls off at high frequencies. 25 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Frequency Response of Two-stage RC Coupled Amplifier At Mid-frequencies (i.e. 50 Hz to 20 KHz) The voltage gain of the capacitors is maintained constant in this range of frequencies, as shown in figure. If the frequency increases, the reactance of the capacitor CC decreases which tends to increase the gain. But this lower capacitance reactive increases the loading effect of the next stage by which there is a reduction in gain. Due to these two factors, the gain is maintained constant. 26 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Advantages of Two Stage RC Coupled Amplifier The frequency response of RC amplifier provides constant gain over a wide frequency range, hence most suitable for audio applications. The circuit is simple and has lower cost because it employs resistors and capacitors which are cheap. It becomes more compact with the upgrading technology. 27 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Disadvantages of Two Stage RC Coupled Amplifier The voltage and power gain are low because of the effective load resistance. They become noisy with age. Due to poor impedance matching, power transfer will be low. 28 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Applications of Two Stage RC Coupled Amplifier They have excellent audio fidelity over a wide range of frequency. Widely used as Voltage amplifiers Due to poor impedance matching, RC coupling is rarely used in the final stages. 29 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  UNIT-5 Tuned Amplifiers #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Tuned Circuit Tuning means selecting. To pick up and amplify the desired radio frequency signal, the resistive load in the audio amplifier is replaced by a tuned circuit. Tuned circuit also called a parallel resonant circuit. The tuner circuit is nothing but a LC circuit which is also called as resonant or tank circuit. The tuned circuit is capable of selecting as particular frequency and rejecting the others. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Tuned Amplifiers The use of tuned circuit in the transistor amplifier circuit, makes possible the selection and amplification of a particular desired radio frequency. Tuned amplifiers are the amplifiers that are employed for the purpose of tuning. The amplifier which amplifies a specific frequency or a narrow band of frequencies is called a Tuned Amplifier. When an amplifier circuit has its load replaced by a tuned circuit, such an amplifier can be called as a Tuned amplifier circuit. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Need of Tuned Amplifiers An audio amplifier is unable to select a particular frequency signal. As it provides a mixture of frequency that lies in the range of 20 Hz to 20 KHz. Thus an audio amplifier is not used for radio frequency signal amplification. For Example: Radio and Television stations transmit at a particular radio frequency assigned to them. The radio or Television receiver is required to pick up and amplify the desired radio frequency signal, while rejecting all others. This job cannot be done by the audio amplifiers. Tuned amplifier is used to pick up and amplify the desired frequency and reject all others. The tuned amplifiers servers the following two purposes: a. Selection of a desired radio frequency signal. b. Amplification of the selected signal to a suitable voltage level. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development Basic Tuned Circuit or Resonance Circuit When at a particular frequency inductive reactance became equal to capacitive reactance and the circuit then behaves as a purely resistive circuit. This phenomenon is called the Resonance and the corresponding frequency is called Resonant Frequency. A tuned circuit can be of two types according to the type of its connection to the main circuit. a. Series tuned circuit (Series resonant circuit) b. Parallel tuned circuit (parallel resonant circuit) #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Series tuned circuit or Series resonant circuit Figure 1: Series Tuned Circuit The inductor and capacitor connected in series make a series tuned circuit. At resonant frequency, a series resonant circuit offers low impedance which allows high current through it. A series resonant circuit offers increasingly high impedance to the frequencies far from the resonant frequency. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Parallel tuned circuit or Parallel resonant circuit Figure 2: Parallel Tuned Circuit #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Parallel tuned circuit or Parallel resonant circuit It consist of an inductor(L) and a capacitor(C) connected in a parallel to each other w.r.t. a supply source. Consider the frequency of the a.c. supply source to be varied suitably. As a result of this ,the circuit will encounter different impedance at different frequencies. As the frequency is increased, the inductive reactance also increased and the capacitive reactance is decreased. There is a certain frequency of the applied a.c. voltage at which the inductive reactance is equal to the capacitive reactance. This frequency is called Resonance frequency. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Resonant Frequency #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Resonance Curve Figure 3 : Impedance Versus Frequency Figure 4 : Current Versus Frequency Fig. Shows the variation of circuit impedance (Zp) with the change in frequency of the applied voltage. As the frequency is changed, above or below the resonance, the value of impedance decreases rapidly. The impedance is maximum at the resonance and it’s value is equal to Zp=(L/CR). Fig. Shows the variation of circuit current with the change in frequency of the applied voltage. As the frequency is changed, above or below the resonance, the value of current increases rapidly. The current is minimum at the resonance and it’s value is equal to V/(L/CR). #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Bandwidth of a parallel resonance circuit The bandwidth of a resonant circuit is given by the band of frequencies, which lie between two points (A and B) on either side of the resonance curve where the impedance drops to 1/√2 or 0.707 of it’s maximum value at resonance. f1 is the frequency corresponding to point A and f2 corresponding to point B, then the bandwidth, BW = f2 - f1 =∆f Figure 5 : Bandwidth of a parallel resonance circuit #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Sharpness of Resonance The resonance curve of a resonant circuit is required to be sharp as possible to provide a high selectivity. It means that the impedance falls off rapidly as the frequency is varied above and below the resonant frequency. Mathematically, The sharpness is the ratio of the bandwidth of the circuit to its resonant Figure 6 : Effect of coil resistance on the sharpness of resonance curves. frequency i.e. Sharpness of resonance= (Bandwidth/Resonant frequency) = (BW/f0) = (f2 - f1 )/f0=1/Q0 #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Q-factor Q-factor is Quality factor Its value is equal to the ratio of inductive reactance or capacitive reactance at resonance to the circuit resistance. Mathematically, the Q-factor, Q0=(XL/R)=(ω0.L/R)=(2∏f0.L)/R Sharpness of resonance depends upon the value of Q-factor. Higher the value of Q0, more selective is the resonant circuit. The value of Q0 depends upon the value of coil resistance, higher is the value Q0. When the coil resistance is small, the resonance curve is very sharp. If the coil resistance is large, the resonance curve is less sharp. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Relation Between Bandwidth and Q-factor Bandwidth is, BW=f0 /Q0 , f0 = BW × Q0 Higher the value of quality factor Q0 , the lower will be the value of bandwidth. Higher value of quality factor provides a higher selectivity, but a smaller bandwidth whereas a lower value of quality factor provides a poor selectivity but a larger bandwidth. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Types of Tuned Amplifiers a. Single tuned amplifier b. Double tuned amplifier c. Stagger tuned amplifier #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Single Tuned Amplifier An amplifier circuit with single tuner section being at the collector of the amplifier circuit is called as Single tuned amplifier. The values of Inductance (L) and Capacitance(C) of the tuned circuit are selected in such a way that the resonant frequency of the tuned circuit is equal to the frequency to be selected and amplified. The BJT used in Tuned amplifier with two different coupling components: a. Capacitively coupled tuned amplifier b. Inductively coupled tuned amplifier #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Single Tuned Amplifier using a Bipolar Junction Transistor Figure 7 : Capacitive Coupled Figure 8 : Inductive Coupled #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Continue.. In Capacitively coupled tuned amplifier, output is taken through a coupling capacitor Cc. In inductively coupled tuned amplifier, output is taken across an inductor. Both these circuits consist of a BJT amplifier and tuned circuit as the load. The resistors R1, R2 and RE are called as Biasing Resistors. These resistors provide the d.c. operating currents and voltages for the transistor. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Working of Single stage tuned amplifier The radio frequency signal to be amplified is applied at the input of the amplifier. The resonant frequency of the tuned circuit is made equal to the frequency of the input signal by changing the value of capacitance (C) or inductance (L). When the frequency of the tuned circuit becomes equal to that of the input signal, a large signal appears across the output terminals. If the input signal is complex wave i.e. it contains many frequency components, in that case the signal with frequency equal to the resonant frequency will be amplified and all the other frequencies will be rejected by the tuned circuit. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development Single Tuned Amplifier using a Field Effect Transistor Figure 9 : Single tuned amplifier using field-effect transistor Note: FET and MOSFET may also be used in tuned amplifier circuit instead of BJT. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Frequency Response of Single Tuned Amplifier The Voltage gain of the tuned amplifier is very high at the resonant frequency and decreases as the frequency changes above or below the resonant frequency. The bandwidth of an amplifier is equal to the range of frequency over which the voltage gain of the amplifier is equal to or greater than the 70.7% of the maximum voltage gain. BW = f2 - f1 =∆f= f0 /Q0 Figure 10 : Frequency Response Curve Where, Q0 is the Quality factor of the tuned circuit. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Limitations of Single Tuned Amplifier Figure 11 : Frequency spectrum of the Figure 12 : Frequency response of the transmitted signal tuned amplifier The tuned amplifiers in communication receivers is used to select the desired carrier frequency and amplifying the complete band of frequencies around the selected carrier frequency. Tunes amplifiers are required to be highly selective, but the high selectivity requires a tuned circuit with a high Q factor. High Q-factor will give a high voltage gain. But at the same time it will give reduced bandwidth. It means that a tuned amplifier with reduced bandwidth may not be able to amplify equally the complete band of transmitted signal. Narrow bandwidth of the amplifier will result in a poor reproduction of the audio signal. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Double Tuned Amplifier The limitation of single tuned amplifier is overcome by double tuned circuit. In this amplifier, two tuned circuits are inductively coupled to each other. An amplifier circuit with a double tuner section being at the collector of the amplifier circuit is called as Double tuner amplifier circuit. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Construction of Double Tuned Amplifier It consist of a transistor amplifier with two tuned circuits. One of the tuned circuits i.e.L1C1 is shown as the collector load and other i.e. L2C2 as the output. The resistor R1,R2 and RE are used to provide d.c. currents and voltages for the transistor operation. Figure 13 : Double tuned amplifier #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Operation of Double Tuned Amplifier The signal to be amplified is applied at the input terminal through the coupling capacitor Cin. The resonant frequency of the tuned circuit L1C1 is made equal to that of the signal by adjusting L1 or C1. Under these conditions, the tuned circuit offers a very high impedance to the input signal. As a result of this, a large output appears across the tuned circuit L1C1. The output from this tuned circuit is inductively coupled to the L2C2 tuned circuit. These double tuned circuits are extensively used for coupling various circuits of radio and television receivers. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Frequency Response of Double Tuned Amplifier The frequency response of a double tuned amplifier depends upon its degree of coupling or coefficient of coupling between the two tuned circuits. The degree of coupling gives us an idea of the amount of energy transferred between the two tuned circuits. There are three coupling: 1. Tight Coupling 2. Critical Coupling 3. Loose Coupling #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development Frequency Response with different degrees of coupling Figure 14.1 : Tight Coupling Figure 14.2 : Critical Coupling Figure 14.3 :Loose Coupling Figure 14: Frequency response of tuned circuits with different degrees of coupling #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Continue… Figure 14.1 shows the response curve for tight coupling. Here the resonance is shown to occur at two new frequencies which are different from the resonance frequency f0. As the degree of coupling is decreased, the two frequencies come closer and merge into one frequency f0 at critical coupling which is shown in figure 14.2. When the degree of coupling is decreased below the critical coupling, a single peak of reduced height is obtained as shown in figure 14.3. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development  Frequency Response of Double Tuned Amplifier Figure 15 : Frequency response of a double-tuned amplifier The tuned circuits of a double tuned amplifier are tightly coupled to each other. This type of response curve provides a high selectivity, high gain and relatively large bandwidth to the tuned amplifier. #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development Compare single tuned and double tuned amplifier #Education with Values Department of Electronics and Telecommunication School of Polytechnic and Skill Development