Electronic Circuit Analysis UNIT II.pptx

ECA

1. UNIT - II Feedback Amplifiers

2. CONTENTS 2.1 Concepts of feedback 2.2 Classification of feedback amplifiers 2.3 General characteristics of Negative feedback amplifiers 2.4 Effect of Feedback on Amplifier characteristics 2.5 Voltage series and Voltage shunt Feedback configurations 2.6 Current series and Current shunt Feedback configurations

3. 2.1 Concepts of feedback Feedback is a portion of the output is returned to the input to form part of the system excitation The purpose of an amplifier is to amplify the input signal without changing its characteristics except its amplitude. The amplifier that works on the principle of feedback is called feedback amplifier Feedback is a process where a fraction of the output (voltage/current) is fed back to the input Dept. of ECE Y.Jeevan 3

4. Types of feedback Positive (regenerative) feed back If the input signal and the feedback signal are in same phase, the signals get added up and the resultant output increases. This is called positive feedback Positive feedback causes distortion and instability in amplifiers and hence it is not used for amplifiers; whereas positive feedback increases the gain and overall power of input signal and hence used in oscillator circuits Dept. of ECE Y.Jeevan 4

5. Negative (degenerative) feed back If the input signal and the feedback signal are in opposite phase, the resultant input signal is the difference of input and feedback signals. This is called negative feedback. Negative feedback is also known as degenerative or inverse feedback Negative feedback induces desirable modifications in circuit performance. Though negative feedback reduces the overall gain of the amplifier, it has numerous advantages and hence widely used in amplifier circuits Dept. of ECE Y.Jeevan 5

6. Feedback gain Open loop gain Feed back Factor Closed loop Gain The amount of feedback is Dept. of ECE Y.Jeevan 6

7. 2.2 General characteristics of Negative feedback amplifiers Properties of Negative Feedback Amplifier Desensitize the gain : It brings stability to amplifier by making gain less sensitive to all kind of variations. Reduce non-linear distortion : The negative feedback makes the output proportional to the input, i.e. reduces non-linear distortion. Reduce the effect of noise : It minimizes the contribution of unwanted electric signals. This noise may be generated by circuit components or by extraneous interference. Control the input and output impedances : It increases or decreases the input and output impedances. This is done by choosing appropriate feedback topology. Extend the bandwidth of the amplifier : By incorporating negative feedback, the bandwidth can be increased. Dept. of ECE 7 Y.Jeevan Dept. of ECE

8. Advantages of Negative Feedback Amplifier In a negative feedback amplifier, the gain of the amplifier reduces. However, it is still used in almost every amplifier due to its various advantages. Some of the advantages are given below: Gain stability Significant extension of bandwidth Very less distortions Decreased output resistance Stable operating point Reduces noise and other interference in amplifier Dept. of ECE 8 Y.Jeevan Dept. of ECE

9. Gain and De-sensitivity Feedback can be used to desensitize the closed-loop gain to variations in the basic amplifier. Assume  is constant. Take differentials of the closed loop gain equation gives, Divided by A v , the close loop gain sensitivity is equal to, This result shows the effects of variations in A on A CL is mitigated by the feedback amount. (1+ A  ) is also called the de-sensitivity amount. Differential respected with A Dept. of ECE 9 Y.Jeevan

10. 2.3 Effect of Feedback on Amplifier characteristics Dept. of ECE 10 Y.Jeevan Gain: V i =V s – V f = V s −βV o The output V o  must be equal to the input voltage (V s  - βV o ) multiplied by the gain A of the amplifier. Hence, (V s −βV o ) A=V o Or AV s −AβV o =V o Or AV s =V o (1+Aβ) Therefore, V o /V s =A/(1+Aβ) Let A f  be the overall gain (gain with the feedback) of the amplifier. The equation of gain of the feedback amplifier, with negative feedback is given by A f =A/(1+Aβ) Hence, gain decreases with feedback

11. Effect of Feedback on Amplifier characteristics Dept. of ECE 11 Y.Jeevan Gain Stability An important advantage of negative voltage feedback is that the resultant gain of the amplifier can be made independent of transistor parameters or the supply voltage variations, A f =A/(1+Aβ) For negative voltage feedback in an amplifier to be effective, the designer deliberately makes the product Aβ much greater than unity. Therefore, in the above relation, ‘1’ can be neglected as compared to Aβ and the expression becomes A f =A/(1+Aβ) = 1/β It may be seen that the gain now depends only upon feedback fraction, β, i.e., on the characteristics of feedback circuit. As feedback circuit is usually a voltage divider (a resistive network), therefore, it is unaffected by changes in temperature, variations in transistor parameters and frequency. Hence, the gain of the amplifier is extremely stable

12. Effect of Feedback on Amplifier characteristics Dept. of ECE 12 Y.Jeevan Bandwidth: BW f = BW(1+Aβ) Negative feedback, increases bandwidth. Distortion: A power amplifier will have non-linear distortion because of large signal variations. The negative feedback reduces the nonlinear distortion. It can be proved mathematically that: D f = D/(1+Aβ) It is clear that by applying negative feedback, the distortion is reduced by a factor (1+Aβ) Noise : There are numbers of sources of noise in an amplifier. The noise N can be reduced by the factor of (1+Aβ), in a similar manner to non-linear distortion, so that the noise with feedback is given by N f = N/(1+Aβ) Input / Output Impedance: The input and output impedances will also improve by a factor of (1+Aβ), based on feedback connection type

13. A feedback amplifier consists of a basic amplifier and a feedback network. A feedback amplifier is sometimes referred to as a closed-loop amplifier because the feedback forms a closed loop between the input and output Block diagram of a general feed back Amplifier Dept. of ECE Y.Jeevan 13 The basic parts of a single-loop feedback connection around a basic amplifier are as follows: Signal source, (b) Feedback network (c) Sampling network, (d) Comparator or mixer network, and (e) Basic amplifier with forward transfer gain.

14. Dept. of ECE 14 Y.Jeevan Signal Source : Signal source is either a signal voltage V s in series with a resistor R s (a Thevenin’s representation) or a signal current I s in parallel with a resistor R s (a Norton’s representation). Feedback Network : The feedback network is usually a passive two-port network which may contain resistors, capacitors, and inductors .

15. Dept. of ECE 15 Y.Jeevan Sampling Network : Two types of sampling networks can be used. These two sampling networks are: Voltage or node sampling : In this type of sampling system the output is sampled by connecting the feedback network in shunt across the output Current or loop sampling : In this type of sampling system the output is sampled by connecting the feedback network in series with the output

16. Dept. of ECE 16 Y.Jeevan Comparator or mixer network : Mixer network circuit is either series ( loop ) input or shunt ( node ) input connections

17. 2.4 Classification of feedback amplifiers Depending on the input signal (voltage or current) to be amplified and form of the output (voltage or current), amplifiers can be classified into four categories. Depending on the amplifier category, one of four types of feedback structures should be used . Dept. of ECE 17 Y.Jeevan Characteristics Types of Feedback Voltage Series Voltage Shunt Current Series Current Shunt Input Type Voltage Current Voltage Current Output Type Voltage Voltage Current Current Feedback Type Shunt Shunt Series Series Input Connection Series Shunt Series Shunt

18. Classification of feedback amplifiers Dept. of ECE 18 Y.Jeevan Voltage series (Vin & Vout ) Current series (Vin & Iout ) Current Shunt (( Iin & Iout ) Voltage Shunt ( Iin & Vout )

19. Based on the magnitudes of the input and output impedances of an amplifier relative to the source and load impedances , respectively, amplifiers can be classified into four broad categories as: Voltage amplifier ( Voltage Series) Current amplifier (Current Shunt) Trans-conductance amplifier (Current Series) Trans-resistance amplifier (Voltage Shunt) 2.5 Voltage series, Voltage shunt, Current series and Current shunt Feedback configurations Dept. of ECE 19 Y.Jeevan

20. A voltage amplifier is defined as an amplifier, which provides an output voltage proportional to the input voltage , and the proportionality factor is independent of the magnitudes of the source resistance ( R s ) and load resistance ( R L ). Thevenin’s equivalent circuit of a two-port network, which represents an amplifier. Here, V s = source voltage, V i = amplifier input voltage, V o = output voltage, R s = source resistance, R i = amplifier input resistance, R o = amplifier output resistance, R L = external load resistance. If R i is larger compared with R s ( R i >> R s ), then V i  V s . Thevenin’s equivalent circuits of a voltage amplifier. Voltage amplifier ( Voltage Series or Series-Shunt ) Dept. of ECE Y.Jeevan 20

21. Voltage amplifier ( Voltage Series or Series-Shunt ) Samples the output voltage and returns a feedback voltage signal Ideal feedback network has infinite input impedance and zero output resistance Increases input resistance and reduces output resistance, makes amplifier closer to ideal Dept. of ECE Y.Jeevan 21

22. Current amplifier A current amplifier is defined as an amplifier, which provides an output current proportional to the input current , and the proportionality factor is independent of the magnitudes of the source resistance ( R s ) and load resistance ( R L ). Norton’s equivalent circuits of a current amplifier. Here, I s = source current, I i = amplifier input current, I o = I L = output or load current, If the amplifier input resistance R i is smaller compared with the source resistance R s ( R i << R s ), then I i  I s . Current amplifier(Current shunt or Shunt-Series) Dept. of ECE Y.Jeevan 22

23. . If the external load resistance R L is smaller compared with the output resistance R o of the amplifier ( R o >> R L ), then I o  A i I i  A i I s . Hence, the output current is proportional to input current. A i = I o / I i with R L =0, and hence represents the short-circuit current amplification, or current gain . If the external load resistance R L is large compared with the output resistance R o of the amplifier ( R o << R L ), then V o  A v V i  A v V s . Hence, the output voltage is proportional to input voltage. A v = V o / V i with R L =  , and hence represents the open-circuit voltage amplification, or voltage gain Current amplifier(Current shunt or Shunt-Series) Dept. of ECE Y.Jeevan 23

24. An ideal current amplifier must have zero input resistance ( i . e . R i =0) and infinite output resistance ( i . e . R o =  ). As the feedback circuit is connected in series with the output, the output impedance is increased and due to the parallel connection with the input, the input impedance is decreased. . Current amplifier(Current shunt or Shunt-Series) Dept. of ECE Y.Jeevan 24

25. A trans-conductance amplifier is defined as an amplifier, which provides an output current proportional to the input voltage , and the proportionality factor is independent of the magnitudes of the source resistance ( R s ) and load resistance ( R L ). A trans-conductance amplifier is represented by a Thevenin’s equivalent in its input circuit and a Norton’s equivalent in its output circuit. If the amplifier input resistance R i is larger compared with the source resistance R s ( R i >> R s ), then V i  V s . Trans-conductance amplifier (Current series or series-series) Dept. of ECE Y.Jeevan 25 If the external load resistance R L is smaller compared with the output resistance R o of the amplifier ( R o >> R L ), then I o  G m V i  G m V s . Hence, the output current is proportional to the input voltage The symbol G m = I o / V i with R L =0, and hence represents the short-circuit transfer conductance (trans-conductance) or gain .

26. An ideal trans-conductance amplifier must have infinite input resistance ( i . e . R i =  ) and infinite output resistance ( i . e . R o =  ). Dept. of ECE 26 Y.Jeevan

27. A trans-resistance amplifier is defined as an amplifier, which provides an output voltage proportional to the input current , and the proportionality factor is independent of the magnitudes of the source resistance ( R s ) and load resistance ( R L ). A trans-resistance amplifier is represented by a Norton’s equivalent in its input circuit and a Thevenin’s equivalent in its output circuit. If the amplifier input resistance R i is smaller compared with the source resistance R s ( R i <<R s ), then I i  I s . If the external load resistance R L is large compared with the output resistance R o of the amplifier ( R o << R L ), then V o  R m I i  R m I s . The symbol R m = V o / I i with R L =  , and hence represents the open-circuit transfer resistance (trans-resistance), or gain. Trans-resistance amplifier (Voltage shunt or Shunt-Shunt) Dept. of ECE Y.Jeevan 27

28. An ideal trans-conductance amplifier must have zero input resistance ( i . e . R i =0) and zero output resistance ( i . e . R o = 0). Dept. of ECE 28 Y.Jeevan

29. Dept. of ECE 29 Y.Jeevan Series feedback connections tend to increase the input resistance , shunt feedback connections tend to decrease the input resistance. Voltage feedback tends to decrease the output impedance , current feedback tends to increase the output impedance. Typically, higher input and lower output impedances are desired for most cascade amplifiers. Both of these are provided using the voltage series feedback connection .

30. Table represents the characteristics and the parameters values in the ideal case. Dept. of ECE 30 Y.Jeevan

31. Dept. of ECE Y.Jeevan 31 Characteristics Types of Feedback Voltage-Series Voltage-Shunt Current-Series Current-Shunt Voltage Gain Decreases Decreases Decreases Decreases Bandwidth Increases Increases Increases Increases Input resistance Increases Decreases Increases Decreases Output resistance Decreases Decreases Increases Increases Harmonic distortion Decreases Decreases Decreases Decreases Noise Decreases Decreases Decreases Decreases