Unit1-Feedback amplifiers in Electronic Circuits

Electronic Circuits

1. FEEDBACK A MPLIFI E RS

2. Outline Introduction to Feedback Feedback Amplifier – Positive & Negative Advantages/Disadvantages of Negative Feedback Basic Feedback Concept Classification of Amplifiers Series – Shunt Configuration Shunt – Series Configuration Series - Series Configuration Shunt – Shunt Configuration

3. Introduction to Feedback   Feedback is used in virtually all amplifier system. Invented in 1928 by Harold Black – engineer in Western Electric Company    methods to stabilize the gain of amplifier for use in telephone repeaters. In feedback system, a signal that is proportional to the output is fed back to the input and combined with the input signal to produce a desired system response. However, unintentional and undesired system response may be produced.

4. Feedback Amplifier  Fe e dback is a technique where a proportion of the output of a system (amplifier) is fed back and recombined with input    Positive feedback Negative feedback A  There are 2 types of feedback amplifier: i n p u t o u t p u t

5. Positive Feedback  Positive feedback is the process when the output is a dde d to the input, amplified again, and this process continues. A  Positive feedback is used in the design of oscillator and other application. i n p u t output

6. Positive Feedback - Example In a PA system get feedback when you put the microphone in front of a speaker and the sound gets uncontrollably loud (you have probably heard this unpleasant effect).

7. Negative Feedback Negative feedback is when the output is subtracted from the input. A  The use of negative feedback reduces the gain. Part of the output signal is taken back to the input with a negative sign. i n p u t o u t p u t

8. Negative Feedback - Example Speed control If the car starts to speed up above the desired set-point speed, negative feedback causes the throttle to close, thereby reducing speed; similarly, if the car slows, negative feedback acts to open the throttle

9. Feedback Amplifier - Concept Basic structure of a single - loop feedback amplifier

10. Advantages of Negative Feedback Gain Sensitivity – variations in gain is reduced. Bandwidth Extension – larger than that of basic amplified. Noise Sensitivity – may increase S-N ratio. Reduction of Nonlinear Distortion Control of Impedance Levels – input and output impedances can be increased or decreased.

11. Disadvantages of Negative Feedback Circuit Gain – overall amplifier gain is reduced compared to that of basic amplifier. Stability – possibility that feedback circuit will become unstable and oscillate at high frequencies.

12. Basic Feedback Concept Basic configuration of a feedback amplifier

13. Basic Feedback Concept Closed-loop transfer function or gain is The output signal is S : o  AS  where A is the amplification factor Feedback signal is S fb   S o where ß is the feedback transfer function S fb At summing node: S   S i i   S o A S 1   A f A  A  A  1 f t he n A  if  A  1

14. Classification of Amplifiers Classify amplifiers into 4 basic categories based on their input (parameter to be amplified; voltage or current) & output signal relationships:     Voltage amplifier (series-shunt) Current amplifier (shunt-series) Transconductance amplifier (series-series) Transresistance amplifier (shunt-shunt)

15. Feedback Configuration Series: connecting the feedback signal in series with the input signal voltage. Shunt: c o nn e c t i n g the feedback signal in shunt (parallel) with an input current

16. Series - Shunt Configuration v v v f A v A  1   A

17. Series - Shunt Configuration if R o  R L then the output of feedback network is an open circuit; Output voltage is: V o  A v V  By neglecting R s due R to i  R s  V fb i V s:   V i v v i vf  ; error voltage  V o A v  A V 1   A feedback voltage is: V fb   v V o where ßv is closed-loop voltage transfer function

18. Series - Shunt Configuration O r   R if with feedback Assume Vi=0 and Vx applied to output O r   Input Resistance, R if Output Resistance, R of V i  V   V fb  V    v ( A v V  ) V (1   v A v ) Input current V    i i i V V i  R R i (1   v A v )  I  i f I i R  R i (1   v A v )  V i v x   V  te V rm   in V a bf l.  V  V     v V x o o R R V x (1   v A v )  Inpu V t c x u  r A re v V n  t I i  x v v R o f wit V h x feedba R c o k R of   I (1   A )

19.   Series input connection increase input resistance – avoid loading effects on the input signal source. Shunt output connection decrease the output resistance - avoid loading effects on the output signal when output load is connected. Equivalent circuit of shunt - series feedback circuit or voltage amplifier Series - Shunt Configuration

20. Feedback transfer function; Series - Shunt Configuration Non-inverting op-amp is an example of the series-shunt configuration. For ideal non-inverting op- amp amplifier  R 2     1   V R i  1  V o A vf  1  1  1   R  R 2   

21. Series - Shunt Configuration Equivalent circuit i i f o v i vf o V i R R 1 R 1 A v A v V A R 1 V fb I V  / R i A v V   R i (1   A v )   V i R   1  2   R 1  V  V       1 2     R  R V i  V  A v 1   A    1 2     R  R 1    V o  V   1 2     R  R V o  A v V  V   V i  V fb

22. Series - Shunt Configuration Example: Calculate the feedback amplifier gain of the circuit below for op-amp gain, A=100,000; R1=200 Ω and R2=1.8 k Ω . Solution: A vf = 9.999 or 10

23. Series - Shunt Configuration Basic emitter-follower and source-follower circuit are examples of discrete-circuit series-shunt feedback topologies. v i is the input signal error signal is base- emitter/gate-source voltage. feedback voltage = output voltage  feedback transfer function, ß v = 1

24. Series - Shunt Configuration Small-signal voltage gain:    e e i vf r r R E V    g  R r m E o  A  1  R E V 1   1    g m  R E  1  Open-loop v  o r  ltage  gain:     R E A v   r  g m  R E  r C l o s e  d  - l oo p  i n p u e t r e s i st an c e :  1        g m  R E  r R if Output resistance:    m  E   1   r   (1  g r ) R  r 1  E of   R  R  r  m  r  R E 1    g m  R E ( 1  g r )  1 

25. Shunt – Series Configuration i i if A  A i 1   A

26. Shunt – Series Configuration Basic current amplifier with input resistance, Ri and an open-loop current gain, Ai. Current I E is the difference between input signal current and feedback current. Feedback circuit samples the output current – provide feedback signal in shunt with signal current. Increase in output current – increase feedback current – decrease error current. Smaller error current – small output current – stabilize output signal.

27. Shunt – Series Configuration if R i  R s i i i i f   A  I o A i I 1   A t hen I i  I  then the output is a short circuit; output current is: I o  A i I  feedback current is: I fb   i I o where ßi is closed-loop current transfer function Input signal current: I i  I   I fb

28. Shunt – Series Configuration O r   R if with feedback Input Resistance, R if I i  I   I fb  I    i ( A i I  ) I (1   i A i ) Input current I    i i i I i R i (1   A ) V i  I  R i  i i i i f R I (1   A )   V i R i Output Resistance, R of Assume I i =0 and I x applied to output terminal. I   I fb  I    i I x  I     i I x V x  ( I x  A i I  ) R o V x   I x  A i (   i I x )  R o V x  I x (1   i A i ) R o R o f with feedback x o f I R  V x  R o  1   i A i 

29. Shunt - Series Configuration   Shunt input connection decrease input resistance – avoid loading effects on the input signal current source. Series output connection increase the output resistance - avoid loading effects on the output signal due to load connected to the amplifier output. Equivalent circuit of shunt - series feedback circuit or voltage amplifier

30. Shunt - Series Configuration Op-amp current amplifier – shunt-series configuration. I i ’ from equivalent source o • f I I  i i a s n n d eg R lig s . ible and s i R >>R ; assume V 1 virtually Current I 1 : i i f b I  I '  I i F o f b F R   I R V   I gro I un  d  ; V / R 1 o 1    1  R  R F  1 i I o  I fb  I  I 1   Ideal curr I e i nt  gain R : 1  A i  o   1   F  Output cu I rre  nt: R

31. Shunt - Series Configuration   1  Output current Ai is open-loop current g I a  in  I i '  I fb  I i  I fb I o  A i I   A i ( I i  I fb ) and Assume V V o 1  is  v I i fb rt R u F ally ground: Closed-loop current gain:   I current: 1  R 1   R   I V F f b o R 1 I    1   R   R  I fb   F  I o  I fb  I 1  I fb     R 1  R A I  1   F  1   i i  I o A i if A

32. Shunt - Series Configuration Common-base circuit is example of discrete shunt-series configuration. Amplifier gain: Closed-loop current gain: R L I o I i I  R L I o I i I  I fb I o / I   A i   i i if A    I o  A i I 1   1  A

33. Shunt - Series Configuration Common-base circuit with R E and R C I o R E I i V - V + R C I o R E I i R C i i if R r A i R r A   E     E  I     I o m  g m r   1      A  1      g r

34. Series – Series Configuration g g gf A g A  1   A

35. Series – Series Configuration The feedback samples a portion of the output current and converts it to a voltage – voltage- to-current amplifier. The circuit consist of a basic amplifier that converts the error voltage to an output current with a gain factor, A g and that has an input resistance, R i . The feedback circuit samples the output current and produces a feedback voltage, V fb , which is in series with the input voltage, V i .

36. Series – Series Configuration Assume the output is a short circuit, the output current: I o  A g V  feedback voltage is: o z g i g f A g   I o  A V 1   A V fb   z I where ßz is a resistance feedback transfer functio Input signal voltage (neglect Rs=∞ ): V i  V   V fb

37. Series – Series Configuration Assume I i =0 and I x applied to output terminal.  R o f with feedback Output Resistance, R of x x g z x o  f b  z x V x  I x (1   z A g ) R o V   I  A (   I )  R I     z I x V x  ( I x  A g I  ) R o I  I  I   I  o z g x of I R  V x  R  1   A  O r   R if with feedback Input Resistance, R if V i  V   V fb  V    z ( A g V  ) V (1   z A g ) Input current V    i i i V V i  R R i (1   z A g )  I  i i f I R  R i (1   z A g )  V i

38. Series – Series Configuration Series input connection increase input resistance Series output connection increase the output r e s i st an c e Equivalent circuit of series - series feedback

39. Series – Series Configuration E i g f V R A  I o  1 The series output connection samples the output current  feedback voltage is a function of output current. Assume ideal op-amp circuit and neglect t V ra i n  s V is f t b or  b I a o s R e E -current:

40. Series – Series Configuration Assume I E  I C and R i  i g f E f b g r A R V m  g E m  g o m  b m  g  A   I o V   V i  V fb  V i  I o R E I o  g m r  A g  V i  I o R E  I   g r I  g r A V V 1   g r A  R

41. Series – Series Configuration

42. Series – Series Configuration i o gf f b C R C V I r R C    R  g m  R    C L  A          g m  R E   V  L       R  R  r  g m V   R E      V   1 1    g m  R E  r    1 V i  V   V fb  V   1   I o   ( g m V  )

43. Shunt – Shunt Configuration z z z f A z A  1   A

44. Shunt – Shunt Configuration The feedback samples a portion of the output voltage and converts it to a current – current- to-voltage amplifier. The circuit consist of a basic amplifier that converts the error current to an output voltage with a gain factor, A z and that has an input resistance, R i . The feedback circuit samples the output voltage and produces a feedback current, I fb , which is in shunt with the input current, I i .

45. Shunt – Shunt Configuration Assume the output is a open circuit, the output voltage: V o  A z I  feedback voltage is: V o I fb   g Input signal voltage (neglect Rs=∞ ): I i  I   I fb g z i z f   V o A z  A I 1   A where ßg is a conductance feedback transfer functio

46. Shunt – Shunt Configuration O r   i f R with feedback Input Resistance, R if I i  I   I fb  I    g ( A z I  ) I I    i (1   g A z ) Input current I R (1   g A z ) V i  I  R i   i i g z i i f R I (1   A )   V i R i Assume Vi=0 and Vx applied to output O r   R with feedback Output Resistance, R of g x   V  te V r  m  in V a bf l.  V  V     g V x o o R R V x (1   g A z )  Input V c x u  rr A e z n V t  I i  g z o x x o f of V R R   I (1   A )

47. Shunt – Shunt Configuration Equivalent circuit of shunt - shunt feedback circuit or voltage amplifier

48. Shunt – Shunt Configuration n. Input current splits between feedback current and error current. Shunt output connection samples the output voltage  feedback current is function of output voltage. 2 I A i z f   R  V o Basic inverting op-amp circuit is an example of shunt-shunt configuratio V o   I fb R 2 where I fb  I i

49. Shunt – Shunt Configuration Az is open-loop transresistance gain factor (-ve value) A I A i zf 1   z R 2 A z   V o where I fb   V o / R 2 V  A I   A  I  I  o z  z i fb

50. Shunt – Shunt Configuration

51. Shunt – Shunt Configuration           F  i   o  zf F  F  o F i F C R F      g m  g m  R  V A V o  I i R   r  R    g m  R R V R r I  R g V R 1  1   1   I  1 1  1  1     R 1    1 1   1 1     R C R F  r  R F   R F   C F    F   V  V   V o   V o  V o  V  m 

52.              F  z o zf F  z F  F   F    F  i o zf R  A z  I i V A R r R R R r R R R  A z  I V A      R r   1  1  A   1      C   1   C  1        A r  R C  A z   1  1    g m r  R C  C    Shunt – Shunt Configuration Open-loop transresistance gain factor A z is found by setting R F =    g m   Multiply by (r π R C )  C Assume R <<R F π & r << R F

53. Feedback Amplifier Input and output Impedances Summary For a series connection at input or output, the resistance is increased by (1+  A). For a shunt connection at input or output, the resistance is lowered by (1+  A).

54. Feedback Amplifier