1. Feedback Amplifier By : Mohanish R. Chaubal – 140950111002
Bachelor of Engineering – 3rd Semester
Electronics & Communication Engineering

2. Introduction
It’s impossible to think of electronic circuits without some forms of feedback.
Negative feedback
Desensitize the gain
Reduce nonlinear distortion
Reduce the effect of noise
Control the input and output impedance
Extend the bandwidth of the amplifier
The basic idea of negative feedback is to trade off gain for other desirable properties.
Positive feedback will cause the amplifier oscillation.

3. The General Feedback Structure
This is a signal-flow diagram, and the quantities x represent either voltage or current signals.

4. The General Feedback Equation
Closed loop and open loop
Closed loop gain
Feedback factor β
Loop gain Aβ
Amount of feedback (1+ Aβ)

5. Some Properties of Negative Feedback
Gain desensitivity
Bandwidth extension
Noise reduction
Reduction in nonlinear distortion

6. The Four Basic Feedback Topologies
Voltage Amplifier (VCVS)
Voltage-controlled voltage source
Current Amplifier (ICIS)
Current-controlled and current source
Transconducatnce Amplifier (VCIS)
Voltage-controlled and current source
Transresistance Amplifier (ICVS)
Current-controlled and voltage source

7. The VCVS Feedback Topologies
voltage-controlled voltage-source topology

8. The Amplifier with VCVS Feedback
voltage-controlled voltage-source topology

9. The ICIS Feedback Topologies
current-controlled current-source topology

10. The Amplifier with ICIS feedback
current-controlled current-source topology

11. The VCIS Feedback Topologies
voltage-controlled current-source topology

12. The Amplifier with VCIS Feedback
voltage-controlled current-source topology

13. The ICVS Feedback Topologies
current-controlled voltage-source topology

14. The Amplifier with ICVS Feedback
current-controlled voltage-source topology

15. Series - Shunt Configuration (VCVS)

16. Series - Shunt Configuration (VCVS)if
then the output of feedback network is an open circuit;
Output voltage is:
feedback voltage is:
where ßv is closed-loop voltage transfer function
By neglecting Rs due to ; error voltage is:

17. Series - Shunt Configuration (VCVS)
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

18. Series - Shunt Configuration (VCVS)
Non-inverting op-amp is an example of the series- shunt configuration.
For ideal non-inverting op- amp amplifier
Feedback transfer function;

19. Series - Shunt Configuration (VCVS)
Input Resistance, Rif
Output Resistance, Rof Or
Input current
Rif with feedback
Assume Vi=0 and Vx applied to output terminal. Or
Input current
Rof with feedback

20. Series - Shunt Configuration (VCVS)
Example:
Calculate the feedback amplifier gain of the circuit
below for op-amp gain, A=100,000; R1=200 Ω and
R2=1.8 kΩ.
Solution:
B = __ R1___ = __200_Ω__
R1+R Ω kΩ
= _200 Ω_ = 0.1 Ω
2.0 kΩ

21. Series - Shunt Configuration (VCVS)
B = 0.1 Ω Now, Av = _1 = _1_ B 0.1 Av = 10

22. 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.

23. 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.

24. Thank You