1. Feedback

2. 8 Feedback
Why Feedback?
Desensitize the gain: make the value of the gain less sensitive to variations in the value of circuit components, such as might be caused by changes in temperature
Reduce nonlinear distortion: make the output proportional to the input
Reduce the effect of noise: minimize the contribution to the output of unwanted electric signals generated, either by the circuit components themselves, or by extraneous interference
Control the input and output impedances: raise or lower the input and output impedance by the selection of an appropriate feedback topology
Extend the bandwidth of the amplifier

3. 8 Feedback
8.1 The General Feedback Structure

4. 8.2 Some Properties of Negative Feedback
8.2.1 Gain Desensitivity
Assume  is constant. Taking differentials of both sides results in
desensitivity factor

5. 8.2 Some Properties of Negative Feedback
8.2.2 Bandwidth Extension
Midband Gain:
3-dB Frequency:

6. 8.2 Some Properties of Negative Feedback
8.2.3 Noise Reduction
Signal-to-Noise Ratio

7. 8.2 Some Properties of Negative Feedback
8.2.3 Reduction in Nonlinear Distortion
 = 0.01
Open Gain: 1000
Open Gain: 100

8. 8.3 The Four Basic Feedback Topologies
8.3.1 Voltage Amplifiers
Sample Voltage
Mix Feedback
Input: Voltage
Output: Voltage
Voltage-mixing voltage sampling
Series - shunt feedback

9. 8.3 The Four Basic Feedback Topologies
8.3.2 Current Amplifiers
Sample Current
Mix Feedback
Input: Current
Output: Current
Current-mixing current sampling
Shunt - series feedback

10. 8.3 The Four Basic Feedback Topologies
8.3.3 Transconductance Amplifiers
Sample Current
Mix Feedback
Input: Voltage
Output: Current
Voltage-mixing current sampling
Series - series feedback

11. 8.3 The Four Basic Feedback Topologies
8.3.4 Transresistance Amplifiers
Sample Voltage
Mix Feedback
Input: Current
Output: Voltage
Current-mixing Voltage sampling
Shunt - shunt feedback

12. 8.4 The Series-Shunt Feedback Amplifier
8.4.1 The Ideal Situation

13. 8.4 The Series-Shunt Feedback Amplifier
8.4.1 The Ideal Situation (cont.)
Series mixing

14. 8.4 The Series-Shunt Feedback Amplifier
8.4.1 The Ideal Situation (cont.)
Shunt sampling

15. 8.5 The Series-Series Feedback Amplifier
8.5.1 The Ideal Situation
Series mixing

16. 8.5 The Series-Series Feedback Amplifier
8.5.1 The Ideal Situation (cont.)
Series sampling

17. 8.6 The Shunt-Shunt and Shunt-Series Feedback Amplifier
8.6.1 The Shunt-Shunt Configuration

18. 8.6 The Shunt-Shunt and Shunt-Series Feedback Amplifier
8.6.3 The Shunt-Series Configuration

19. 8.6 The Shunt-Shunt and Shunt-Series Feedback Amplifier
8.6.3 Summary
Ri  Rif: Mixing
Voltage (series) mixing always increases the input resistance.
Current (shunt) mixing always reduces it.
Ro  Rof: Sampling
Voltage (shunt) sampling always reduces the output resistance
Current (series) sampling increases it

20. Homework:
10.1, 10.3, 10.8, 10.16, 10.24, 10.26, 10.31, 10.32