1

Figure 8.1 General structure of the feedback amplifier. This is a signal-flow diagram, and the quantities x represent either voltage or current signals.

The General Feedback Structure

FeedbackFeedback

1o

fs

x AA

x A

2

Gain Desensitivity

2(1 )f

dAdA

A

1

1f

f

dA dA

A A A

1fA

3Figure 8.2 Illustrating the application of negative feedback to improve the signal-to-noise ratio in amplifiers.

Noise Reduction

1 2 1

1 2 1 21 1o s n

A A AV V V

A A A A

4

Figure 8.3 Illustrating the application of negative feedback to reduce the nonlinear distortion in amplifiers. Curve (a) shows the amplifier transfer characteristic without feedback. Curve (b) shows the characteristic with negative feedback ( 0.01) applied.

Reduction in Nonlinear Distortion

5

Voltage-Mixing Voltage-Sampling (Series–Shunt) Feedback

voltage amplifier

6

Current-Mixing Current-Sampling (Shunt–Series) Feedback

current amplifier

let Is increase …

7

Voltage-Mixing Current-Sampling (Series–Series) Feedback

transconductance amplifier

8

Current-Mixing Voltage-Sampling (Shunt–Shunt) Feedback

transresistance amplifier

9

Figure 8.8 The series–shunt feedback amplifier: (a) ideal structure and (b) equivalent circuit.

The Series–Shunt Feedback Amplifier

10Figure 8.10 Derivation of the A circuit and circuit for the series–shunt feedback amplifier. (a) Block diagram of a practical series–shunt feedback amplifier. (b) The circuit in (a) with the feedback network represented by its h parameters.

The Practical Situation

11

Figure 8.11 Summary of the rules for finding the A circuit and for the voltage-mixing voltage-sampling case of Fig. 8.10(a).

12

Figure 8.12 Circuits for Example 8.1.

Example 8.1