Transcript Document

Feedback
The General Feedback Structure
xo
A
Af  
xs 1  A
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.
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Gain Desensitivity
dA
dAf 
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(1  A )
 dA f

A
 f

1  dA 
 
 
1

A

A



Af 
1

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Noise Reduction
Vo  Vs
A1 A2
A1
 Vn
1  A1 A2 
1  A1 A2 
Figure 8.2 Illustrating the application of negative feedback to improve the signal-to-noise ratio in amplifiers.
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Reduction in Nonlinear Distortion
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.
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Voltage-Mixing Voltage-Sampling (Series–Shunt) Feedback
voltage amplifier
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Current-Mixing Current-Sampling (Shunt–Series) Feedback
current amplifier
let Is increase …
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Voltage-Mixing Current-Sampling (Series–Series) Feedback
transconductance amplifier
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Current-Mixing Voltage-Sampling (Shunt–Shunt) Feedback
transresistance amplifier
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The Series–Shunt Feedback Amplifier
Figure 8.8 The series–shunt feedback amplifier: (a) ideal structure and (b) equivalent circuit.
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The Practical Situation
Figure 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.
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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).
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Example 8.1
Figure 8.12 Circuits for Example 8.1.
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