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FEEDBACK AMPLIFIER ANALYSIS

1.Identify the topology:

(a) Is the feedback signal Xf a voltage or a current signal? i.e. is Xf applied in series or in shunt with external excitation?

(b) Is the sampled signal Xo a voltage or a current signal? i.e, is Xo taken at the output node or from the output current loop?

 

2. Separate feedback network from basic amplifier, A, and find Input & Output resistance, R𝛽i & R𝛽o, of feedback circuit and also, feedback factor,𝛽 , from feedback circuit. Detail

 

3. Draw the basic amplifier circuit without feedback network but with R𝛽i & R𝛽o obtained in step 2. Detail

 

4. Use a Theveninʹs equivalent source input if the feedback signal Xf is a voltage or a Nortonʹs equivalent source if Xf is a current. 

 

5. Replace each active device in Aʹ circuit by a proper model (i.e. h‐ parameter or hybrid‐p model) and evaluate amplifer gain Aʹ of the Aʹ circuit with R𝛽i & R𝛽o included. Also, find the Input & Output resistance of the Aʹ circuit (Ri & Ro)

 

6. From (i) amplifier gain, Aʹ with feedback network I/O resistance & (ii) feedback factor,𝛽. Find Af, Rif & Rof

  • Af = Aʹ/(1+ A‘𝛽)

 

  • Rif = (1+Aʹ𝛽) Ri {series mixing ‐ V}

  • Rif = Ri / (1+Aʹ𝛽) {shunt mixing ‐ I}

  • Rof = (1+Aʹ𝛽) Ro {series sampling ‐ I}

  • Rof = Ro / (1+Aʹ𝛽) {shunt sampling ‐ V}

Input resistance of F/B circuit, R𝞫i

Output resistance of F/B circuit, R𝞫o

First, separate the whole F/B Amplifier into TWO blocks, the basic amp A and the F/B network. Apply the following rulestofindRi &Ro. 

1. For voltage (series) mixing, SEVER the output node of the F/B circuit. 

 

 

 

 

 

2. For current (shunt) mixing, SHORT the output node of F/B circuit. 

1. For voltage (shunt) sampling, SHORT the input node of F/B circuit. 

 

 

 

 

 

2. For current (series) sampling, SEVER the input node of F/B circuit. 

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NANYANG TECHNOLOGICAL UNIVERSITY • SCHOOL OF ELECTRICAL AND ELECTRONIC ENGINEERING

 

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