GATE Electrical Engineering - Complete Master Library

 

Analog Electronics – Page 42

Current Series Feedback Amplifier – Complete Derivation

                                      

                                             

Current series feedback is a feedback configuration where:

• Output current is sampled
• Feedback signal is applied in series with input

This configuration is commonly used in transconductance amplifiers.

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Closed Loop Gain Derivation

Let

• A = Open loop gain
• β = Feedback factor
• Af = Closed loop gain

Feedback current:

If = βIo

Input current to amplifier:

Ii = Is − If

Output current:

Io = A Ii

Substitute feedback:

Io = A(Is − βIo)

Expand equation:

Io = AIs − AβIo

Rearranging:

Io + AβIo = AIs

Io(1 + Aβ) = AIs

Closed loop gain:

Af = Io / Is = A / (1 + Aβ)

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Input Resistance with Feedback

Series mixing increases input resistance.

Rif = Ri (1 + Aβ)

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Output Resistance with Feedback

Current sampling increases output resistance.

Rof = Ro (1 + Aβ)

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Key Characteristics

• Input resistance increases
• Output resistance increases
• Gain stabilizes
• Bandwidth improves

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Important GATE Points

• Current series feedback → Transconductance amplifier
• Closed loop gain = A / (1 + Aβ)
• Input resistance increases
• Output resistance increases