Analog Electronics – Page 40
Voltage Series Feedback Amplifier – Complete Derivation
Voltage series feedback is one of the most widely used feedback configurations in amplifiers.
In this method:
- Output voltage is sampled
- Feedback signal is applied in series with input
Closed Loop Gain Derivation
Let
- A = Open loop gain
- β = Feedback factor
- Af = Closed loop gain
Feedback voltage:
Vf = βVo
Input to amplifier:
Vi = Vs − Vf
Substitute feedback:
Vi = Vs − βVo
Output voltage:
Vo = AVi
Substitute Vi:
Vo = A(Vs − βVo)
Expand equation:
Vo = AVs − AβVo
Rearranging:
Vo + AβVo = AVs
Vo(1 + Aβ) = AVs
Closed loop gain:
Af = Vo / Vs = A / (1 + Aβ)
Input Resistance with Feedback
Voltage series feedback increases input resistance.
Rif = Ri (1 + Aβ)
Output Resistance with Feedback
Voltage sampling reduces output resistance.
Rof = Ro / (1 + Aβ)
Key Characteristics
- Input resistance increases
- Output resistance decreases
- Gain becomes stable
- Bandwidth increases
Important GATE Points
- Voltage series feedback → Voltage amplifier
- Closed loop gain = A / (1 + Aβ)
- Input resistance increases
- Output resistance decreases
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