Voltage series feedback, also called series–shunt feedback, is a commonly used feedback topology where:
- Output voltage is sampled (shunt sampling)
- Feedback is applied in series with the input (series mixing)
It is widely used in voltage amplifiers such as op-amp circuits.
Table of Contents
Basic Concept
The feedback signal is subtracted from the input:
$$
V_{in} = V_s – \beta V_o
$$
V_{in} = V_s – \beta V_o
$$
- \(V_s\) = input voltage
- \(V_o\) = output voltage
- \(\beta\) = feedback factor
Closed-Loop Gain
$$
A_f = \frac{A}{1 + A\beta}
$$
A_f = \frac{A}{1 + A\beta}
$$
- \(A\) = open-loop gain
- \(A_f\) = closed-loop gain
For large loop gain (\(A\beta \gg 1\)):
$$
A_f \approx \frac{1}{\beta}
$$
A_f \approx \frac{1}{\beta}
$$
This shows that gain becomes independent of amplifier parameters.
Input Impedance
Due to series mixing:
$$
Z_{in(f)} = Z_{in}(1 + A\beta)
$$
Z_{in(f)} = Z_{in}(1 + A\beta)
$$
Result: Input impedance increases.
Output Impedance
Due to shunt sampling:
$$
Z_{out(f)} = \frac{Z_{out}}{1 + A\beta}
$$
Z_{out(f)} = \frac{Z_{out}}{1 + A\beta}
$$
Result: Output impedance decreases.
Effect on Performance
1. Gain Stability
Gain becomes less sensitive to parameter variations.
2. Bandwidth Increase
$$
A_f \times BW = \text{constant}
$$
A_f \times BW = \text{constant}
$$
As gain decreases, bandwidth increases.
3. Reduced Distortion
Improves linearity and reduces harmonic distortion.
4. Noise Reduction
Reduces internal noise effects.
Applications
- Operational Amplifiers (Op-Amps)
- Audio amplifiers
- Voltage amplifiers
- Instrumentation circuits
Advantages
- High input impedance
- Low output impedance
- Improved stability
- Increased bandwidth
- Reduced distortion and noise
Disadvantages
- Reduced gain
- Possible instability if poorly designed