- 2072 Magh · 4 marks
Short note: Positive and negative feedback
Short answer
Positive feedback adds a portion of output in phase with input, increasing gain and enabling oscillation when Aβ=1; used in oscillators. Negative feedback subtracts, reducing gain but improving stability, bandwidth, and linearity; used in amplifiers. Both are defined by gain A and feedback fraction β.
Write a short note on positive and negative feedbacks.
Feedback is the process of returning a portion of the output signal to the input of the amplifier. With gain A and feedback fraction β, there are two types.
Vs --> (+) --> [ A ] --+--> Vo
^ |
+---- [ β ] <--+
Positive (regenerative) feedback
- The feedback signal is in phase with the input and adds to it.
- Gain: Af = A/(1 − Aβ). The gain increases.
- It reduces stability. If Aβ = 1, the circuit oscillates.
- Uses: oscillators (Wien bridge, square wave generator), Schmitt trigger, flip-flops and comparators with hysteresis.
Negative (degenerative) feedback
- The feedback signal is opposite in phase to the input and subtracts from it.
- Gain: Af = A/(1 + Aβ). If Aβ >> 1, Af ≈ 1/β, which depends only on the feedback network.
- Advantages: stable gain, reduced distortion and noise, increased bandwidth, better input and output impedances.
- Disadvantage: reduced gain.
- Uses: amplifiers, inverting and non-inverting op-amp circuits, voltage regulators.
| Point | Positive feedback | Negative feedback |
|---|---|---|
| Phase | In phase with input | Out of phase with input |
| Gain | Increases | Decreases |
| Stability | Poor | Good |
| Use | Oscillators | Amplifiers |
✓ Answer checked by Claude Opus 5.5 (high effort)
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