Amplifiers and Feedback
This domain tests amplifier gain, frequency response, impedance, coupling, amplifier classes, and the effect of negative feedback. Expect both descriptive and numerical questions.
Core rules
- Voltage gain Av = Vout/Vin; current gain Ai = Iout/Iin; power gain Ap = AvAi.
- Voltage gain in decibels: Av(dB) = 20 log10(Av). Power gain: Ap(dB) = 10 log10(Ap).
- Input impedance affects loading; a low output impedance helps drive a load.
- Class A conducts for the full cycle, is linear but inefficient. Class B uses two devices for half-cycles and suffers crossover distortion. Class AB reduces that distortion. Class C is efficient but highly non-linear and used in tuned RF stages.
- Negative feedback reduces gain by the factor 1 + Aβ, widens bandwidth, reduces distortion and noise, and changes input and output impedance.
- An op-amp has very high open-loop gain; with negative feedback, the closed-loop gain is set by external components.
Worked example
An amplifier with open-loop gain A = 1000 uses negative feedback with feedback fraction β = 0.01. Loop gain Aβ = 10. Closed-loop gain Af = A/(1 + Aβ) = 1000/11 = 90.9. If the input is 10 mV, output ≈ 0.909 V. The gain is much lower than 1000 but far more stable.
Marks-losing traps
Use 20 log for voltage and 10 log for power; mixing them is a common error. Negative feedback does not eliminate distortion, it reduces it. Class B efficiency is higher than Class A, but crossover distortion must be addressed. Do not ignore loading: a measured gain can fall when a low input impedance loads the source.