Amplifiers and Feedback
What this domain tests
Single-stage amplifier configurations, gain and phase behaviour, frequency response, power amplifier classes, and the four negative-feedback topologies with their effect on gain, bandwidth and impedance. Numerical questions on Af, desensitivity and dB gain are common.
Core rules
- CE amplifier: high voltage and power gain, 180° phase inversion, moderate input impedance. CC (emitter follower) has gain ≈ 1, high input and low output impedance; CB has current gain below 1 but works best at high frequency.
- Voltage gain in decibels: Av(dB) = 20 log Av.
- Maximum theoretical efficiency: class A 25 % (50 % with transformer coupling), class B 78.5 %, class AB between the two with reduced crossover distortion, class C highest but heavily distorted and used with tuned RF loads.
- Negative feedback: Af = A/(1 + Aβ). The factor (1 + Aβ) is the desensitivity, so gain falls by it while bandwidth, gain stability and linearity improve by the same factor.
- Voltage-series feedback raises input and lowers output resistance; voltage-shunt lowers both; current-series raises both; current-shunt lowers input and raises output resistance.
Worked example
An amplifier with A = 1000 and β = 0.01 is given negative feedback. 1 + Aβ = 1 + 10 = 11, so Af = 1000/11 = 90.9. The gain drops about eleven times, the bandwidth rises about eleven times, and a 10 % drift in A now produces only about 0.9 % change in Af.
Traps that cost marks
- Using Aβ = −1 or dropping the 1 in the denominator.
- Claiming feedback increases gain; it always reduces it while stabilising it.
- Mixing up the input/output resistance rules for the four topologies.
- Quoting class A efficiency as 78.5 %.