Amplifiers and Oscillators
What this domain tests
Amplifier and oscillator items test gain, feedback and the conditions for sustained oscillation. Expect dB conversions, class-of-operation comparisons, and identification of RC, Hartley, Colpitts and crystal circuits.
Core rules
- Voltage gain Av = Vo/Vi; in decibels, 20 log₁₀(Av). Power gain uses 10 log₁₀(Ap).
- Classes: A conducts 360°, best fidelity but at most 25% efficiency; B conducts 180° and suits push-pull; AB sits between; C conducts for less than 180°, is highly efficient and is used in RF.
- Negative feedback: gain falls by the factor (1 + Aβ), while bandwidth and input impedance rise and distortion and output impedance fall.
- Barkhausen criterion: loop gain Aβ ≥ 1 and total loop phase shift of 360° (or 0°). An amplifier plus a feedback network meeting this oscillates.
- RC phase-shift oscillator: three RC sections of 60° each with an inverting amplifier; f = 1/(2πRC√6).
- Hartley uses a tapped inductor, Colpitts a split capacitor, and a crystal a piezoelectric resonator with very high Q and therefore high frequency stability.
Worked example
An amplifier delivers 2 V output for a 20 mV input. Av = 2/0.02 = 100, which is 20 log 100 = 40 dB. For an RC phase-shift oscillator with R = 10 kΩ and C = 1 nF, RC = 10 µs, so f = 1/(2π × 10 µs × 2.449) = 1/(153.9 µs) ≈ 6.5 kHz.
Traps that cost marks
Using 10 log for voltage ratios; claiming negative feedback causes oscillation; forgetting the 60° per RC section; swapping Hartley (inductor) and Colpitts (capacitor); assuming class A is the most efficient.