Amplifiers and Oscillators
What this domain covers
Amplifiers and Oscillators tests gain calculations, decibel conversions, amplifier classes, feedback and the conditions for sustained oscillation in RC, LC and crystal oscillators.
Core rules
- Voltage gain A_v = V_out/V_in; power gain in dB is 10 log₁₀(P_out/P_in); voltage gain in dB is 20 log₁₀(V_out/V_in). Cascaded stages add their dB gains.
- Class A conducts for the whole cycle, is linear but inefficient (under 50 %); class B conducts for half a cycle and needs push-pull output; class AB reduces crossover distortion; class C is high-efficiency and used in tuned RF stages.
- Negative feedback reduces gain but improves bandwidth, distortion, input impedance and stability. Gain with feedback is A_f = A/(1 + Aβ).
- Barkhausen criterion: loop gain Aβ = 1 and total phase shift around the loop = 0° (or 360°). An RC phase-shift oscillator needs three RC sections giving 180°, plus 180° from the transistor.
- Hartley uses a tapped inductor, Colpitts a tapped capacitor, and a crystal has the highest Q and frequency stability.
Worked example
A two-stage amplifier has gains of 20 dB and 26 dB. Total gain = 46 dB, a voltage ratio of 10^(46/20) ≈ 200. If negative feedback of β = 0.01 is applied to a stage with A = 1000, then A_f = 1000/(1 + 10) ≈ 91.
Traps that cost marks
Using 10 log instead of 20 log for voltage; saying negative feedback increases gain; forgetting the extra 180° needed in an RC oscillator; and confusing Colpitts (capacitive divider) with Hartley (inductive divider).