Amplifiers and Oscillators
Amplifiers and Oscillators tests biasing, small-signal gain, input and output impedance, feedback, power classes, op-amp circuits, and the conditions for sustained oscillation. Numerical gain and frequency questions are common.
Core rules
- Common-emitter gain is Av ≈ −gm(RC || RL); the minus sign means phase inversion. Emitter bypassing raises AC gain but reduces negative feedback.
- Common-collector (emitter follower) has gain near 1, high input impedance, low output impedance; common-base has low input impedance and no current gain.
- Negative feedback trades gain for bandwidth, linearity, and impedance control.
- Barkhausen criterion: loop gain Aβ = 1 and loop phase shift = 0° or 360° at the oscillation frequency.
- RC phase-shift oscillator needs three RC sections for 180°; Wien bridge oscillates at f = 1/(2πRC) with gain ≈3.
- Op-amp golden rules apply only with negative feedback: input terminals at equal voltage, no input current.
Worked example
A CE stage has gm = 40 mS, RC = 2.2 kΩ, and RL = 10 kΩ. The AC load is RC || RL = (2.2 × 10)/(12.2) = 1.80 kΩ. |Av| = 40 mS × 1.80 kΩ = 72. The output is inverted, so Av ≈ −72. If input is 10 mV peak, output is about 0.72 V peak, provided the supply and bias allow that swing.
Traps
Using RC alone instead of RC || RL overstates gain. Forgetting that coupling capacitors set low-frequency cut-off, or that bypass capacitors affect gain, loses marks. An oscillator with Aβ slightly below 1 will die out; far above 1 it may clip. Op-amp outputs cannot exceed supply rails.