Semiconductor Devices, Amplifiers, Oscillators and Power Supplies
Devices
Know diode behaviour: forward conduction above roughly 0.6–0.7 V for silicon, reverse blocking, Zener regulation in reverse breakdown, and varactor capacitance variation with reverse bias. Bipolar transistors are current-controlled (gain hFE, base-emitter drop near 0.7 V); FETs are voltage-controlled with high input impedance. Candidates should distinguish NPN/PNP and N-channel/P-channel polarities, and know that a MOSFET gate is insulated. Traps include assuming a transistor is off when base current flows, and confusing Zener and varactor roles.
Amplifiers
Understand classes: A (conducts full cycle, linear but inefficient), B (half cycle, crossover distortion), AB (common RF/audio compromise), C (short conduction angle, high efficiency, used in RF with tuned output), and D (switching). Know gain, bandwidth, negative feedback benefits (wider bandwidth, lower distortion, changed input/output impedance), and stability. Recognise oscillator requirements: positive feedback with loop gain of one and 360° total phase shift. Colpitts uses a capacitive divider, Hartley an inductive divider; crystal oscillators give high Q and stability.
Power supplies
Trace transformer, rectifier, filter and regulator. Half-wave and full-wave rectification give different ripple frequencies (f and 2f for a 50 Hz mains supply). Larger filter capacitance and lighter load reduce ripple. Linear regulators dissipate (Vin − Vout) × I as heat; switch-mode supplies are more efficient but generate RF noise needing screening and filtering.
Rule of thumb
A class C RF amplifier can exceed 70% efficiency but must feed a tuned circuit to restore the sine wave; do not expect linear AM or SSB amplification from it.