Analog Electronics
Analog Electronics combines diode circuits, bipolar and MOS biasing, small-signal amplifiers, feedback, oscillators, and op-amp applications. GATE EE favours operating-point arithmetic and ideal-op-amp node equations.
Diodes
- Silicon diode conducts with about 0.7 V forward drop; an ideal diode is a short when forward biased and an open when reverse biased.
- Half-wave average = V_m/π; full-wave average = 2V_m/π.
BJT and MOSFET
- Active region: I_C = βI_B and g_m = I_C/V_T with V_T ≈ 26 mV at room temperature.
- Emitter degeneration stabilises the bias point against β spread.
Op-amps
- Ideal op-amp: infinite gain and input impedance, zero output impedance. With negative feedback, the input terminals form a virtual short.
- Inverting gain −R_f/R_in; non-inverting gain 1 + R_f/R_1; integrator −1/(sRC).
Worked example: a Zener regulator with V_in = 15 V, V_Z = 10 V, R_S = 100 Ω, R_L = 1 kΩ. Load current = 10 V/1 kΩ = 10 mA. Current through R_S = (15 − 10)/100 = 50 mA. Zener current = 50 − 10 = 40 mA, so P_Z = 10 V × 40 mA = 0.4 W. If the load demanded more than 50 mA the Zener would fall out of regulation, since R_S must supply both branches.
Traps that cost marks
- Expecting an op-amp output beyond its supply rails.
- Assuming a virtual short without negative feedback.
- Confusing β (h_FE, current gain) with the feedback factor β of a feedback amplifier.