Analog Electronics

Diode circuits are analysed piecewise using the ideal/constant-drop model: a full-wave rectifier gives V_dc = 2V_m/π, and a Zener diode holds a constant voltage for regulation once above its knee current.

Transistor amplifiers first set a stable DC operating point (Q-point), then use the small-signal parameters (g_m = I_C/V_T for a BJT) to find gain; the common-emitter/common-source stage gives high voltage gain.

Negative feedback reduces gain to A_f = A/(1+Aβ) but in return improves gain stability, bandwidth, distortion and impedance — all by the same factor (1+Aβ), the amount of feedback.

With negative feedback the ideal op-amp forces its two inputs to the same voltage (virtual short) and draws no input current; from these two rules the gain of any op-amp circuit follows immediately.

An oscillator sustains a signal without input by satisfying the Barkhausen criterion — loop gain magnitude of 1 and total phase shift of 0° (or 360°) — with the frequency set by the phase-shift network.