How it works
The Wien bridge RC network provides 0° phase shift (not 180°) at f_0 and a voltage division of exactly 1/3 at that frequency. Because the RC network does not invert, the op-amp must be in non-inverting mode. For oscillation to start, the non-inverting amplifier gain A_v = 1 + R_f/R1 must equal 3 (since feedback fraction = 1/3 and loop gain Aβ must equal 1). Using R_f/R1 = 2 sets A_v = 3 exactly. In practice, gain slightly above 3 allows oscillation to build up; an automatic amplitude control circuit — typically a thermistor or an AGC loop with a JFET in the feedback path — then reduces gain to exactly 3 once the amplitude reaches the target, preventing saturation.
Key points to remember
Wien bridge oscillators are purely RC — no inductors — making them compact and low-cost for audio frequencies (20 Hz to 20 kHz). Frequency can be varied by simultaneously changing both R or both C; using a dual-gang potentiometer controls frequency while maintaining constant amplitude. Phase margin of the Wien bridge: below f_0 the phase is positive (leading), at f_0 it is 0°, above f_0 it is negative (lagging) — the op-amp only oscillates at the unique frequency where phase is 0°. Distortion is the main quality metric: well-designed Wien bridge oscillators achieve THD below 0.01% with good amplitude stabilisation.
Exam tip
The examiner always asks you to prove that the RC Wien bridge network provides exactly 1/3 voltage division and 0° phase shift at f_0 — set up the voltage divider with series RC and parallel RC, substitute jω = j/(RC), and show that V_out/V_in = 1/3 at ω = 1/(RC).