When you need a clean sine wave below 1 MHz — for audio testing, calibration, or as a reference tone — the Wien bridge oscillator is the classic go-to. Unlike LC oscillators (which need bulky inductors) or RC phase-shift oscillators (which produce ~5% THD), a properly built Wien bridge delivers sub-0.01% distortion with just resistors, capacitors, and an op-amp.

The core idea: A Wien bridge uses an RC network as a frequency-selective positive feedback path around an op-amp, combined with a negative feedback path that sets gain. At one specific frequency, the RC network has exactly zero phase shift and an attenuation of 1/3. If the amplifier provides a gain of exactly 3, the loop gain is 1 and the circuit oscillates.

The frequency-setting network is a series RC (R and C) feeding a parallel RC (same R and same C) to ground, driving the op-amp's non-inverting input. The oscillation frequency is:

f = 1 / (2π·R·C)

Worked example: Want 1 kHz? Pick C = 16 nF (standard value), then R = 1/(2π·1000·16e-9) ≈ 9.95 kΩ. Use a 10 kΩ resistor — close enough. The negative feedback divider needs Rf/Rg = 2 (since non-inverting gain is 1 + Rf/Rg = 3).

The hard part — amplitude stabilization: If gain is exactly 3, any drift kills the oscillation or sends it into clipping. The classic solution (from HP's first product, the HP200A in 1939) is to replace Rg with a small incandescent lamp. As amplitude rises, the filament heats up, resistance increases, and gain drops back toward 3. A #327 lamp works well for op-amp circuits. Modern designs use a JFET as a voltage-controlled resistor driven by a peak detector, or two anti-parallel diodes with a series resistor for soft limiting.

Real-world application: Audio test equipment like the Krohn-Hite 4400 and bench function generators in "low distortion" mode use Wien bridges. They're also the basis for the Twin-T notch oscillator variant used in seismology preamp testing.

Rule of thumb: For best stability, use 1% metal-film resistors and C0G/NP0 ceramic or polypropylene capacitors — these have low tempco and low dielectric absorption. Avoid X7R ceramics; their voltage coefficient will modulate the frequency. Keep R between 1 kΩ and 100 kΩ; below that you load the op-amp output, above that input bias currents cause errors.

Tuning: A ganged dual potentiometer (replacing both Rs simultaneously) lets you sweep frequency over roughly a 10:1 range per RC value.