Wien-bridge oscillator with lamp stabiliser
A series RC and a parallel RC pass exactly one third of the signal, in phase, at f = 1/(2πRC). A non-inverting amplifier with a gain of 3 sustains it. The trick is the lamp: as the amplitude grows it heats up, its resistance rises and the gain drops back to exactly 3, so the sine stays clean.
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How it works
At f = 1 ÷ (2πRC) = 1.592 kHz the Wien network passes exactly ⅓ with zero phase shift.
The loop gain is 1 when 1 + R_f ÷ R_lamp = 3, so the lamp settles at R_f ÷ 2 = 110 Ω.
Try it on the bench
- Replace the lamp with a fixed 110 Ω resistor: the output either dies or grows into a clipped square wave, because a fixed gain is never exactly 3.
- Change both capacitors to 4.7 nF: about 3.4 kHz.
Parts on the bench
- 2 × Bench power supply
- NE5532 low-noise op-amp
- 3 × Resistor
- 2 × Film capacitor
- Pilot lamp
In the course
Electronics Engineering, module “Oscillators and timing”.