In this answer it is said that the bulb in a Wien bridge oscillator is for temperature compensation.
Is that true? (I thought it is a kind of AGC to stabilize the output level.)
Answer
When first powered up, the lamp is cold and has a low resistance. This makes the circuit gain greater than 1 and helps kick-start the oscillator. As the lamp warms, with bigger amplitudes, its resistance gets a bit bigger. This reduces the gain of the circuit and stabilizes the oscillator amplitude.
The lamp posseses a fair degree of thermal lag and for most frequencies above low audio it's resistance doesn't change much over a cycle of the oscillator waveform and hence the signal isn't distorted much.
When the frequency gets low (10Hz or less typically) there is noticeable distortion on the top of the waveform compared to the bottom because, the bottom of the waveform generates less heat and at low frequencies the bulb cools down more compared to the top of the waveform when more heat is generated.
Modern Wien oscillators use a JFET for amplitude control by converting the peak amplitude of the output to a DC level (rectification and smoothing) but they still suffer from low frequency distortion because at low frequencies there will be a pronounced ripple on the DC level controlling the amplitude via the JFET.
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