If a circuit like the Wien bridge oscillator below uses an incandescent bulb for automatic gain control, how does this actually vary the gain?
I know that the resistance of the bulb increases as the filament heats up, but I've been told that it can be used as a variable gain element for gain stabilisation. What I don't understand is how it varies.
As far as I can tell, the bulb in this circuit is being used in place of another resistor that forms a voltage divider with R3 which outputs to the op-amp's inverting terminal. If the bulb was replaced with a resistor, R4, I could understand how selecting values for the resistors would allow the user to control the gain, but I don't understand how the lamp does it.
Maybe I'm misunderstanding, but from what I could tell, the lamp was able to control gain automatically, instead of needing to be a chosen value of a resistor. Is it because incandescent bulbs heat up so much that the resistance is constantly increasing? And if this is the reason, how is this property related to the gain of the circuit?
I'm also wondering what is the actual advantage of using these lamps instead of other gain stabilisation methods like JFET or diodes. With most incandescent bulbs not being used any more due to their inefficiency, is there any particular advantage that would mean someone might prefer to use it in their circuit?