# Constructing a block diagram for amplitude control of an oscillator

I'm building a Wien Bridge oscillator. The oscillation part works just fine, but it currently limits on the supply rails.

I want to add amplitude control, so I've added a precision rectifier which feeds an integrator that controls an LDR to change the gain of the circuit.

This setup makes the output amplitude oscillate. To get a better understanding of what happens I want to construct a block diagram of the amplitude control and use control theory to improve the design.

I'm already a bit stuck on building the block diagram. I've the following: Where the 3 is the ideal gain, to compensate the $\frac{1}{3}$ of the frequency selective network. The $\delta R$ is the change in the gain resistor due to the LDR. Since the frequency selective network has 0 phase shift at the oscillation frequency I believe there is no need to put in an actual transfer function.

I've modelled the rectifier with a gain of 1, as it doesn't really change the amplitude it just presents it in a different form. The $\frac{1}{sC}$ is the integrator.

Is this somewhat correct? How can I now add the fact that the integrator output alters the $\delta R$?

• " I've added a precision rectifier which feeds an integrator that controls an LDR to change the gain of the circuit." I suspect that deleting the precision rectifier and the integrator would fix the problem, Back in the day, people just used the nonlinear resistance of an incandescent light bulb filament to reduce the gain as the amplitude increased. Adding an integrator has introduced another time constant into the system and (presumably) that time constant has created a second oscillator that is modulating the Wein bridge amplitude. – alephzero Aug 10 '18 at 10:49
• Yes, it would fix the amplitude oscillation problem, but the distortion is then very high. I want the output to be as clean as possible. The question is also more about applying/learning control theory rather than actually fixing the oscillation problem (although I'll celebrate with a beer once it's fixed :) ). – Remco Poelstra Aug 10 '18 at 10:54