Automatic Gain Control (AGC) for a Wien Bridge Oscillator can be realized using a JFET for stabilization. Does someone know of a design that uses a MOSFET instead? If so, I would be thankful if you could provide a paper or other kind of source.
The JTET function in AGC is a voltage controlled resistor. Therefore, search a MOSFET circuit that does the same functionbz: Oscillator Voltage dependendent output resistance (rds)
Muhammad Hamza El-Saba Thank you for your reply.
It is clear that this would be basically a MOSFET operated in deep-triode region (linear I/V relation). Can you recommend a source that takes advantage of such an exemplary design (e.g. a paper) ?
I have not tried, but one can propose the attached circuit. R1 and C1 are responsible by the oscillation frequency, as usual. With the increase of the output voltage, the on resistance of M1 decreases, lowering the resultant resistance and thus the gain. R2A should be a small fraction of R2B, in order the distortion caused by the nonlinear resistance of M1 should be small and assure that this device is in the ohmic region.
R3 should be at the order of half of R2A + R2B, but it may be necessary some adjustment due to the action of M1. In the same way, R4 should be about twice the value of R5, in order to "bootstrap" the variation at the source of M1 and thus reducing the modulation of the resistance of M1, which could appear as a modulation of the output signal. Both should be of high value to guarantee the adequate filtering of the rectified voltage in association with C2.
Antonio Carlos da Costa Telles Thank you for your answer. Can you provide a source for the circuit you gave, so I can reference it in my research?
Frederik Lesniewski
I designed the circuit by myself. To the best of my knowledge, it is a novel one.
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