Clapp oscillator signal distortion.

FvM said:

Besides the said faults, I won't consider the posted two transistor circuit as amplitude control loop example, because it misses a rectifier and averaging filter. It's just another case of more or less soft clipping, with respective harmonic distortions.

I'd also emphasize that -40 dBc distortion isn't bad for a LC oscillator without gain control.

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How would you implement gain control in a simple transistor oscillator circuit? I'm familiar with AGC using op-amps but I'm not seeing how it could implemented in my sample circuit. I may actually need it. I optimized my circuit to yield a pretty clean oscillator signal. However, any changes in the coil impedance due to disruptions in the coil eddy currents (I am using the coil as an eddy current sensor so this is expected) causes a significant decrease in the amplitude of the oscillator signal. It would be much easier for the any post processing circuitry to deal with an osciallator signal with a constant amplitude.

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Many thanks to everyone who made suggestions. I have a lot of useful information and have already significantly improved my circuit design.

I think that a FET as controllable resistor is a promising building block, preferably with a gate-drain voltage divider that cancels even non-linearity. The control voltage should be derived from the rectified and low-pass filtered oscillator voltage.

FvM said:

I think that a FET as controllable resistor is a promising building block, preferably with a gate-drain voltage divider that cancels even non-linearity. The control voltage should be derived from the rectified and low-pass filtered oscillator voltage.

Click to expand...

Which resistance makes the most sense to control, in series with the emitter resistor or in parallel with the collector resistor? What are the advantages and disadvantages of one over the other? Do you have a link to an example so that I can understand how this all fits together? The only other issue I have is that a buffer is required between the output signal and the rectifier and filter circuits which takes up more board space.

Emitter resistance seems promising to me. It must dimensioned to allow a variation between <1 and >1 loop gain at φ=0.

The three resistors serve to isolate the LC tank from the rest of the circuit. They should be adjusted so they provide a small kick per cycle.

By changing the collector and emitter resistors, you shift the range of output V up and down.

By increasing the emitter resistor, you reduce gain. It may make it easier to find the precise operating point which maintains oscillations at the amplitude you want, and minimum distortion.

I have never seen an article explaining all this. In fact I would not be surprised if it could fill a book.

An oscilloscope will quickly make it all obvious.