I want to make a cheap variable gain amplifier using an op-amp, using a MOSFET as the variable resistance. The circuit is for an audio application at line (signal) levels (ballpark of 1v peak to peak).
I need help in finding a suitable MOSFET. At the moment I'm having to look at datasheet one at a time, looking at its graph of Vds vs Id (with multiple plots for different Vgs values). I have no idea how to refine what datasheet I choose to look at. Tips? What criteria can I use to narrow search results?
Also, the graphs often neglect to show the curves for Vgs values close to the gate threshold voltage - which is important to know, since the linear region for those values span the smallest Vds range. (If that makes sense).
What am I doing wrong? I'm sure this is a very common use case, yet it seems an uphill battle.
Edit: I'm making a so-called audio compressor that maintains a more constant volume when the input varies. I believe this is also called AGC, although a compressor tends to offer more control, such as user settable attack/release rates, thresholds and so on. The gain will be varied by about a factor of 5. (For my application, the input audio source is already normalised. The output of this stage will go to a totally separate stage that actually controls the volume in the regular fashion. (From muted up to full volume.)
An example of what I want to do is here, but I want to explore other ways of doing it, for educational purposes really. Also, amplifiers with voltage controlled gain seem to be very expensive, and I don't trust myself not to blow it up. A design with more numerous but cheaper components means mistakes will be significantly cheaper!