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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!

Jodes
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    I doubt that you will find a suitable MOSFET as most of them are designed to be used as a switch with low Rds_on. To achieve that low Rds_on they have a very large W/L so a small change in Vgs will result in a large change in conductivity between drain and source. It will be a huge challenge to keep that Rds in the region where you need it to be. – Bimpelrekkie Aug 19 '17 at 19:23
  • Don't be so sure – Tony Stewart EE75 Aug 19 '17 at 19:23
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    @TonyStewart.EEsince'75 What a useful comment, care to elaborate so that I and everyone else here can learn something ? – Bimpelrekkie Aug 19 '17 at 19:25
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    Maybe a MOSFET designed for RF use would be more suitable. The "oldfasioned" way of doing what you want is using a JFET. Another way which might result in less distortion is by using an OTA, for example LM13600. And the "modern" way is of course to just use a chip designed for volume control. – Bimpelrekkie Aug 19 '17 at 19:28
  • Class A requires constant bias and high distortion if low gain https://www.infineon.com/dgdl/an-948.pdf?fileId=5546d462533600a40153559ed34311b1 – Tony Stewart EE75 Aug 19 '17 at 19:30
  • What makes you think this'll be cheaper than a designed-for-purpose VCA IC? – ThreePhaseEel Aug 19 '17 at 19:34
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    Still not getting your point, those IRF532/IRF9532 have a very low Rds_on. In that amp they're source followers so mainly used in saturation mode, then you actually want large W/L to keep Vgs small (and not lose much headroom). Still unclear how that would help when used as a variable resistor for volume control other than on/off. – Bimpelrekkie Aug 19 '17 at 20:32
  • What control range do you need? I have had success with JFETs as series elements - don't forget to put a compensating voltage into the gate. I have also used junction diodes as shunt control elements or using a Gilbert cell arrangement with bipolar transistor. – Kevin White Aug 19 '17 at 22:14
  • How are you going to control the gate voltage in order to maintain Rds at the desired value? I think you will need feedback control of some sort just to do that. Anyway, I would probably first look at a 2N7002 or BSS138 as they are very cheap and have been in production for a long time, and their Rds is not that low, even when biased to 5 or 10V Vgs. – user57037 Aug 20 '17 at 00:59

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Check out the CA3080 CA3080A, this might be the best way to do what you want. Bob K.

robertkondner
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    Note that the CA3080 is **ancient**, it is an OTA just like the LM13600. – Bimpelrekkie Aug 19 '17 at 21:29
  • I didn't know an OTS was what I needed, thanks for the tip! LM13600 is half the price per channel, and surprisingly cheap in the first place - perfect :) – Jodes Aug 20 '17 at 03:12