In this VCA example (I'm ignoring the variation with positive control voltage at source), the control voltage (Vgs) is being "mixed" with the signal into the gate, am I right?
Yes, that's one way to look at it (if by "mixing" you mean "adding together", as there is another use for the term "mix"). Essentially, we're creating an offset that the AC signal swings around (i.e., placing it on different portions of the transconductance curve).
The Source resistance will reduce gain but it should tend to linearize the amplifier across all conditions. Are there other ways to reduce distortion? I am assuming that wrapping feedback around this would hamper the variable gain, until it runs out of gain.
Yes, negative feedback both gives and takes away. This is not a high gain circuit to start with so you don't have a lot to sacrifice. You could do tandems (i.e., cascade a couple of them with a common control voltage). If you really need a wide gain variability with only modest distortion, you might be better off going with an OTA (operational transconductance amplifier). I have a video on OTAs slated for the future.
@@ElectronicswithProfessorFiore I was about to ask about the distortion caused by this circuit, because I got a bit confused. Why would I not use this instead of an opamp. If I get it correctly - and please correct me if I am wrong! - the curve in the beginning of the video shows that this circuit is rather unlinear, hence the introduced distortion might be too high, let's say for a simple volume adjusting circuit in an audio context. Of course who knows, maybe this distortion does actually sound good? :)
@@krakamak I assume the curve to which you are referring is the transconductance (gm) curve. Yes, that's non-linear (square law, to be precise). gm sets the gain of the amplifier, and as long as the signal remains small, the distortion will not be very large. In the circuit shown, THD is in the area of 0.1 to 0.2 %, depending on gain. Not super hi-fi, but not gross, either. This circuit would not be used for something like a simple volume control because it does not have a wide enough range. There are better ways of doing that. It can be used as part of a signal feedback system, though. For example automatic gain control (AGC) which is used to keep the output signal at a relatively consistent level in spite of changes in the input signal strength. A classic example of this is in radio receivers where the input signal is a function of how far you are from the broadcast antenna. And yes, you might like the distortion when the circuit is pushed! Guitar fuzz boxes have used all manner of devices over the years, including BJTs, FETs, and op amps.
Yes, but notice how we did that in the second half of the video. It really depends on how you want the direction of the control voltage change to work, not its polarity (e.g., making the voltage more positive yields greater gain).
In this VCA example (I'm ignoring the variation with positive control voltage at source), the control voltage (Vgs) is being "mixed" with the signal into the gate, am I right?
Yes, that's one way to look at it (if by "mixing" you mean "adding together", as there is another use for the term "mix"). Essentially, we're creating an offset that the AC signal swings around (i.e., placing it on different portions of the transconductance curve).
@@ElectronicswithProfessorFiore Yes, I meant adding : ) Thank you very much!
The Source resistance will reduce gain but it should tend to linearize the amplifier across all conditions. Are there other ways to reduce distortion? I am assuming that wrapping feedback around this would hamper the variable gain, until it runs out of gain.
Yes, negative feedback both gives and takes away. This is not a high gain circuit to start with so you don't have a lot to sacrifice. You could do tandems (i.e., cascade a couple of them with a common control voltage). If you really need a wide gain variability with only modest distortion, you might be better off going with an OTA (operational transconductance amplifier). I have a video on OTAs slated for the future.
@@ElectronicswithProfessorFiore I was about to ask about the distortion caused by this circuit, because I got a bit confused. Why would I not use this instead of an opamp. If I get it correctly - and please correct me if I am wrong! - the curve in the beginning of the video shows that this circuit is rather unlinear, hence the introduced distortion might be too high, let's say for a simple volume adjusting circuit in an audio context.
Of course who knows, maybe this distortion does actually sound good? :)
@@krakamak I assume the curve to which you are referring is the transconductance (gm) curve. Yes, that's non-linear (square law, to be precise). gm sets the gain of the amplifier, and as long as the signal remains small, the distortion will not be very large. In the circuit shown, THD is in the area of 0.1 to 0.2 %, depending on gain. Not super hi-fi, but not gross, either.
This circuit would not be used for something like a simple volume control because it does not have a wide enough range. There are better ways of doing that. It can be used as part of a signal feedback system, though. For example automatic gain control (AGC) which is used to keep the output signal at a relatively consistent level in spite of changes in the input signal strength. A classic example of this is in radio receivers where the input signal is a function of how far you are from the broadcast antenna.
And yes, you might like the distortion when the circuit is pushed! Guitar fuzz boxes have used all manner of devices over the years, including BJTs, FETs, and op amps.
@@ElectronicswithProfessorFiore Great, thank you for the reply! And the videos are great, keep them coming! :)
Could I use a P channel FET in order to use a positive control voltage?
Yes, but notice how we did that in the second half of the video. It really depends on how you want the direction of the control voltage change to work, not its polarity (e.g., making the voltage more positive yields greater gain).