Worth noting if people did not notice: All signals was positive, not symmetrical around 0, the JFET will not be a happy little controllable resistor when the Drain voltage is lower than Source voltage
I did some experiments years ago with JFET attenuators and a distortion analyser. It helps a lot to add half of the Drain voltage into the Gate control, typically using a 1M resistor between Drain and Gate, and then another 1M resistor from the Gate to the control voltage. Of course this means the control voltage now has to swing twice as far for the same effect, but that is easy to set up with an op-amp. I have not done the maths, but suspect that this helps to cancel out the square law transfer characteristic.
Really cool! Perhaps one way to reduce distortion would be to attenuate the signal prior to this voltage-controlled stage and then reamplify it by the same amount following this stage. That may add a little extra noise, but it may be worth it depending on application.
To maintain a decent linearity you have to keep the signal as low as possible. With 1V peak you are really pushing the jFet. Do the same with 0.1V peak to peak. Of course you'll have to amplify with a second stage and this is not ideal for noise.
couldn't you make an extremely simple AM transmitter with this? audio goes on the gate, carrier signal goes on the resistor, the magnitude of the carrier will change with the audio signal, hence amplitude modulation
You are missing a key benefit of JFETs in your circuit. JFETs have a very high input impedance. You are feeding it with 50 ohms. You want to impedance match this, which will give you a huge increase in amplification and decrease in noise (reducing the SNR by adding passive gain in front of the FET).
Others have also pointed out mostly:
Worth noting if people did not notice: All signals was positive, not symmetrical around 0, the JFET will not be a happy little controllable resistor when the Drain voltage is lower than Source voltage
I did some experiments years ago with JFET attenuators and a distortion analyser. It helps a lot to add half of the Drain voltage into the Gate control, typically using a 1M resistor between Drain and Gate, and then another 1M resistor from the Gate to the control voltage. Of course this means the control voltage now has to swing twice as far for the same effect, but that is easy to set up with an op-amp. I have not done the maths, but suspect that this helps to cancel out the square law transfer characteristic.
Yes, they liked it, distortion gives the synth character.
I did not know there was a distortion analyzer test box. I learned something new.
If I recall correctly the jfet is more of a programmable current sink than a variable resistor.
I always drool over your lab kit...especially when you break out the keysights.
Merci. I love this video.
The FET is the reason why a transistor is named transistor, because Transistor is the short form of tranformeble resistor.
Really cool! Perhaps one way to reduce distortion would be to attenuate the signal prior to this voltage-controlled stage and then reamplify it by the same amount following this stage. That may add a little extra noise, but it may be worth it depending on application.
Oh so very interesting. Thank you.
Youre Videos educational valuable.
To maintain a decent linearity you have to keep the signal as low as possible. With 1V peak you are really pushing the jFet. Do the same with 0.1V peak to peak. Of course you'll have to amplify with a second stage and this is not ideal for noise.
Using resistors, H&H greatly improved the voltage controlled resistance of FETs.
How do we maintain low THD at high amplification in this circuit?
The amount of distortion depends on the signal level. You would get less distortion if you decreased the signal level.
couldn't you make an extremely simple AM transmitter with this? audio goes on the gate, carrier signal goes on the resistor, the magnitude of the carrier will change with the audio signal, hence amplitude modulation
You are missing a key benefit of JFETs in your circuit. JFETs have a very high input impedance. You are feeding it with 50 ohms. You want to impedance match this, which will give you a huge increase in amplification and decrease in noise (reducing the SNR by adding passive gain in front of the FET).
How does a Gilbert cell multiplier like the MC1496 perform in that application?
If even JFET introduces so much distortion what is HiFi AGC are made from? 😕