# My very basic voltage-controlled amplifier gives an asymmetric waveform?

I want to build a voltage-controlled amplifier. The Wikipedia pages on voltage-controlled amplifiers and voltage-controlled resistors suggested using the following approach: first I would simply build a voltage amplifier using an opamp and this familiar topology:

I would then replace one of the two resistors with a voltage-controlled resistor to yield a voltage-controlled amplifier. Wikipedia suggested using a JFET, which to me seemed quite reasonable. I thus made the following circuit in LTSpice:

I've attached the LTSpice-file at the bottom.

My problem now is the following: by varying the voltage on the gate of the JFET, I can indeed vary the gain of this circuit. However, when I input a sine-wave with no DC-offset, the circuit outputs a sine wave with more gain in one direction than in the other. In the LTSpice circuit on the picture, for instance, a 200 mVpp input sine gives an output sine going from 750 mV in the positive direction to only -700 mV in the negative direction. Obviously something is wrong here!

I can't quite make out whether it is just an offset or the output is really not a sine wave anymore. My main question would be why is my output waveform asymmetric with respect to the time-axis? but if this circuit is a dumb idea in general I wouldn't mind hearing too.

LTSPice file: http://g2f.nl/002hube

• if the SOURCE is tied to the opamp, then you are varying the gate-source voltage which varies the FETs on-resistance. – analogsystemsrf Oct 6 '19 at 22:39
• Increase your R1 value by 10 to reduce FET current – Tony Stewart Sunnyskyguy EE75 Oct 7 '19 at 1:28