I've designed this Voltage Controlled Amplitude circuit and I've simulated it in Multisim.
When simulating I get a waveform like this:
The green waveform is the modulator signal and the red is the output of the circuit. This seems to work OK.
But building it on a breadboard, I get this on my oscilloscope:
The green signal is a square wave that is the input signal. This is modulated by a low frequency sine wave. The yellow signal is the output signal, that is obviously not the expected square waveform.
I can't find the error on the breadboard. Any advice on how to fix this?