Differential Transistor Voltage Control Amplifier with inverting input
Courtesy/Thanks to: Rene Schmitz
Description: VCA-1 & VCA-2 are build arround differential transistor pairs. The circuits are suitable for input voltages of 5Vpp, the output level of my VCOs, however the clipping level is somewhat higher, arround 7Vpp. If you use higher voltages in your system you might raise the input resistors to 150k. Both circuits have linear control voltage response. VCA-2 is a more streamlined version of VCA-1, the circuit has also an inverting input to simulate ringmodulation via a nulling method. One problem of these discrete VCAs is thermal drift of the transistors, this causes the CV rejection to vary (As was pointed out by Jörgen Bergfors). The best cure for this would be to use a dual transistor pair as in a CA3086 or (my favourite) 2SC1583. In my latest VCA-2s I have used a 2SC1583 instead of the BC550s with good results. Feedtrough was lower than 2mV, and the offset was very low. But heat radiation could still ruin the stability. A styrofoam insulation for the dual transistor might be a good idea.
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