Viability of Voltage Controlled Amplifier circuit

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Viability of Voltage Controlled Amplifier circuit

Postby Leonardo19 » Mon Aug 01, 2016 9:11 am

Hello, this is my diy amplifier.
As part of the synthesizer project I have been working to design a simple voltage controlled amplifier. Due to the complexity of other parts of the circuit and the amount of time I've spent on this project overall I am seriously considering opting for a variable amplifier chip ; an OTA perhaps. Still, working with what I have now, I have drawn up what I think might function as a voltage controlled amplifier from discrete components and an op amp.

As I understand it the circuit should behave as follows: T1 acts as a low impedance emitter follower, biasing the incoming signal at VCC/2 so as to allow for continuous operation of T2. This second transistor functions as a current source with the right end of R4 effectively at GND. The amount of current flowing through this resistor is determined by voltage across the base and emitter junctions of that transistor (Vbe/R4). Because of op amp rules the current flowing through T2's collector/emitter must be mirrored across R5. This produces a voltage at the output proportional to the voltage at the collector of Tr2.

During testing, however, the circuit didn't exactly behave as above. To account for possible errors in the values I chose for that test I have ignored assigning values to this particular circuit. Does this circuit make sense? Is my analysis correct.
P.S. Perhaps this is a different topic but please feel free to critique my EAGLE labelling conventions and circuit syntax as well. It's still new to me.

Any advice will be very appreciated.
Best regards
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Joined: Wed Jul 27, 2016 6:07 am

Re: Viability of Voltage Controlled Amplifier circuit

Postby jeff-der-experte » Sun Nov 06, 2016 6:25 pm

Hello Leonardo19,

for the volume control circuit of my self-built theremin I needed a very good VCA. In the beginning I experimented with the LM13600 (or equivalent LM13700) but did not get good results.

Later I got acquainted with the SSM2164, a 4-channel VCA for 4 independent music sources. It works with control voltages between +3.3 V for -100 dB signal damping and -0.66 V for amplification with +20 dB. Its sensitivity is -33 mV/dB.

If you want only one signal to be controlled I can recommend you the SSM2018T by Analog Devices. Its data sheet is available in the net.

Because my theremin has three voices to be mixed on a small mixing desk I need the SSM2164 for the three voices (three demodulator circuit boards with different timbres: ring modulator, cascode and pentode). The fourth channel is controlled by the voltage generated by the player's hand on the loop-shaped volume antenna. This channel contains the mixing product of the three voices. So I can get a wide variety of timbres.

I wish you good luck.

Kind regards
Posts: 2
Joined: Sun Nov 06, 2016 2:12 pm

Re: Viability of Voltage Controlled Amplifier circuit

Postby SteveThack » Thu Feb 16, 2017 12:55 pm

Hi Leonardo19,

I'm a bit puzzled by your circuit, as it would seem to be a way of varying the DC output voltage of the op-amp. I'm not sure how this will help if you want the control voltage to control the audio gain.

Could you explain a bit more, and in particular explain what you are seeing that is unexpected?
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Joined: Sat Jun 28, 2014 8:22 pm

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