# Viability of Voltage Controlled Amplifier circuit

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.

EDIT: Fixed an error in my drawing in which the non-inverting pin of the op amp was erroneously tied to GND and not 1/2 Vcc.

• How fast do you need the amplifier to respond to the gain signal? Commented Apr 11, 2015 at 20:10
• @OlinLathrop Hmm, I hadn't thought about that. Let's see: I intend to use a VCA in two places, once to set the cutoff frequency of a voltage controlled filter and again as a stand-alone amplifier. In the first instance I will need to accommodate frequencies up to 20KHz but for the later stand-alone section the requirements are more forgiving. There the gain will be routed to a low frequency oscillator with a maximum frequency of only 30 Hz or so. Does this help? Commented Apr 11, 2015 at 20:37
• Um, OK, so how fast do you need the amplifier to respond to the gain signal? Commented Apr 11, 2015 at 21:41
• Oh, was that not helpful? Sorry :P I suppose instantaneously would be best but with the 20KHz limit I believe 50 microseconds is the highest delay time. But I'm not sure I understand what you're asking. Commented Apr 11, 2015 at 22:09
• I was asking about the response time to the gain signal, not the signal that the amplifier is amplifying. Commented Apr 12, 2015 at 12:37