Adding Modulation to Voltage Signal

Hello, I just started sketching ideas to add LFO to VCA modulation in a device that already has a VCA circuit employed, but I've run into a snag... I can design a circuit to give proper modulation at full open VCA voltage, but as the signal voltage drops from 4.5v to 0v, the moduation signal falls and begins clipping the lower portion of the wave.

The first picture below is the stock circuit. 4.5V Key Down/0V key up (can be attack/release contoured) to 15k resistor feeding a 3906 transistor with base to ground. Collector to 10k resistor then to the input bias of a 3080 transconductance op amp. Voltage on the other side of the transistor before the 10k is -15V Key Up and -12V Key Down.

Second picture is one my rough sketches. The LFO (1.6 to -1.6 swing) with a bias from the A/R voltage (which is fed through a buffer) feeds an amplifier that connects to the base of the transistor that was originally grounded. With this circuit I get a good modulation swing from ~-15V to ~-12V on the other side of the transistor at max A/R voltage, but as the 4.5 falls back to 0 the bottom begins to clip, as in picture three. I would like the amplitude to decrease steadily as the 4.5 falls to zero and then stop sending signal to base of transistor.

I tried using biased transistors as variable resistors in the gain loop of the LFO amplifier but my knowledge of transistor switching and operation are not that great. If anybody out there could help it would be greatly appreciated.

What are the specifications of what you want the circuit to do?

Input signal voltage and current

Output signal voltage, current, and impedance: a voltage source (low Z) or a current source (high Z)?

Maximum frequency (48 Hz?)

Power supply rail voltages

Here are the specs of what I want to do:

*Stock operation*:
Incoming signal '*A*':* 0V-4.5V*
Output from collector '*C*': When incoming signal *A*=*0V*, *C*=*-15V*. If *A*=*4.5V*, *C*=*-12V*.

*What I want to do*:
Incoming signal '*A*':* 0V-4.5V*
Use LFO triangle signal with a swing of *+/-1.6V* to modulate *C* from *-15V* to *-12V*
As *A* rises or falls from *4.5V* to *0V*, voltage swing at *C* changes accordingly (eg, swing -15V/-12V at 4.5V, -15V/-13V at 3V, -15/-14 at 1V, etc). Numbers for that aren't exact as long as 0V at A = 0V at C, and 4.5V at A = -15/-12 swing at C.

Frequency of my LFO has quite a large range, from .2hz to audio range. Power supplies in the machine are +12, -12, -15, +5.

Patrick, it sounds like you aks for one gain and offset then ask for a different one. Is this what you want?

Input 0V, output 0V
Input 4.5V, output -12V

If so, all that is needed is a gain of -12/4.5 = 2.67. The circuit below will do this.

Here are the input and output waveforms.

Yes, I want the gain of the modulation signal to be dependent on the 4.5v going into it. The waveform seems to be what I'm looking for, but I'm not quite sure how to translate the circuit schematic you supplied to mine.

The Op Amp output provides the same voltage that you want at the transistor collector. A 10k ohm resistor between the Op Amp output and the -15 V node (is this a node on the LM3080?) will drive the same current into the node as the transistor circuit. To the node it looks like a voltage source having a 10k ohm impedance.

If you want to retain the transistor to have a current source we can do that. To the node is will then look like a current source having the transistor collector resistance in parallel. There will be an error due to transistor base current. This error can be eliminated by using a P-channel MOSFET rather than the PNP BJT.