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I've got an LM13700 in this VCA configuration, and for whatever reason the gain control input (I'm applying a triangle LFO, ranging from 2-7v) is appearing as a voltage at the output. Does anybody know why this could be happening? : enter image description here

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  • \$\begingroup\$ Are you applying LFO to gain input or Vin? \$\endgroup\$ – JaySabir Aug 17 '20 at 6:36
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With change the bias current (gain) of the amplifier, also changes the input bias current and the input offset current. The input offset current generates voltage at 470 Ohm resistors, which is more visible at the output of the amplifier.

It will not be perfect, but it can be improved:

-With smaller resistors at the input (470 Ohm > 47 Ohm).

-By zeroing the initial offset voltage. (Minimum triangle signal at the output without input signal.)

13700

13700-1

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  • \$\begingroup\$ Hmmmm. Lower value resistors on the inputs to Vref did not appear to do anything. However, what did help a bit was instead of having two 470 ohm resistors, I used a 1M ohm to Vref from the noninverting input, and then wired the inverting input straight to Vref. That improved things about 25%, and then the key after that was to adjust Vref down by about a volt. Perhaps in doing that I achieved what csabahu meant by zeroing the initial offset voltage? Curious for anyone's further input on this; I kinda got a solution but I don't really understand it. I'm using a single supply if that matters. \$\endgroup\$ – Jonathan Donald Aug 18 '20 at 4:17

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