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Quick question here about adding control voltage and attenuation into a circuit.

Say I have a simple op-amp configuration such as below, to vary the gain of an audio signal, where P1 is a variable resistor or potentiometer.

Suppose I wanted to add an input signal, say a ±5V low frequency signal, that would vary the resistance of P1 (i.e. using this low frequency signal to, in essence, turn the knob of the potentiometer instead of doing it by hand). And suppose I wanted to attenuate this input signal as well (for example, having a potentiometer that, when turned, varies the amplitude of the low frequency input signal, say to ±2V).

What exactly would I need to add to the circuit below to make this possible?

enter image description here

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    \$\begingroup\$ Try: ti.com/lit/an/sbaa146a/… \$\endgroup\$
    – glen_geek
    Feb 11 '21 at 18:30
  • \$\begingroup\$ One way is a motorized potentiometer, literally a geared electric motor driving P1. \$\endgroup\$ Feb 11 '21 at 19:38
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One approach is to use an OTA; operational transconductance amplifiers like LM13700 have a third input pin, that alters the gain of the amplifier. If one wishes, in addition to gain, an attenuator on the input signal, that's just a good use for a second OTA (the LM13700 contains two OTAs, so it's not much trouble or cost); gain can be greater or less than 1.

The gain-setting input is a current-sensing input, diode-clamped to the negative supply rail.

OTAs have some disadvantages; they aren't temperature-independent nor entirely linear for large input signals, and input currents may be significant.
OTA output is a current source/sink, not a low-impedance voltage source like ideal op amps, so a resistor to ground and follower (maybe just the built-in Darlington follower) convert to a voltage source if that's important.

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  • \$\begingroup\$ Yes @Whit3rd, after some research into the LM13700 I think it is the way to go. I'm drawing up a circuit that I think will solve my problem which uses the LM13700, but that will be a separate post, as I do have some questions about the OTA. \$\endgroup\$
    – JTaft121
    Feb 13 '21 at 0:02
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One crude solution is to replace P1 with an LDR illuminated (only) by an LED or lamp powered by the control signal. When P1 > R1 the |gain| is > 1 and vice versa.

There are also circuits that use a JFET.

There are log or linear digital pots that will work from a bipolar supplies such as the DS1881 but then you would have to digitize the control signal.

You could also use a 4-quadrant analog multiplier.

All of these options have various disadvantages, such as added distortion, unpredictable and perhaps unstable control voltage range, high cost etc.

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A compact solution is to use a VCA chip, such as the SSI2164 or THAT 2180.

Modifying an opamp circuit to be voltage controllable is highly non-trivial and usually not the best way, which is why these chips exist.

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