I'm thinking of circuits that feature an operational amplifier like the venerable 741. Extremely often, you see a variable resistor in the final output acting as a potential divider to adjust the level of the output signal. The feedback resistor would be fixed in this case to provide the appropriate gain.

Why do you never instead see a variable resistor as the feedback resistor for the op amp? It seems that it would achieve the same objective. You would be adjusting output level by changing gain instead. Is there some reason to always have a fixed feedback resistor and adjust output level through other means?

  • \$\begingroup\$ In audio, variable resistors are used in feedback networks for tone controls. One issue for volume is that if you want to go down to zero volume, it's only possible in inverting mode. Under non-inverting feedback, the gain is at least one (\$1 + R1/R2\$). \$\endgroup\$ – Kaz Aug 10 '16 at 1:07
  • \$\begingroup\$ You do see it. See Douglas Self, Small Signal Audio Design, p231&ff, also the Quad 34 and 44 preamplifiers and no doubt many others. \$\endgroup\$ – user207421 Aug 10 '16 at 1:07

Whether or not you're right about your anecdotal observation, there are some reasons not to make the feedback circuit tweakable with a variable resistor. At least one some "front panel" one intended as a primary control.

One reason is the stability considerations of the amplifier, which vary with the amount of feedback. The greater the feedback, the more difficult it is for amplifiers to be stable. That is why you hear amplifiers being touted as "unity gain stable": it means that even if you configure them with a gain of only 1 (as opposed to 2, 3, ...) they will still be stable. If the feedback circuit of op-amp that is not unity-gain stable is allowed to be controlled by a variable resistor, then it could be taken into a region of unstable operation.

Secondly, see this question about noise gain. The noise gain of an op-amp follows the same formula as the non-inverting gain, whether or not the topology is inverting or non-inverting. If you try to use the inverting configuration with a variable resistor such that you allow gains below one, you actually end up with a situation in which you have signal gain less than one, and noise gain greater than one. That is not to say that a voltage divider after the output is noise free. Sure the noise relative to the signal rises as it as attenuated toward zero, but at least that passive network isn't actively amplifying noise while attenuating signal.

These aren't reasons to reject the idea of having some variable control in the feedback of amplifiers. In audio amplifiers, it has been a common technique for decades to implement tone controls in the feedback circuit. See the 1952 Negative Feedback Tone Control paper by P. J. Baxandall for instance.

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  • \$\begingroup\$ There is also a Baxandall active volume control, cited and explained in Self above. \$\endgroup\$ – user207421 Aug 10 '16 at 3:11

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