Op Amp's gain controlling by voltage DAC - Page 1
Forum section: electronics / beginners

author: Katanara
author_rank: Contributor
timestamp: December 09, 2022, 10:42:00 am
content: Hello! I have seen many schemes where a AC signal (e.g. sine wave) amplifying by op amp and it's using amplitude adjustment with multiplying DAC (pict. 1). But is it possible to control a gain of Op Amp with common voltage DAC?Could it work (pict.2) ? And why if not?Surely this is the basic issue and previous topics probably already exists. So the answer in the form of a link is also a very good answer. Thanks!

author: Kleinstein
author_rank: Super Contributor
timestamp: December 09, 2022, 10:56:06 am
content: The OP-amp circuits with the normal resistors, capacitors and inductors are linear or clamping (if the amplifier is driven to saturation). So that can not perform the nonlinear function of adjusting the amplitude. So one would need at least 1 more (actuall usually more) additional nonlinear element, like a diode or nonlinear capacitor. The main part is done by that extra part / not really the OP-amp.

author: ledtester
author_rank: Super Contributor
timestamp: December 09, 2022, 11:09:42 am
content: There are analog multipliers/dividers. Together with op-amps they form the basis of "analog computers".From:https://www.analog.com/en/product-category/analog-multipliers-dividers.htmlQuoteAlso see:https://en.wikipedia.org/wiki/Analog_computer

author: Katanara
author_rank: Contributor
timestamp: December 09, 2022, 11:16:42 am
content: Sorry for not quite correct pics. I would like to control input AC signal by DC signal of voltage DAC in fb of amp. If I understood you exactly.

author: pqass
author_rank: Super Contributor
timestamp: December 09, 2022, 11:46:34 am
content: Quote from: Katanara on December 09, 2022, 11:16:42 amLookup Eurorack VCA (voltage controlled amplifier) schematics.ie. Where a steady or varying DC "control voltage" (0-5V or +/- 2.5V) controls the amplitude of an audio signal through an amplifier. They typically use "transconductance" op amps like the LM13700 (https://www.digikey.ca/en/products/filter/linear/amplifiers/instrumentation-op-amps-buffer-amps/687?s=N4IgTCBcDaIDIFkCMBmA7ABgyAugXyA)eg. See:http://musicfromouterspace.com/analogsynth_new/DUALVCA/DLLVCA001.htmlhttps://www.lookmumnocomputer.com/11831184-quad-vca-mixerAlternatively,you could use a "vactrol" (or home-made green LED and LDR optocoupler) controlled VCA.See:https://damienclarke.me/effects-pedals/posts/building-a-vactrol-vca/However,you didn't specify your requirements for how fast the controlled signal or control voltage needs to be. The above works at slow audio frequencies and lower CV.

author: Katanara
author_rank: Contributor
timestamp: December 09, 2022, 12:27:34 pm
content: The input signal is under 100 khz. The speed of regulating is unimportant.

author: Whales
author_rank: Super Contributor
timestamp: December 09, 2022, 12:34:10 pm
content: Some alternatives that might or might not suit:A digital potentiometer can be used directly to replace a resistor around the opamp.Some all-in-one stereo audio amplifier ICs accept a voltage or single pot as a volume control.

author: Katanara
author_rank: Contributor
timestamp: December 09, 2022, 12:54:34 pm
content: In the past I have encountered serious problems with digital potentiometers in the feedback of Op Amp. The passing signal was clipped from below. I came to the conclusion that the digipot has a voltage at its pins, which introduces distortion into the feedback. And it's had a low resolution anyway (most of them are 8-bits).

author: dmills
author_rank: Super Contributor
timestamp: December 09, 2022, 02:29:58 pm
content: Not an opamp exactly, but for audio type doings a Blackmer cell VCA is commonplace, dB linear control at 6mV/dB and easy to use (Current in, current out, inverting so that when you connect the output to an I/V converter opamp you get an overall non inverting response).THAT Corp make the common ones in use today.LM13700 transconductance opamp is another possibility, if about 0.5% THD is acceptable, this time with a more or less linear control law.A 4 quadrant multiplier is another approach, maybe a gilbert cell mixer or such?Regards, Dan.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 09, 2022, 02:49:15 pm
content: Yes. Its a classic multiplying DAC applcation https analog.com/en/analog-dialogue/articles/high-resolution-multiplying-dacs.htmlIt has many advantages over linear multiplier VCAs. They are temperature stable and have very tight control linearity for a start.

author: David Hess
author_rank: Super Contributor
timestamp: December 09, 2022, 02:53:41 pm
content: Quote from: Katanara on December 09, 2022, 10:42:00 amIf the voltage output DAC uses an external reference, then it can be used to adjust the feedback factor adjusting the gain. Or it can be used as an attenuator. Some variable gain circuits use a CMOS DAC wired backwards.The alternative is to use the DAC to adjust the gain of an operational transconductance amplifier or multiplier.