[sdiy] An unusual opamp only four quadrant multiplier

cheater cheater cheater00social at gmail.com
Sat Apr 3 03:19:49 CEST 2021

This kind of crazy stuff often ends up sounding either very bad or
very unique or very good. I'd say go for it.

On Sat, Apr 3, 2021 at 2:27 AM Didrik Madheden via Synth-diy
<synth-diy at synth-diy.org> wrote:
> I watched one of Aaron's latest lectures, the one about Buchla's
> Diodeless Deadband Circuits. (Hi Aaaron. Yes, I'm "GG" over there in
> case you're wondering.) That circuit got me thinking about another
> circuit I saw once, and finally found my way back to. It's an analog
> four quadrant multiplier (ring modulator) using only jellybean opamps
> as active elements, ie no OTAs or similar.
> Two opamps are connected as unity followers with their outputs
> connected together through a resistor, to create a squaring circuit.
> Another similar cell is connected with one of the input signals
> inverted. Then the current consumption of those two cells is measured
> by putting a resistor in series with the power supply (!) which forms
> a simple current to voltage converter. The difference of those signals
> is taken with another opamp, and that is the result of the
> multiplication.
> This was published in 2011 by Vanchai Riewruja & Apinai Rerkratn as
> "Four-quadrant analogue multiplier using operational amplifier". There
> is, what seems like a first draft available for free online which
> doesn't have the final circuit with two opamps per cell, and shows
> pretty bad circuit performance. The final version is available in the
> International Journal of Electronics volume 98, issue 4. If you have
> academic access, or $53 burning in your pocket you can get direct
> access to it. If you have neither of those, contact me off list.
> So my question is, has anything like that ever been used in a
> practical circuit? Either in a musical synthesizer, or some other
> context that you're aware of? It's a bit naughty connecting the opamp
> power supply through a resistor, but it seems to work according to the
> paper.
> --
> /Didrik
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