[sdiy] An unusual opamp only four quadrant multiplier

[Didrik Madheden]

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