Electronics > Projects, Designs, and Technical Stuff
Poor mans analog multiplier
GK:
I've got a design on the bench that requires eight linear, DC-coupled and stable analog multipliers. That many AD633's starts to get expensive. Many people have tried to use a Gilbert Cell MC1496 for this role with only very average results.
Well after a little head scratching, here is my solution to the problem, using three MC1496's to essentially replicate the internal topology of the AD633 and similar multipliers.
IC1 and IC2 form the exponential voltage converter that kills 3 birds with one stone:
1) DC biases the twin differential transistor pair of IC3
2) Temperature compensates the gain of the twin differential transistor pair of IC3
3) Linearises the twin differential transistor pair of IC3
I quickly built the circuit on breadboard this evening and it appears to work exceptionally well. Without any offset trimming whatsoever the DC offset error at the output is under 10mV! Better than some samples of my AD633's!
I've done some measurements and the individual transistors inside the MC1496 are remarkably well matched. Only 2 transistors inside IC1 are actually utilized in the circuit, but I wouldn't bother substituting the package with a dual NPN transistor. You won't find a dual with better device matching than those two transistors inside the MC1496 for anywhere near the same price - mine measured a better than 50uV Vbe match.
GK:
BTW, here is the circuit that is replicated:
See application note:
http://www.analog.com/static/imported-files/tutorials/MT-079.pdf
moffy:
Can you provide any harmonic distortion/FFT graphs, figures? Would be interesting.
GK:
I think an FFT in SPICE would be useless as the transistors in the virtual world are all perfectly matched, and the linearity of the multiplier depends very much upon the matching of the transistors.
I think the best way to simply and effectively asses the static transfer linearity in the real world would be to set one input to full scale and then sweep the other the full range in accurately measured discrete steps and tabulating the results. Then the inputs can be swapped and the test repeated.
I'll get around to that maybe in several days.
moffy:
That would be interesting. When I mentioned FFT I was meaning on a realtime signal not a simulation. Sorry for the confusion. But I would also be interested in the BW performance. THD will get worse with frequency. But then that is a whole new method of testing which requires good oscillators and a spectrum analyser. I know I don't have that.
It was neat the way you used the MC1496 as a matched npn pair. Nice!
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