Anyone know where to find some easy to understand info about analog multipliers.

[Brian Marshall]

I'm doing some reading in don lancasters active filter cook book, and there are some references to using analog multipliers in the state variable filter.

I was under the impression that analog multipliers output varried voltage depending on the inputs... not current.  The state variable filter would need variable current at the output to really work this way.  Am I missing something here?

already did a forum search and didnt find much.

[George Giblet]

The majority of multipliers use current, at least internally.  To the outside world it can be either.  If you take an OTA like an LM3080 it only has current output whereas an LM13600 has both current and voltage.  The thing is the current output can be converted to a voltage simply by driving it into a load,  Vout = Iout* Zout.   The output impedance can be high so often you have a buffer at the output like the LM13600.  BTW, an OTA is a two-quadrant multiplier and four-quadrant multiplier can flip the sign of the output when the "control" goes negative.

There's some good applications notes on OTA's from Harris-Semiconductor (now defunct) and Intersil.  One that comes to mind is AN6668,
http://www.intersil.com/data/an/an6668.pdf

There's another one with 4 and 7's in the application note number that I can't remember, and perhaps this one,
http://www.intersil.com/data/an/an6077.pdf

See also National Semi stuff on the LM13600/LM13700.

Unfortunately to understand filters using OTA's you have to understand OTA and go through the filter math.  You should be able to find a lot of stuff on the web regarding filters and OTAs but unfortunately a large percentage are academic paper dealing with certain aspects.

[toneman]

google "balanced modulators".

check app notes for LM13600/13700.

check www.PAIA.com for their 9710 module for a 1496 balanced modulator.

"analog multipliers" & "balanced modulators"  are "close" to the same, but not.

also, check "ring modulators"....this is how frequency doubling all started.

afn
T

[Paul Perry (Frostwave)]

The majority of multipliers use current, at least internally.  To the outside world it can be either. 

Yeah, I've noticed that EEs think in terms of current, while mortals (like me) think of voltages, which I guess puts us one step further away from reality.
BTW I was pretty excited to see someone named Gilbert answered this one, seeing as it was another Gilbert - Barrie Gilbert - that invented the integrated multiplier!

[Brian Marshall]

ok, all good info.  I didn't realize that OTA's were analog mulipliers...  I've read the 13700 datasheet about 100 times. 

I guess I just never actually understood what an analog multiplier was, and had made some presumptions from the small amount i had read before.  Some datasheet i read a while back (dont remember the part exactly) only mentioned the output in terms of voltage, and from what i understood baically was a voltage controlled amplifier, and litterally was an analog multiplier because the output voltage was equal to x*y...  didnt mention much about output impedance or loading in regards to the output.

I'm guessing analog multipliers must have some serious issues with off set, or distortion, or we would all be using them in phasers and filters and such.

[R.G.]

I'm guessing analog multipliers must have some serious issues with off set, or distortion, or we would all be using them in phasers and filters and such.

Another winner.

Let me congratulate you on having the turn of mind that says "OK, I see it's that way. But ... WHY is it that way? Hmmmm...."

Your supposition is correct. It's like how we know that no one can correctly predict the stock market for any long period of time. It's simple - if they could, they would have, and they would already own everything.

Analog multipliers need careful trimming to prevent feedthrough of one signal to the output when the other signal is 0. For audio, this is critical because the periods of silence are important, and the human ear is very good at detecting soft sounds in the middle of intermittent loud sounds. And you are spot-on about distortion. The simple multipliers can only accept a signal of about 25-50mV before they distort. So we divide the signals down to that range and get noise when we amplifiy it back up.

Curing the problems with the basic multiplier also tends to need more circuitry, so good multipliers are either complex, or expensive, or both; bad prescription for a stompbox.

Good mind you got there. Keep it turning.

[Paul Perry (Frostwave)]

Those "X and Y voltage in, Z=XxY voltage out" multiplier chips like the AD633 have a (very carefully balanced) current multiplier core & wrap everythig up to make it easy to apply the chip, say in a ring modulator (but, you still need external trimpots to tune out bleed through).

It isn't easy trying to make say the LM13700 act as a multiplier (following the circuit in the app notes), but if you do, note that it won't stay balanced if the power voltage changes. The full range multiplier (traditionally they are called "four quadrant" because both X and Y can be +ve or -ve - geddit?) relies on a bit of algebra where you add a constant to one of the variables so it is always positive, then multipliy it, then subract the constant times the multiplier.

There are other ways to multiply, if you don't need to be too accurate.
1. generate a string of rectangles X high & Y wide, then average them out.

2. If you have a diode square function generator (think a few dozen op amps, resistors & diodes) then take the difference between (X+Y) squared and (X-Y) squared - yeah, thats 4 XY!!

oh yes, analog computers, those were the days... but, some of those old analog tricks could be turned into hellish noisemakers, I bet!

[Sir H C]

Barrie Gilbert of Analog Devices is the man on analog multipliers.  Google him/analog multiplier and you can find some well written papers on them and the drawbacks already mentioned.

[Brian Marshall]

thanks for the info and taking me to school on all this stuff.

i got some SSM2164's from analog devices a few weeks ago.  They call them a VCA, but from the definition as i understand it now they are analog multipliers.  The VCA terminology is probably aimed at specific people they think will be looking to use them (marketing) I guess.

I used 2 stages of an ssm2164, and a TL074 to make a state variable filter.  It seems to work pretty well, although they are really only rated down to 8 volts.  It seemed to sound a lot better at 15 V though.