# analog multiplier

Discussion in 'Electronic Basics' started by [email protected], Jul 19, 2006.

1. ### Guest

Can you make an analog multiplier with an operational amplifier or a
biased transistor?

Thanks.

2. ### Phil AllisonGuest

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** You can make one if you use both.

........ Phil

3. ### BanGuest

View\fixed font
.---------+------o
| | V+
.-. .-.
| |R1 | |R1
| | | | ___
'-' '-' .--|___|-.
| | | R2 |
| | | |\ |
| +----+---|-\ |
| | | >-+--o
+---------)----+---|+/ Vout
| | | |/
| | | ___
| | '--|___|-GND
|/ \| R2
o-| LM394 |-GND
Vx |> <|
o | |
| '----+----'
=== |
GND .-. Vout~ R2/Ry * VxVy/2Vt
| |Ry
| | with Vy<0, Vt=26mV
'-'
| +/-Vx<<Vt 6mV for 1%THD
o
Vy dual transistor for low
o offset drift
|
=== R1 chosen for proper bias
GND
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

4. ### Abstract DissonanceGuest

The only method I know of uses the fact that log(f*g) = log(f) + log(g)

hence you convert your signals to there logs then add and take the
exponential.

i.e.

f*g = exp(log(f) + log(g))

5. ### Greg NeillGuest

You can make logarithmic and exponential output
configurations (employing a diode in the op-amp
circuit), and a summing amplifier. So you could
put together multiplying and dividing circuits
by combining them appropriately. Have a look
here:

http://en.wikipedia.org/wiki/Operational_amplifier_applications

John

7. ### ChrisGuest

Hi, Bob. You can, but I wouldn't.

Multipliers made from op amps and discrete components are subject to
major drift and non-linearity issues. Also, less complex designs have
the limitation of only working in one or two quadrants (meaning one or
both of the multiplier inputs can only be positive).

You can get a good four-quadrant IC analog multiplier (Analog Devices
AD633) for several dollars in single quantity. It has differential
inputs, better than 2% error across the input voltage range, works from
DC to well above audio frequencies, and doesn't require *any* external
components. Perfect for newbies.

If you have requirements for higher frequency, more accuracy, or
special functions (division, square root), other products which can

http://www.analog.com/en/index.html
http://www.ti.com/

a question, the headache of trying to cobble together a working
multiplier from discrete components just isn't worth it.

Good luck
Chris

8. ### Bob MastaGuest

Others have given you good info regarding multipliers
that use device nonlinearities. These are the methods
of choice for high-speed applications (Gilbert cell especially).
But for low-speed situations it's possible to build a
multiplier based on pulse-width modulation. The basic
idea is that you feed a fast triangle wave into one input of
a comparator, and one of the values to be multiplied into
the other. The output is a rectangular wave whose duty
cycle is proportional to the input value. You then use this
to modulate (chop) the second input, and average the
results (low-pass filter) to get the product output. This
works well at low frequencies and can be made very
linear. As you try to move to higher frequencies things
get trickier.

Best regards,

Bob Masta