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analog multiplier

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

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  1. Guest

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

    Thanks.
     
  2. Phil Allison

    Phil Allison Guest

    <>

    ** Groper alert.

    ** You can make one if you use both.



    ........ Phil
     
  3. Ban

    Ban Guest

    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. 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 Neill

    Greg Neill Guest

    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
     
  6. John  Larkin

    John Larkin Guest

    Also google "Gilbert cell multiplier."

    John
     
  7. Chris

    Chris Guest

    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.

    http://www.analog.com/UploadedFiles/Data_Sheets/277093686AD633_e.pdf

    If you have requirements for higher frequency, more accuracy, or
    special functions (division, square root), other products which can
    help you are available from Analog Devices and Burr-Brown (now TI).

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

    If you're thinking about actually making one instead of just answering
    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 Masta

    Bob Masta Guest

    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
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
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