HOW TO design antilog/log circuit?

Discussion in 'The Projects Forum' started by kachung30, Feb 3, 2010.

  1. kachung30

    kachung30 Thread Starter New Member

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    l need to do multiplier by OP-AMP. But l don't know suitable ic value . ie transistor, resisitor ,etc
    thank you:)
     
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  2. t06afre

    t06afre AAC Fanatic!

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    I have an old book from Burr Brown at home. I shall see if I can find it. But you should be aware of that is many problems with such circuits. As an example they do not work very well at low inputs level. What are your trying to make. It could that is better solution to your problem.
     
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  3. kachung30

    kachung30 Thread Starter New Member

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    I am trying to design an voltage multipliers to calculate output voltage. output is a^b
     
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  4. Papabravo

    Papabravo AAC Fanatic!

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    As you are no doubt aware, multiplication is a non-linear operation. The construction and use of non-linear circuits have numerous problems including the need to scale the output to prevent saturation.

    The use of a logarithmic amplifier is straight forward, but the inverse operation of raising to a power has the problems of saturation and output scaling.

    Check application notes from National Semiconductor for ideas on these beasties.
     
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  5. MikeML

    MikeML Senior Member

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  6. jans123

    jans123 New Member

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    Right, National was my "tutor" many years ago, long before the application note http://www.national.com/an/AN/AN-311.pdf was published. Look at it, it might give some hints.
     
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  7. steveb

    steveb Senior Member

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    What frequency bandwidth, (or response time) do you need? If you need high speed (MHz for example) you will likely need an analog approach as discussed above. To avoid the difficult design work, you can buy an existing chip, but they are expensive.

    However, if you don't need high bandwidth, you can use a cheap microprocessor with built in A/D and D/A converters. All types of nonlinear operations (including multiplication) can be easily handled in programming.
     
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  8. KL7AJ

    KL7AJ Senior Member

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    Might also look at a Gilbert cell. These are true multipliers, but they have limited dynamic range, so need careful signal conditioning beforehand.

    Gilbert cells can be made with six transistors.

    eric
     
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  9. kachung30

    kachung30 Thread Starter New Member

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    The op-amp must be used uA741.and l want to know the model of diode and value of resistor.:confused:
     
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  10. beenthere

    beenthere Retired Moderator

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    As long as you are using obsolete op amps, you can use the great old National Semiconductor op amp circuits collection - www.national.com/analog/amplifiers/application_notes- select AN-31

    Just about any old diode will work. Try a 1N914. The resistance value has to do with the end point of the input voltage. As the current through the resistor has to be matched by current through the diode, a reasonable resistance will keep the current below the output current limit of the op amp at the upper range of the input voltage.

    Be aware that the log response of your circuit is temperature dependent. For analog computation to have any repeatability, you will need to use something like the op amp collection circuit with temperature compensation. Notice there is an analog multiplier circuit in there, too.
     
    Last edited: Feb 4, 2010
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  11. Ron H

    Ron H AAC Fanatic!

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  12. supratim

    supratim New Member

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    If u still have the problem, u may use AD538 which is an analog computation unit. U can also try MPY534 or MPY634. These are multipliers.
     
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