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Log amplifier schematics

Discussion in 'Electronic Design' started by Sergey Kubushin, Sep 9, 2003.

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  1. Hi, everybody!

    I'm looking for a schematic for a logarythmic amplifier. It should work in
    audio range (20 Hz - 20 KHz) and has 50-80 dB range. It doesn't have to be
    very precise, it's intended use is audio amplifier's output indication with
    a linear neon tube, but it must be reasonably stable. The simplistic opamp
    with two germanium diodes won't do. Monolythic IC is preferrable if its
    cheap enough (the cheapest I can find from AD are $10 apiece that is kinda
    high) and is not packed into a case unvisible without a microscope (SO-xx
    are fine and preferrable). Relatively simple opamp schematic will also do.

    All the information is highly appreciated.

    P.S. I don't have "The art of electronics" on my shelf here in the US and
    I'm not going to buy it for a one schematic. I do have my Russian edition
    that I bought when living out there but unfortunately its still collecting
    dust out there, I haven't brought all my treasures to the US so far...
  2. Robert Baer

    Robert Baer Guest

    Back in the old daze of CP/M, Solid State Music made a number of these
    under the Exar brand; the part numbers are (and sitll appear to be)
  3. Ban

    Ban Guest

    this is a nice and temperature compensated log amp working in the first
    quadrant, so both Vin and Vref have to be positive.

    Vout = -Ut(R3+R4)/R4 ln((R2*Vin)/(R1*Vref)

    +--------------->-+ +-<------------------+
    | | | |
    | Ic2| |Ic1 |
    | ||C1 10p | | |
    +--||--+ | | |
    | || | | | |
    100k | | | | C2|| |
    ___ | |\ | T2 | | T1 +--||--+
    Vino-|___|-+-|-\ | |/ \| | || |
    R1 | >-+ GND-| dual NPN|--+ | 100p | 100k
    GND-|+/ | |> <| | 33k | /| | ___
    |/ | | | | ___ | /-|-+-|___|-oVref
    | +-----+----)--|___|-+-< | R2
    | | R5 \+|-GND
    | ___ | \|
    | R3 |
    | .-.
    o | |+0,3%/K
    Vout | |R4
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    ciao Ban
  4. Robert Baer

    Robert Baer Guest

    ...and National has an IC which is a very well matched pair that
    should be an excellent choice...
  5. Thanks a lot. Can you tell me that part number?
  6. Ben Bradley

    Ben Bradley Guest

    He surely means the LM394, but it's not cheap ($5 or so at
    digikey), and surely overkill for your app. I'd use the 'matched'
    transistors in a LM3046/CA3046 transistor array, and use the savings
    to buy lunch.
  7. Robert Baer

    Robert Baer Guest

    Except that they have an excessive bandwidth and have a poor log
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