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Attn: John Popelish--what results from your tanh circuit?

Discussion in 'Electronic Design' started by The Phantom, May 19, 2005.

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  1. The Phantom

    The Phantom Guest

    Tell us what happened.
     
  2. Guest

    Setting up the tanh amplifier with a maximum voltage gain of 5 was a
    failure, because the output saturated at 7 volts and I was trying for a
    +- 10 volt peak capability. I had to go below about a peak gain of 3
    before the output exceeded 10 voltes before saturating, and at that
    compression factor, the view wasn't worth the climb.

    My latest version has a peak tanh voltage gain of 3.75 and parallels
    the tanh function with a linear gain of 1/4, for a peak voltage gain of
    4 (to give me two extra effective A/D bits for small signals) so that
    once the tanh output saturates around +-9 volts, the gain falls to no
    lower than 1/4, effectively throwing away 2 bits of resolution for the
    largest signals. This looks quite good. This is a better
    approximation of a sort of symmetrical reciprocal gainfunction that
    would provide a constant relative resolution over the broadest voltage
    range.

    I will post some graphs of calculated versus test data and the A/D
    resolution (both absolute and relative) on abse, tonight.
     
  3. Phil Hobbs

    Phil Hobbs Guest

    Came in late because I've been in a wheat field in northern France for a
    week. Diff amps can have the effects of extrinsic emitter resistance
    taken out by applying a little bit of positive feedback to the
    bases--but it's hard to adjust unless you have some definite null
    indication. This turns out to be very useful in laser noise cancellers,
    where it can get you another 20-30 dB SNR at high photocurrents.

    There's a picture at
    http://users.bestweb.net/~hobbs/canceller/noisecan.pdf, top of P. 912.

    Cheers,

    Phil Hobbs
     
  4. Not really applicable to my circuit, I think, but a good reference
    paper. Thanks.
     
  5. Jim Thompson

    Jim Thompson Guest

    Finally had time to add linear asymptotes, all nicely temperature
    compensated....

    Newsgroups: alt.binaries.schematics.electronic
    Subject: Follow-up - TANH Compressor with Linear Asymptotes -
    TANH-Compressor.pdf
    Message-ID: <>

    ...Jim Thompson
     
  6. I have redrawn and simulated your circuit and think I, at least
    superficially, understand what you are doing. I think your thermistor
    does all the temperature canceling. It looks like Q3 and Q4
    contribute no temperature sensitive effect, because their emitter
    currents are controlled in a high gain feedback loop. If this is
    right, I assume I could replace Q3 and Q4 and their opamps with a
    linear addition of the input to a subtracter that is used to combine
    the currents from Q1 and Q2 and still have a compensated design.

    If that is correct, I could parallel collector currents from several
    separately compensated pairs like Q1 and Q2, but run at different
    currents and with different divider gains at the front end, to
    generate various arbitrary but temperature compensated transfer functions.

    In fact, I think I could do pretty well eliminate the U2 opamp and
    connect a compensated (but, perhaps lower impedance version of the)
    signal divider directly to Q1. But the position of use negative
    tempco thermistors would have to move to the input side of the divider.
     
  7. Jim Thompson

    Jim Thompson Guest

    Provided each has its own TC'd divider.
    Negative tempco thermistors aren't very linear, making good
    compensation nasty to attain. The QTI PTC thermistor I used is pretty
    linear, as is the TC of the diff-pair.

    ...Jim Thompson
     
  8. Understood. I need fair correction over a fairly narrow range.
    Perhaps 15 to 30 C. I have also arrived at a better understanding of
    the actual transfer function I want to try for. It is essentially
    linear over the input voltage range of +-.1 volt, and has an
    incremental gain inverse to the amplitude (a 1% change of the input
    produces a 20 mV change in the output) over about .2 to 10 volts and
    -.2 to -10 volts. Combining 2 or 3 tanh functions and a linear
    component, I think I can come very close to this response.

    I have been using the two differential amplifiers in the LM13700 as my
    tanh generators, because I can temperature control the chip with the
    two darlington devices also on the chip and the two collector currents
    are already subtracted, but with your (also suggested by Ban)
    compensation scheme, I can simply subtract paralleled differential
    collector current pairs and add the linear component, though I do also
    have to come up with a few stable current sources. Especially handy
    if I need 3 pairs, instead of 2.
     
  9. Jim Thompson

    Jim Thompson Guest

    One other thought: Eliminating OpAmps may get you into base current
    sensitivities, screwing your TC and linearity.

    ...Jim Thompson
     
  10. Guest

    But they do have large tempcos, so I can add a series and parallel
    resistor to program one to a wide range of tempcos over some
    temperature range. The narrower the range, the better the fit. For
    instance, I could use one of these:
    http://www.panasonic.com/industrial/components/pdf/ARG0000CE2.pdf
    say the 10,000 ohm ERTD2FHL103S @ $1.06 from Digikey, parallel it with
    9.1 k and series that pair with 24 k and achieve a resistor that has
    about 29 k at 25 C and has about the right negative temperature
    coefficient over a 0 to 50 C range, to act as the input side of a
    divider with a much lower ground resistance for my differential
    amplifier. If the grounded resistor was actually a string of low
    resistance values, I have a series of different divided ratio signals
    all with the about the same compensation, I think.
     
  11. Got it. That is where the "perhaps lower impedance version of the
    signal divider" came from.

    High beta transistors don't hurt, either.
     
  12. Jim Thompson

    Jim Thompson Guest

    I think you can create a single linear temperature compensation, then
    scale it with additional dividers.

    ...Jim Thompson
     
  13. Now for my next branch of thought:

    What is the expression for the transfer function of a differential
    amplifier with emitter degeneration resistors? Does this take me into
    the realm of the Lambert W or is it still some form of the hyperbolic
    tangent?

    I am suspecting that my best use of two differential stages in
    parallel with a small fixed gain to produce my desired transfer
    function will be most efficient with one undegenerated stage (high
    gain, low saturation current) and one with degeneration (lower gain
    and higher saturation current) but I am having trouble expressing this
    in Mathcad to have it show me what the optimum combination looks like.
     
  14. Jim Thompson

    Jim Thompson Guest

    I think it takes you into "Lambert W" land.
    Did you not note that the extra differential pair I added is pure
    linear, because of the feedback from the emitters.

    In Mathcad I'd sum TANH + linear with various weightings until I
    achieved "optimum", whatever that might be, since it's really just
    your choice.

    ...Jim Thompson
     
  15. Jim Thompson

    Jim Thompson Guest

    On Fri, 27 May 2005 14:09:37 -0700, Jim Thompson

    [snip]
    I should have added... Design your single linear attenuator such that
    the extrapolated slope passes thru 0°K, to match diff pairs.

    ...Jim Thompson
     
  16. So I have another educational experience to look forward to.
    I have little problem setting up a figure of merit for the fit I want,
    so that Mathcad can search for the optimization I want, once I have an
    expression or even an implicit description of the functions involved.

    I have already done quite a few different optimizations for two tanh
    functions (both in parallel and in series) and also in parallel with a
    linear gain. I also looked into adding positive feedback around one
    or both tanh functions. (That makes my curve fit considerably better,
    if I could put up with the tolerance exaggerations. I had a Duh!
    moment when I realized that if negative feedback improves linearity,
    positive feedback enhances nonlinearity.) So I have a pretty good idea
    what I can do with those cases.

    Now I want to explore how adding emitter resistors to one or both
    differential pairs alters the optimizations. (My hunch is that one
    tanh function will be best with nonlinear enhancement [positive
    feedback] and one will be best with some linear enhancement and input
    range expansion [emitter degeneration]. But I am having difficulty
    producing an expression for this transfer function.

    I am pretty sure I once knew how to do this. But I am having a senior
    moment about it, now.
     
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