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Limiter

Discussion in 'Electronic Design' started by Tim Williams, Mar 16, 2013.

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  1. Tim Williams

    Tim Williams Guest

    Nobody really makes a Magic-Joerg-Block where the output saturation is
    adjustable (and preferably, accurate), so naturally, I made my own.
    http://www.seventransistorlabs.com/Images/Limiter.png
    Threshold between inputs is, of course, exponential (tanh more precisely,
    I suppose). The comparator output crosses threshold over 10s of mV
    difference, good enough for most purposes. Put an op-amp or comparator in
    if you'd like it sharper?

    I find 31mV offset with fresh-out-of-the-bag transistors (say, that's
    about a V_TH). So unless you get lucky with a monolithic matched set,
    you'll need the offset trimmer, or a fancy op-amp setup, to do better.

    Tim
     
  2. Tim Williams

    Tim Williams Guest

    When I need a fast logic function, and TinyLogic is unsuitable, I favor
    discrete CML or ECL style over RTL/DTL/TTL style saturating stuff. :)
    Of course one of the nice things about CML/ECL is the analog nature.
    Hence circuits like this...
    I'm not doing radio here (as you might guess from the resistors, it'll go
    to... oh, probably a few MHz, which isn't bad, but a long way from FM
    BCB), so AM-PM isn't a problem.

    To which kind of FET limiter are you referring? FETs will suck if used
    directly in a circuit like this, of course. The cascade-of-amps is
    time-tested, although if your FETs have a lot of Cds at low voltage,
    you'll see a lot of sloppiness.

    I may investigate a log amp or something like that one day. Perhaps build
    the better part of a spectrum analyzer. Those damn things sure jacked up
    in price the last couple years.

    Tim
     
  3. Bill Sloman

    Bill Sloman Guest

    It looks as if a lot of your +90ppm/C temperature drift is in the
    temperature dependent current gain of the MMBTH81. You could get more
    current gain out of a PNP/NPN complementary Darlington, but IIRR they
    have a nasty tendency to oscillate. If you ran one with just 0.5mA in
    the MMVTH81 it would be slower, but still fairly quick (if it didn't
    oscillate).

    The green LED version is unexpectedly horrible - did the MMVTH81 still
    have enough Vce left in your test circuit?
     
  4. Nico Coesel

    Nico Coesel Guest

    What is the purpose? You'll just add a lot of distortion this way.
     
  5. Joerg

    Joerg Guest


    Sure they do. For example in the audio world:

    http://en.audiofanzine.com/guitar-combo-amplifier/crate/GT65/user_reviews/r.36076.html

    Quote "Setting the tones, bass mid and treble, volume for clean and
    crunch (+ saturation) of a single knob Tone for channel distortion with
    adjustable saturation and volume".

    In the ham radio world you have clippers. I made a transistor clipper
    sometime in the late 70's, after I got my license.

    [...]
     
  6. Am 16.03.2013 16:04, schrieb John Larkin:

    Or TI OPA698/699. Work nicely here in a slope amplifier in
    the Oliver Collins style.

    On some OpAmps one can clamp the compensation pin. That limits the drive
    to the output followers.

    regards, Gerhard
     
  7. Jamie

    Jamie Guest

    Lets see now. I just quickly picked up the calculator and it appears the
    current sink for the high side NPN maybe over driving it a bit..

    At a 50 ohm load I would say that is fine however, at a short circuit
    output, it looks like it forcing the high side output (NPN) to sit
    somewhere in the 200ma+ region.

    Maybe you don't plan on output being low Z or shorted but I would be
    concerned with the livelihood of the NPN. who knows..

    Jamie
     
  8. Tim Williams

    Tim Williams Guest

    Ten bucks, you're kidding right?

    Tim
     
  9. Bill Sloman

    Bill Sloman Guest

    Why? 15ppm/C resistors are widely available, if not quite as cheaply
    as 100ppm/C parts.
    Sure, but you've cancelled that out.
     
  10. Bill Sloman

    Bill Sloman Guest

    It might have been cheaper to decouple the power supplies of each of
    the cascaded amplifier stages with a properly damped non-wound
    inductor/resistor/capacitor networks. Most amplifiers have very little
    common mode rejection at high frequencies, and an output stage with a
    releatively high output current can feed quite a lot of signal back
    into the input stage of the first amplifier in the chain through the
    power supply pins if you aren't very careful.

    Regular decoupling capacitors crap out about 300MHz, so you can end up
    having to by-pass them with something less inductive. I didn;t have to
    go to the trouble of buying some 1nF porcelain capacitors, but I
    wasn't looking for your kind of GB product.
     
  11. Bill Sloman

    Bill Sloman Guest

    Precisely enough that your crummy emitter resistor or the temperature
    dependence of the PNP base current are enough to explain the residual
    tempco.

    If you'd made them a bit better, you might have a better idea of how
    precisely the LED cancells the PNP's Vbe.
    And it would still glow in the dark.
     
  12. Bill Sloman

    Bill Sloman Guest

    A little difficult. I'm in Christchurch, New Zealand, at the moment,
    and my electronic gear - such as it is - is out of reach.

    You were interested eniugh to measure the temperature stability of the
    circuits you'd put together. Why weren't you interested enough to work
    out where the temperature variation was coming from?
     
  13. Bill Sloman

    Bill Sloman Guest

    It can't be perfect, but it could have been better than you achieved
    with fairly minor modifications, none of which demand pages of algebra
    or a Spice sim - though it's often quicker (if less reliable) to run a
    Spice sim than it is to solder in the extra/differemt components.
    The obvious technique is to make it a "ring of two" circuit, and use
    the LED voltage to set up a current source to bias the LED at a more
    nearly constant current. The traditional way would add three more
    transistors - two of them in a current mirror.

    <snipped the abuse>
     
  14. Bill Sloman

    Bill Sloman Guest

    But ground loops put noise on the power supply rails.

    You called it oscillation, rather than electromagnetic interference,
    so the implication is that the extra feedback paths through the supply
    rails (including the ground connection) were creating the instability.
     
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