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Protecting inputs of transistors of a low noise amp

Discussion in 'Electronic Design' started by [email protected], Aug 4, 2006.

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  1. Guest

    Hi
    I have designed a low noise trransimpedance amplifier which has a GaAs
    FET as an input transistor. I want to protect the input (gate) of these
    fets from high voltage transients due to a RF source in proximity.
    If i add Transient voltage suppressor/ zener - is that going to
    deteorate my noise performance. Or can there be leekage issues in the
    zener which can suck my signal current away from the FETs.
     
  2. Watch out for onionization, too.
    Probably not, since your FETs probably don't draw appreciable input
    current.
     
  3. John Larkin

    John Larkin Guest

    That was a very shallot thing to say.

    John
     
  4. John Larkin

    John Larkin Guest

    Why not regular diodes?

    How much capacitance can you stand? How much leakage?

    John
     
  5. Guest

    Why not regular diodes?
    C'ld use if they are more suitable!
    No more than couple of pFs. The leakage has to be less than few nAs.
     
  6. Joerg

    Joerg Guest

    What would be unsuitable about diodes?

    If you run them the against the rails and the bias voltage is far enough
    away from there diodes should be able to fulfill that.
     
  7. Guest

  8. In message <>, dated Fri, 4
    Yes, I'm a real rapscallion, aren't I?
     
  9. John Larkin

    John Larkin Guest

    Or just back-to-back diodes to ground, if the TIA input is nominally
    at zero volts.

    John
     
  10. Joerg

    Joerg Guest

    I'd look for lower capacitance versions and SMT. Even the old standby,
    the BAV99, is around 1.5pF. It is one of my favorites for such purposes,
    mostly because of low cost.
     
  11. Joerg

    Joerg Guest

    Hello John,
    That is often done in radio applications but leads to poor 3rd order
    intercept, harmonics and all kinds of nasties. I'd pre-bias them in that
    case.
     
  12. Mike Monett

    Mike Monett Guest

    But the op is not using a 10 element beam on 20 meters - it's a TIA.

    Wouldn't pre-bias increase the leakage currents and reduce the SNR?

    Regards,

    Mike Monett

    Antiviral, Antibacterial Silver Solution:
    http://silversol.freewebpage.org/index.htm
    SPICE Analysis of Crystal Oscillators:
    http://silversol.freewebpage.org/spice/xtal/clapp.htm
    Noise-Rejecting Wideband Sampler:
    http://www3.sympatico.ca/add.automation/sampler/intro.htm
     
  13. John Larkin

    John Larkin Guest

    Oh, that was a real pearl.

    John
     
  14. Joerg

    Joerg Guest

    Hello Mike,

    True but he wrote "high voltage transients due to a RF source in
    proximity". That can modulate in and make a pudding out of the signal to
    be measured. Unless it's a pulsed source and he can measure in between
    pulses.

    Very little, at least at room temperature. For the BAV99 it's a few nA.
    The nice thing is that with a few volts reverse bias the typical
    capacitance is around 0.5pF. Not bad for such a low cost part.
     
  15. Mike Monett

    Mike Monett Guest

    OK, I just got the "high voltage transients" part and skipped the RF
    source. But that doesn't say much about what kind of transient he wants to
    protect against.
    Isn't that about the same order as the signal he is detecting? I thought
    TIA used gigohm feedback resistors to get some kind of useful signal from
    very small currents from a photodiode. He mentioned

    "Or can there be leekage issues in the
    zener which can suck my signal current away from the FETs."

    So leakage noise might be an issue.
    I wonder if the input of a 74AC04 CMOS gate might be useful as a low-
    leakage clamp. The spec claims the max leakage is pretty high, say about
    100na. But everyone has seen how a floating input can hold a voltage on an
    input pin constant for a long time. This might work if the fault current is
    not too high.

    Regards,

    Mike Monett

    Antiviral, Antibacterial Silver Solution:
    http://silversol.freewebpage.org/index.htm
    SPICE Analysis of Crystal Oscillators:
    http://silversol.freewebpage.org/spice/xtal/clapp.htm
    Noise-Rejecting Wideband Sampler:
    http://www3.sympatico.ca/add.automation/sampler/intro.htm
     
  16. Bill Sloman

    Bill Sloman Guest

    That isn't a a very low leakage current. Check out the Calogic PAD5 - 5pA
    leakage - and the cheaper JPAD50 with 50pA leakage current.

    Farnell stock both.

    The PAD5 offers a worst case capacitance of 0.8pF at 5V reverse bias, while
    the JPAD50 can go up to 2.0pF.
     
  17. Joerg

    Joerg Guest

    Hello Mike,
    If it's a radar pulse then often all you can do is gate it out and
    protect against it so nothing will be fried.

    It can be but he wrote in the answer to John Larkin that it should be
    less than a few nA. So the old BAV99 might still fit the bill.
    Maybe that could be done. I don't know his circuit but maybe he could
    even re-design it for a MOSFET tetrode from the BF9xx series. Those are
    very low capacitance and often include protection diodes because their
    market segment is TV tuners which need similar protection. IIRC the
    BF998 contains diodes and those are pretty cheap.
     
  18. John Larkin wrote...
    It's been rather peasful around here
    with John Woodgate away.
     
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