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Differential amplifier Debugging

Discussion in 'Electronic Design' started by [email protected], Aug 27, 2006.

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  1. Guest

    Hello Everyone,
    I have designed a simple differential amplifier for cryogenic (4
    Kelvin) operation. The circuit is posted on the following link:

    The resistors are metal film and all the capacitots are ceramic X7R
    type besides the 1.2pF, 10pF and 50 pF. These are high frequency
    Silicon capacitors from Vishay (HPC0402 The FETs are GaAs
    The circuit worked fine and room temperature and then i inserted it
    into the vacuum system and starting cooling. 77 kelvinn all well, 4 K
    also everything was ok for few hrs. Untill suddenly the VDD (drain
    bias) power supply went in the constant current mode. I measure the
    resistance from VDD to GND pin, it is merely 18 ohms.
    Here i have only access to the pins outside the black box, rest of it
    is in the cryogenic vacuum system which at this point i dont want to
    open. The pins on the amplifier are connected using a magnet wire which
    has almost 5-10 ohms of resitance between the amplifier and the
    connector. I have measured resistance between all the terminals
    available to me on the connector:
    VDD to GND = 18 ohms (could be just due to the connecting magnet wire)
    VSS to GND = 1.384 kohms
    VDD to Out(+) = 403 ohms
    VDD to Out(-) = 382 ohms
    I am simply having tough time determining what can cause that.
    1. If the FETs/FET is fried still there is a 1 kOhm resistance between
    VDD and GND.
    2. Could any of my capacitor is short circuited, perhaps one of the
    bypass capacitor on the VDD line. But i haven't used any electrolitic
    Please help, i am in deep trouble.
  2. no_one

    no_one Guest

    What kind of solder did you use? If there was tin or zinc in any of the
    components you could get "whisker" growth that would cause a short circuit.
    This happens in space applications unless the proper care is used.
  3. Guest

    Did you look at the operating temperature range of your components?
  4. Even if you could figure out for sure what the problem was, you'd still
    have to open it to fix it, wouldn't you? The only other option I can
    think of is to call its bluff and see how much current it will carry.
    Teach it a lesson. You might burn away a short.
    Could there have been an almost-short in the circuit board that closed
    up because of physical contraction?
  5. vasile

    vasile Guest

    VDD to GND could be 18 ohm only if your connector is short circuited
    (VDD to GND) or one of the decoupling capacitors crashed...

  6. colin

    colin Guest

    I was going to suggest that, if it doesnt start working again and youve
    therefore got nothing to lose just take off the current limit,
    It might open up the short,
    If your lucky it may do it gently,
    at worst if it does it violently and blows it off the board then at least
    you know wich component it is.
    It might heat it up so that it works again but this probably isnt any good.

    ive burnt out a short 5v->gnd on a fualty pcb before now, took quite a lot
    of amps.

    Colin =^.^=
  7. no_one

    no_one Guest

    you also might contaminate your chamber with the by products of such a
    method. Take care.
  8. vasile

    vasile Guest

    I have two questions (not expecting to get answers):

    1. which is the differential input impedance in this amplifier ?
    2. based on answer to 1. why do you need to be cryogenic colled ?

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