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Differential amplifier Debugging

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

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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

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. The FETs are GaAs MESFETs. 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 caps!! Please help, i am in deep trouble.
Reply to
archiees
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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.

Reply to
no_one

Did you look at the operating temperature range of your components?

Reply to
cbarn24050

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?

-- John

Reply to
John O'Flaherty

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

greetings, Vasile

Reply to
vasile

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 =^.^=

Reply to
colin

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

Reply to
no_one

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 ?

greetings, Vasile

Reply to
vasile

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