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.
- If the FETs/FET is fried still there is a 1 kOhm resistance between VDD and GND.
- 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.