best way to generate Gate-Drain or Gate-Source short in electret capsule JFET? - diyAudio
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Old 24th September 2002, 03:19 PM   #1
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Default best way to generate Gate-Drain or Gate-Source short in electret capsule JFET?

I want to deliberately kill a JFET, preferably without using the gate and hoping to obtain a short between gate and one of the other electrodes.

Reason: electret microphones always have a built-in FET, which can be used in common source or (after some surgery in the case of Panasonic capsules) common drain mode. One mike electrode is always connected to the gate and there is no way to connect to this electrode directly. In the case of the Sennheiser capsules, there is also a protection diode from source to gate.

I suspect much better distortion performance can be obtained from the capsule if a FET Op-amp was used as a follower. Or one could think about current mode, i.e. using a current to voltage configuration.

Anyway, the prerequisite is getting a direct connection to the second electrode (the first is the case). I guess trying to lift the small pcb will kill the internal connectors, so the most elegant way would be to make the FET fail with a short.

Anybody know if they fail as open circuits or shorts between the channel and the gate if too high a voltage is applied across source and drain?

I suspect applying a voltage pulse to the gate through the membranes (which do form a cap) is not a good idea?

Eric
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Old 25th September 2002, 05:00 PM   #2
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Default Electretrickery

I've opened duff ones in the past to have a look, but i have not tried what you suggest.
Cheap electrets are real cheap so why don't you just open a couple at the risk of destroying them.
Please report back if you get good results.
IME ther are 2 terminal and 3 terminal electrets.

Eric.
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Old 27th September 2002, 08:07 AM   #3
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Yes, the better ones are 3-terminal, so at least you can configure them as a source follower. Siegfried Linkwitz has something on his website (www.linkwitzlabs.com) on how to cut a trace in Panasonic capsules to make them 3-terminal.

Greetings,

Eric
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