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Old 13th December 2003, 10:15 PM   #1
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Default MOSFET Questions

I have read that MOSFETs will pass current from drain to source in either direction. Is there a voltage drop from D to S? Also is the gate voltage to Drain-to-Source impedance relationship linear or exponential? I see on datasheets gate voltage vs drain to source amps but I need to know exact ohm values for given gate voltages.
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Old 14th December 2003, 07:18 AM   #2
azira is offline azira  United States
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Location: Near Seattle
That's partially true. Technically there is no difference between the Drain and Source of a MOSFET, it's symetrical and can be used either direction (whereas BJTs are not symetrical). But there is also a base "terminal" and if there is a Vbase-source, then the threshold voltage and some other parameters change. For this reason, the base is usually tied to a pin labeled as the source. After that happens, the source terminal is fixed.

As far as resistance, there is an apparent Rds which can be viewed as (Vds / Ids). But the equation for Rds is not a linear property. Since this is tied to the transconductance, you can't figure out Rds of a FET without measuring Gm.
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