I'm trying to analyze a trivial common-source amplifier based on an IRFP9240 power P-channel MOSFET (datasheet at
The circuit is simply this:
-24V | | .-. | | Load | | 100R '-' | | ||-+ ||-> IRFP9240 Vin ---||-+ | | === GND
Now, I know that voltage gain = gm * Rd. But how do I find gm? I'm interested in the condition where the MOSFET will be operating in its linear region, with Vgs close to Vt; Vds around 1V, Id around 200mA.
The datasheet specifies forward transconductance of 4.2S, but that's at 7.2A and 50V, in the saturation region. The transfer characteristic curves only go down to 400mA, and anyway they're at 50V also. Similarly, the output characteristic curves don't show the region I'm interested in.
Given the available data, how can I determine the transconductance at the operating point of interest?
Thanks for any help!