• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

IRFBC20 as source follower drivers for EL34s in AB1 PP

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#21
sawreyrw said:
Eli,

Where did you get the notion that the transconductance of a MOSFET is equal to Id?

Rick
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The approximation is crude. However, MOSFET gfs definitely increases and decreases in step with ID. Other folks have posted about the ID numerical relationship and I've fallen into step.

BTW, the same sort of effects are observed with tubes. Max. gm is obtained at high IB.
 
#22
As a first order aproximation, FET small signal transconductance is proportional to sqrt(Id). The proportionality constant is linearily related to the transconductance coefficient K (units are A/V²), the equation becoming then:
Yfs = sqrt(2K*Id)

This is also obvious from the transfer characteristic, which is a parabola, a quadratic function.

- Klaus
 
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