Common Emitter Amplifier

@LvW

More than that - it is simply false because re is NOT a resistive element. It is the inverse of gm and, thus, has the unit V/A. That`s all.

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OK, my teacher said me that transconductance (gm) means how much anything allows to pass anything then ofcourse its inverse should be resistance that how much anything blocks anything to get pass it through. but you are saying inverse of transconductance(gm) can't be resistance(re).

Eshal said:

@LvW
OK, my teacher said me that transconductance (gm) means how much anything allows to pass anything then ofcourse its inverse should be resistance that how much anything blocks anything to get pass it through. but you are saying inverse of transconductance(gm) can't be resistance(re).

Click to expand...

Eshal - I know what you mean, however, I am afraid you are mixing "conductance" with "transconductance", do you?
A two-pole called "resistor R", of course has an inverse quantity which is called "conductance G". This applies also to differential values r and g, respectively.

However, the "transconductance gm" of a four-pole device (note the subscript "m") is something else. It is NOT the inverse of a resistive element (with two nodes).
Rather, it is a differential ratio dI/dV - and it is important to realize that the current I goes NOT through the path with the voltage V. This is because I goes through the output node and the voltage V is the voltage across the input nodes. Do you see the difference? Therefore, we add the subscript "m" and speak about the "transfer-conductance" (in short form: transconductance).
Hence, it is not a resistive path between input and output. Therefore, it is a misconception to use a resistive symbol re for 1/gm.

what a great explanation you provide. :-*