2.2. Fundamentals of Signal Amplification: The Linear Circuit
The most fundamental property of a useful electronic voltage amplification device is that it possess a transconductance that leads to the possibility of voltage gain. Transconductance is defined as the ratio of the signal (ac, incremental) current out, iout ≡ δiOUT, and the applied input signal voltage, vin ≡ δvIN. That is, transconductance gm is
Equation 2.3
For the BJT, iOUT ≡ iC and vIN ≡ vBE, while for the NMOS, iOUT ≡ iD and vIN ≡ vGS. Thus, (2.1) and (2.2) can be used for the BJT and MOSFET, respectively, to obtain an expression for gm. The results are
Equation 2.4
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