Analog Electronics with LabVIEW®

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, i_out ≡ δi_OUT, and the applied input signal voltage, v_in ≡ δv_IN. That is, transconductance g_m is

Equation 2.3

For the BJT, i_OUT ≡ i_C and v_IN ≡ v_BE, while for the NMOS, i_OUT ≡ i_D and v_IN ≡ v_GS. Thus, (2.1) and (2.2) can be used for the BJT and MOSFET, respectively, to obtain an expression for g_m. The results are

Equation 2.4