BJT—MOSFET transconductance comparisons

Abstract

Through normalization it becomes possible to construct curves of considerable generality for comparing the transconductance properties of BJT's and MOSFET's. Three comparisons are given. The first presents transconductance as a function of output current; the second, transconductance divided by output current as a function of input voltage; and the third, transconductance as a function of input voltage. The BJT advantage increases rapidly with output current and (or) input voltage. The curves approach each other under extremely low-level conditions, but appreciable deviations from the simple theory upon which the present analysis is based also occur in the low-level regime--especially deviations connected with subthreshold conduction in the MOSFET, as well as excess conduction near but above threshold. For these reasons, the moderate-level portions of the simple-theory comparisons are the most meaningful. Recently developed subthreshold analysis is also used in the second comparison.