Extended analysis of input impedance control of an NMOS-transistor with an inductive series feedback

An accurate control of the input impedance can be obtained by an inductive series feedback of a transconductance. Legitimately or not the internal capacitive shunt feedback is traditionally ignored. An extended investigation was made to determine the effect of the capacitive shunt feedback as well as the effects of other internal parasitics. To keep the model fairly simple the inductor was simulated as an inductance with a series resistance with a Q-value of three. Simulation results shows that for a required input resistance and operating frequency a maximum transconductance of the transistor can be found. A rough approximation of the input resistance was found, which is valid for low frequencies and for small transconductances. The major conclusion of this paper is that the shunt feedback cannot be ignored and that good design methods therefore require a more extended analysis