3.8 Discrete Active Component Equivalent Noise Sources

Discrete active components such as bipolar and field effect transistors can be characterized by voltage and current noise spectral densities and in the same way as opamps. The input-equivalent noise sources of bipolar junction transistors are the shot noise of the base current and the thermal noise of the effective base and emitter resistance. These are given by:

I _b and I _e are the transistor base and emitter currents, and r _b, r _e are the base spreading resistance and emitter small-signal resistance. The transistor current gain is represented by ?.

For field effect devices the equivalent noise densities are:

Here g _m is the mutual conductance of the FET, I _g is the gate leakage current, and C _gs is the input capacitance of the FET. The operating radian frequency is ? = 2 ?f.