Common-Emitter Output Impedance

Output impedance of common-emitter (CE) amplifier.In this post, a circuit equivalent model of the output impedance of a common-emitter amplifier will be developed. To begin, a low-frequency non-reactive model of the output resistance of a common-emitter (CE) amplifier will be solved. Proceeding models will include the capacitive elements of a CE amplifier. Separating the High-Frequency (HF) model into two distinct cases will allow for circuit analysis that is reasonable to perform by hand. The first HF model will be for when the base is ac-grounded and the emitter see a degeneration resistor. A second HF model is developed with a grounded emitter and with source resistance to the input (base) terminal. Concluding the post with a spice simulation to compare/validate the circuit equivalent models developed. The desired outcome of this post is to gain an intuition into what “it looks like” electrically speaking into the output of a CE amplifier.


– Review of output resistance/impedance
– Low frequency model
– High Frequency Model – Grounded Base
– High Frequency Model – Grounded Emitter
– Spice Simulation

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