Output resistance of the common-emitter amplifier

Contrary to a long-held belief, the output resistance of the common-emitter (CE) amplifier is not independent of its signal-source resistance. Although it is widely accepted that the output resistance of the CE amplifier is approximately equal to the ratio of the Early voltage to the collector current V-A/I-C, independent of the resistance of the signal source, it is shown herein that this belief is incorrect. The hybrid Pi model gives the output impedance ratio for high-impedance versus low-impedance sources as being equal to unity (i.e., no effect due to the source impedance). On the other hand, the Early model of the bipolar transistor gives a ratio equal to precisely 2. Lastly, the charge injection theory of the bipolar transistor gives a ratio equal to 1.51 in excellent agreement with the experimental value of 1.517.