Output Resistance

Output resistance is an indication of a source's ability to drive a load impedance. An ideal voltage source has zero output resistance, and an ideal current source has infinite output resistance. To find the output resistance of the CE amplifier, we ground the input and drive the output. From the circuit in Fig. 2.7, the output resistance is then R_C||R_TC, where R_TC is the resistance looking into the collector. Since, at this time, we are considering the collector to be modeled as an ideal current source, the output resistance of the CE amp is then simply R_C.