To form a transistor amplifier it is only necessary to connect an external load and signal source as indicated in fig. 1 and to bias the transistor properly.
Consider the two-port network of CE amplifier. RS is the source resistance and ZL is the load impedence h-parameters are assumed to be constant over the operating range. The ac equivalent circuit is shown in fig. 2. (Phasor notations are used assuming sinusoidal voltage input). The quantities of interest are the current gain, input impedence, voltage gain, and output impedence.
For the transistor amplifier stage, Ai is defined as the ratio of output to input currents.
The impedence looking into the amplifier input terminals ( 1,1' ) is the input impedence Zi
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