A single-stage amplifier circuit is shown in Figure 15. The amplifier consists of a transistor, the circuit for matching the source termination and the transistor, and that for matching the load termination and the transistor. The biasing circuit is ignored for brevity. The source and load terminations are usually 50 . The function of the matching circuits is to provide suitable source and load conditions for the transistor such that the transistor can generate power gain for the input signal under a stable condition.
Figure 15: Small-signal high-frequency amplifier
Note that is the input reflection coefficient of the amplifier when output port
is connected to the load with a reflection coefficient of
Similarly, is the output reflection coefficient of the amplifier when input port is connected to the source with a reflection coefficient of
The amplifier can be simplified to the network shown in Figure 16. Note that the network can be represented by the flow graph in Figure 13 if S-parameter is used to model the transistor.
Figure 16: Simplified amplifier network