1.8 POWER GAIN

In this section, we will provide power gain definitions for a two-port network described by its S-parameters when it is connected between an arbitrary generator and load. There are basically three different definitions of the power gain of a two- port network: power gain, available power gain, and transducer power gain. They are given as follows ^[13]:

In this section, we will concentrate on the transducer power gain because it is the most practical definition. The transducer power gain provides a measure of the power gain when a supposedly existing passive matching circuit between the generator and load is replaced by an active two-port network (e.g., amplifier) to provide gain. Note that since the passive matching circuit can extract all the available power from the generator, the active two-port network must provide some additional gain in order to be useful. This gain is given by the transducer power gain ( G _T).

If the transducer gain is unity for an amplifier circuit, then the active circuit isn't needed, and the same result could be achieved by just doing complex- conjugate matching using a passive matching circuit.

To derive the transducer power gain, we use the signal flowgraph of a two-port amplifier connected between a generator and load as shown in Figure 1.27(a). In the figure, ? _S and ? _L represent the generator and load reflection coefficients, respectively. We assume unity source amplitude without loss of generality. The first step is to...