The criteria for selecting bipolar or field-effect transistors in the microwave range depends on application. The importance of noise figure, for example, becomes apparent in the front end of a microwave satellite receiver system. In other applications, power gain and output might be more important. Obviously, gain and noise figure are both important especially in the front ends of receiver systems. For example, earth communications or receiver terminals typically use a parabolic dish antenna with a low-noise amplifier ( LNA ) at the feedpoint. When selecting devices for such applications, however, beware of seemingly self-serving transistor specification data sheets. For example, consider the noise figure specification. As you saw in Fig. 22-28, there is a strong frequency dependence regarding noise figure. Yet device data sheets often list the maximum frequency and minimum noise figure even though the two rarely coincide with one another! When selecting a device, consult the N _F-vs- F curve.

Figure 22-28: Noise figure vs frequency for several devices.

Device gain can also be specified in different ways, so caution is in order. There are at least three ways to specify gain: maximum allowable gain ( G _max), gain at optimum noise figure (GNF), and the insertion gain. The maximum obtainable gain oc- curs usually at a single frequency where the input and output impedances are conjugately matched (i.e., reactive part of impedance canceled out and resistive part transformed for maximum power transfer). The noise figure gain is an impedance-matching situation in which noise figure...