Microwave Ampliﬁer Design
Signal ampliﬁcation is one of the most basic and prevalent circuit functions in modern RF and microwave systems. Early microwave ampliﬁers relied on tubes, such as klystrons and traveling-wave tubes, or solid-state reﬂection ampliﬁers based on the negative resistance characteristics of tunnel or varactor diodes. However, due to the dramatic improvements and innovations in solid-state technology that have occurred since the 1970s, most RF and microwave ampliﬁers today use transistor devices such as Si BJTs, GaAs or SiGe HBTs, Si MOSFETs, GaAs MESFETs, or GaAs or GaN HEMTs [1–5]. Microwave transistor ampliﬁers are rugged, low-cost, and reliable and can be easily integrated in both hybrid and monolithic integrated circuitry. Transistor ampliﬁers can be used at frequencies in excess of 100 GHz in a wide range of applications requiring small size, low noise ﬁgure, broad bandwidth, and medium to high power capacity. Although microwave tubes are still useful for very high power and/or very high frequency applications, continuing improvement in the performance of microwave transistors is steadily reducing the need for microwave tubes.
Our discussion of transistor ampliﬁer design will primarily rely on the terminal characteristics of the transistor, as represented by either scattering parameters or one of the equivalent circuit models introduced in the previous chapter. We will begin with some general deﬁnitions of two-port power gains that are useful for ampliﬁer design and then discuss the subject of stability. These results will then be applied to single-stage transistor ampliﬁers, including designs for maximum gain, speciﬁed gain, and low noise ﬁgure. Broadband balanced and distributed ampliﬁers are discussed in Section 12.4. We conclude with a brief treatment of transistor power ampliﬁers.
TWO-PORT POWER GAINS
In this section we develop several expressions for the gain and stability of a general twoport ampliﬁer circuit in terms of the scattering parameters of the transistor. These results 558
12.1 Two-Port Power Gains
( Z0 )
A two-port network with arbitrary source and load impedances.
will be used in the following sections for ampliﬁer design and in Chapter 13 for oscillator design.
Definition of Two-Port Power Gains
Consider an arbitrary two-port network, characterized by its scattering matrix [ S ], connected to source and load impedances Z S and Z L , respectively, as shown in Figure 12.1. We will derive expressions for three types of power gain in terms of the scattering parameters of the two-port network and the reﬂection coefﬁcients, S and L , of the source and load.
Power gain = G = PL/Pin is the ratio of power dissipated in the load Z L to the power delivered to the input of the two-port network. This gain is independent of Z S , although the characteristics of some active devices may be dependent on Z S . Available power gain = G A = Pavn/Pavs is the ratio of the power available from the two-port network to the power available from the source. This assumes conjugate matching of both the source and the load, and depends on Z S , but not Z L . Transducer power gain = G T = PL/Pavs is the ratio of the power delivered to the load to the power available from the source. This depends on both Z S and Z L .
These deﬁnitions differ primarily in the way the source and load are matched to the twoport device; if the input and output are both conjugately matched to the two-port device, then the gain is maximized and G = G A = G T .
With reference to Figure 12.1, the reﬂection coefﬁcient seen looking toward the load is L
Z L − Z0
Z L + Z0
while the reﬂection coefﬁcient seen looking toward the source is S
Z S − Z0