Satellite Communications Payload and System

Chapter 6

Preamplifier and High-Power Amplifier

6.1 Introduction

The high-power amplifier (HPA) provides the RF power for a payload downlink. Before the signal goes to the HPA, the preamplifier boosts the signal to a level proper for input to the HPA. We may call these two together the HPA subsystem. There are two types of HPA subsystem: the traveling-wave tube amplifier(TWTA) subsystem and the solid-state power amplifier (SSPA). A subsystem named for the TWTA is not normally identified, but for the purposes of this book it must be so. The TWTA subsystem is more common than the SSPA. One of the leading global satellite manufacturers, which makes its own TWTA subsystems and SSPAs, reports that in the periods 1980–2004 and 2004–2008, the number of its TWTAs on orbit on commercial payloads was twice the number of SSPAs (Weekley and Mangus, 2004; Nicol et al., 2008).

The HPA subsystem has the following functions:

• Channel preamplification for the HPA, with the flexibility to make the downlink power independent of the uplink power over a wide range of uplink power
• Predistortion (optional) to counteract the HPA's nonlinear amplification characteristics
• High-power amplification
• Power supply for rest of the functions.

The rest of this chapter addresses the following topics:

• Section 6.2: HPA concepts and terms different from those used for other units: the HPA's nonlinearity description and specification, as well as power efficiency
• Section 6.3: Summary of the differences and similarities ...