High-Frequency Amplifiers

Abstract

Today, in the high- and intermediate-frequency assemblies of telecommunication systems, amplifiers composed of discrete transistors are still used in addition to modern integrated amplifiers. This is particularly the case in high-frequency power amplifiers employed in transmitters. In low-frequency assemblies, on the other hand, only integrated amplifiers are used. The use of discrete transistors is due to the status quo of semiconductor technology. The development of new semiconductor processes with higher transit frequencies is soon followed by the production of discrete transistors, but the production of integrated circuits on the basis of a new process does not usually occur until some years later. Furthermore, the production of discrete transistors with particularly high transit frequencies often makes use of materials or processes which are not (or not yet) suitable for the production of integrated circuits in the scope of production engineering or for economic reasons. The high growth rate in radio communication systems has, however, boosted the development of semiconductor processes for high-frequency applications. Integrated circuits on the basis of compound semiconductors such as gallium-arsenide (GaAs) or silicon-germanium (SiGe) can be used up to the GHz range. For applications up to approximately 3 GHz bipolar transistors are mainly used, which, in the case of GaAs or SiGe designs, are known as hetero-junction bipolar transistors (HBT). Above 3 GHz, gallium-arsenide junction FETs or metal-semiconductor field effect transistors (MESFETs) are used.¹ The transit frequencies range between 50 . . . 100 GHz.

The construction of an HBT corresponds to that of a conventional bipolar transistor. Here, however, different material compositions are used for the base and emitter regions in order to enhance the current gain at high frequencies. The construction of a MESFET is shown in Fig. 3.26b on page 196.