2.2: Linear Amplifier Design Strategies

Last updated
Save as PDF

Page ID: 46024

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

Linear amplifier design requires that the transistor(s) be biased in a high-gain region and that input and output matching networks be used to provide good power transfer at the input and output of the transistor stages. This circuit arrangement is shown in Figure 2.3.1. The DC bias control circuit is fairly standard; it does not involve any microwave constraints. The lowpass filters (in the bias circuits) can have one of several forms and are often integrated into the input and output matching networks. Synthesis of the input and output matching networks (and occasionally a feedback network required for stability and broadband operation) is the primary objective of any amplifier design.

RF transistors used to amplify small signals should have high maximum available gain and low noise characteristics. For transistors used in transmitters, where the efficient generation of power is critical, it is important to linear amplifier design that the transistor characteristics be close to linear in the central region of the output current-voltage characteristics so that distortion is minimized. The ultimate limit on output power is determined by the breakdown voltage at high drain-source voltages and also by the maximum current density that can be supported. Finally, for efficient amplification of large signals, the knee voltage (where the current-voltage curves bend over and starts to flatten) should be low.

Manufacturers of discrete transistors and amplifier modules provide substantial information, including \(S\) parameters and, in some cases, reference designs. An extract from the datasheet of a pHEMT transistor is shown in Figure 2.3.2. The intended application is provided and the device structure has been optimized for the application.

Design examples presented in the next few sections will use the pHEMT transistor documented in Figure 2.3.2. This discrete transistor is described as a low-noise, high-frequency, packaged pHEMT that can be used in amplifiers operating at up to \(18\text{ GHz}\). It shares a common characteristic of FET devices in that \(S_{21}\) is highest at low frequencies and the feedback parameter, \(S_{12}\), is lowest at low frequencies. This means that gain is harder to achieve at higher frequencies and the higher-level feedback means that stability is often a problem at higher frequencies. However, the loop gain described by \(S_{21}S_{12}\) is large at low frequencies so stability is also a problem at low frequencies.