2.10: Summary

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This chapter addressed the design of narrowband amplifiers, but this forms the basis of wideband and power amplifier design to be considered in the following chapters. The bandwidth of an amplifier is dictated by the frequency-dependent characteristics of the active device and at microwave frequencies the device parasitic capacitances are usually significant. Without special broadbanding concepts, the narrowband approach covered in this chapter is good, usually, for amplifier designs with up to \(5\%\) bandwidth.

The basic topology used in amplifiers is an input matching network, an active device, and an output matching network. This arrangement is one of three cascaded two-ports. Sometimes an additional two-port is used in parallel with the active device to provide feedback and ensure stability or a flat gain response over frequency. With narrowband design the use of a feedback network is rarely required. The input matching network provides near-maximum power transfer from the system impedance to the usually higher input impedance of the active device. The near-maximum qualification is used since the requirements for maximum power transfer at the input conflict with the conditions for best noise performance. This originates because the active device has multiple partially correlated physical noise sources and the input matching network affects how the correlated noise sources are combined so that it is possible to minimize their contributions. The active device is followed by an output matching network that matches the output impedance of the active device to the usually higher system impedance. In narrowband amplifier design the device capacitive parasitics are often incorporated into the matching networks. The topology of the input and output matching networks is chosen to ensure out-of-band stability and provide bias with minimum additional components.

Amplifier design is driven by metrics for the power gains at various optimum conditions, and the various gain metrics are used at various stages in design. The gain metrics are also used in choosing an active device and in estimating the design complexity that can be expected.

Amplifier design is a major endeavor and many books have been written about particular aspects of RF and microwave amplifier design. This chapter covered the main topics and also presented treatments that are broadly applied. The reader is directed to references [11, 14, 29, 30, 31, 32, 33, 34, 35, 36] for specialized aspects of amplifier design. Numerous references are available for understanding, analyzing, and characterizing distortion in greater depth than covered here [37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48].