The design and analysis of amplifiers is as old as the field of electronics. Since the invention of active devices, engineers have striven to maximize the performance of amplifiers to achieve higher gain, lower noise, wider bandwidths, and higher dynamic range. Over the years two distinct styles of amplifier design have emerged. Analog amplifier design styles dominate in the frequency range of DC to approximately 1 GHz. The upper frequency limit is really arbitrary and has been a strong function of technology fT and improved integration and miniaturization. Most analog IC amplifiers are designed to drive high-impedance loads and/or mostly capacitive loads. Only the output stage may be designed to drive an external load resistance. Furthermore, since the signals tend to be baseband, the analysis of such amplifiers is based on the voltage or current gain transfer function and pole/zero analysis. Feedback is almost always employed since large gain can be extracted at low frequency and traded off to achieve higher speed, robustness against process tolerance, and linearity.

At the other extreme, we have microwave amplifier design based on two-port theory. The active element such as the transistor is treated as a two-port device and the device is carefully impedance matched to obtain the optimal performance. In the course of the design, extensive use of the Smith Chart enables the designer to trade off gain, noise, bandwidth, and matching.