Chapter 2
Models For Active Devices
Because we are dealing with amplifiers, both small-signal and power, oscillators and active mixers, active devices will play a major role in circuit design. At our frequency range of interest (about 1000 MHz and above), distributed elements predominate, and even in device modeling, we must consider some distributed effects such as packaging.
As we dive into providing functional explanations of the active devices used in practical circuits, there is some temptation not to provide enough information on semiconductor behavior. Because manufacturers of microwave products will use the services of a foundry or specify semiconductors from their suppliers, we will cover the device operation in depth. That said, for the beginner looking into designing circuits from high frequencies to low microwave frequencies, we will provide examples of a variety of devices unique to this frequency range. Some treatment of device physics will therefore be useful in providing a basic understanding of the nonlinear behavior of active devices and modeling in preparing for practical circuits using computer-aided-design (CAD) tool. For CAD applications, we assume the user will have access to nonlinear CAD software, such as a high-performance SPICE program or a harmonic-balance program, such as Ansoft Designer or its derivatives. The general trend is that models for CAD are provided, in addition to the datasheets. For some devices, S-parameter sets, general-purpose SPICE ...
