Universal oscillator circuits

This chapter discusses numerous practical oscillators. Most of the active devices that will be found associated with the oscillator circuits are three-terminal amplifiers. There are negative-resistance diodes and there are devices with more than three terminals such as the dual-gate MOSFET. However, these stand out as exceptions. In actual practice, it is convenient to deal with the fact that most solid-state devices function as three-terminal amplifiers, and a few two terminal devices (diodes) behave as if they already have internal positive feedback paths. Of interest, too, is the fact that circuits can be devised in which any of the terminals of the three-terminal devices function as the common terminal. Thus, while it is true that the source of a JFET is the intended common terminal, either the gate or the drain can be appropriately connected to serve as the common (or grounded) terminal in the oscillator circuit. It should also be recalled that the active device in an oscillator must provide power gain. It is sometimes erroneously thought that voltage or current gain will suffice. If this were true, an ordinary transformer would be able to provoke and sustain oscillation in a resonant circuit or its equivalent. This power requisite is predicted on fundamentals and applies to negative resistance devices and switching devices as well as linear amplifiers.