A method of designing transistor switching circuits

THE DESIGN of most transistor circuitry is done by means of a-c small signal parameters and a-c equivalent circuits. This procedure is quite similar to that used for vacuum tube circuit design where the familiar parameters μ, gm, and rp are used. However, when one wishes to design a circuit in which operation occurs over a greater range than that for which the a-c equivalent circuit applies, some other means of representing the transistor must be found. This article describes a method which uses equivalent circuits to represent the transistor in switching applications, where the transistor is either cut-off or driven to saturation. As with most equivalent circuit representations, this method allows a better visualization of the transistor operation and, as a result, circuit simplifications may be made which might otherwise be overlooked.