Bias Considerations in Transistor Circuit Design

It is well known that the operating point of a transistor has a tendency to shift with temperature. An analysis is made of the factors contributing to this shift. It is found necessary to define two stability factors for an adequate formulation of the bias problem. These factors, together with the voltage and current defining the operating point, are used in the design of bias networks. Various stabilization circuits are considered in detail, and design procedures set up for these schemes. A comparison is made of the various schemes; in particular, direct and feedback stabilization are compared on the basis of battery drain, ac gain, and variation of operating point with battery voltage. The stabilization of direct-coupled stages is also discussed, and the design of a bias circuit for a two-stage direct-coupled pair is taken up in detail. Nonlinear techniques for temperature stabilization are also discussed. These are useful in special applications such as high-power output stages, where the temperature changes are extreme, or in low-level circuits where extremely stable operation is required, e.g., in oscillators.