Self-Bias

Author: Leonard Krugman

Unfortunately, transistors are temperature sensitive devices; in addition some variation usually exists in the characteristics of transistors of a given type. These factors may cause a displacement of the constant base current lines along the collector current axis. Figure 5-3 illustrates the effect of this variation; the abnormal cases are purposely exaggerated. Notice the effect of this shift on the relative positions of the d-c operating point. In the low I_co unit (Fig. 5-3B) the collector voltage is too high; in the high I_co unit (Fig. 5-3C) the collector voltage is too low. To overcome this, the circuit needs degeneration, similar to that produced by an unbypassed cathode bias resistor in a vacuum-tube circuit. In transistor circuitry, this method of degeneration is a form of automatic control of the base bias, known as self bias.

Fig. 5-3. Variation of operating point: normal case (A), lo I_co (B) high I_co (C)

A simple method for establishing automatic control of the base bias requires the base bias resistor to be tied directly to the collector, as in Fig. 5-4. Thus, if the collector voltage is high (Fig. 5-3B), the base current is increased, moving the d-c operating point downward along the load line; conversely, if the collector voltage is low (Fig. 5-3C), the base bias current is decreased, moving the d-c operating point upward along the load line. The value of the selected base bias resistor is different in the self-bias case from that computed in the fixed-bias connection. For self bias, the resistor is tied to the collector voltage, which in this case is 10 volts. Then ohms. The base bias resistor performs the double duty of determining the value of I_b and preventing those excessive shifts in the collector d-c operating point due to temperature change and transistor interchange. The principal limitation of self bias is that it still allows some variation of the d-c operating point, since the base bias resistor is fixed by the required operating point, and the stabilization produced by it is only a secondary effect. In addition, self bias also introduces a-c negative feedback which reduces the effective gain of the amplifier. Despite its limitations, however, self bias is very useful and works well in many applications.

Fig. 5-4. Self-bias operation.

The importance of temperature stability with respect to the d-c operating point cannot be taken lightly. One of the effects of a temperature rise is to increase the saturation current I_co, which, in turn, increases the collector dissipation. The increased collector dissipation increases the temperature, which increases I_co, and so on. Thus, poor temperature stability almost certainly will cause transistor burnouts, particularly if the transistor is operated near its maximum dissipation limit.