Fixed Bias

Author: Leonard Krugman

Fig. 5-2. Fixed-bias operation.

The collector bias conditions, then, fix the d-c bias current I_b of the input base electrode; conversely, the base bias current fixes the collector bias for a given load and supply voltage. The desired base bias current can be obtained by connecting a resistor between the base and the collector terminal of the supply voltage as shown in Fig. 5-2. For E_bb = 20 volts, and I_b = 400 microamperes, the total series resistance is ohms.

This value includes the emitter to base resistance, but since r_e + r_b is generally only a few hundred ohms, they can be neglected. The resulting circuit, with the calculated values, is illustrated in Fig. 5-2 for a N-P-N transistor. If the same characteristics were applied to a P-N-P type, the only circuit change would be a reversal of the supply battery potentials. The transistor bias indicated in this figure is called fixed bias,