Methods of Transistor Biasing

In the interest of simplicity and economy, it is desirable that transistor circuit should have a single source of supply—the one in the output circuit (i.e. V_CC). The following are the most commonly used methods of obtaining transistor biasing from one source of supply (i.e. V_CC ) :

(i) Base resistor method

(ii) Emitter bias method

(iii) Biasing with collector-feedback resistor

(iv) Voltage-divider bias

In all these methods, the same basic principle is employed i.e. required value of base current (and hence I_C) is obtained from V_CC in the zero signal conditions. The value of collector load R_C is selected keeping in view that V_CE should not fall below 0.5 V for germanium transistors and 1 V for silicon transistors.

For example, if     = 100 and the zero signal collector current I_C is to be set at 1mA, then I_B is made equal to I_C / = 1/100 = 10 µA. Thus, the biasing network should be so designed that a base current of 10 µA flows in the zero signal conditions.