Hunter-Goodrich Bias Method

Author: Leonard Krugman

A method of establishing tighter control on the base bias current illustrated in Fig. 5-5 is the Hunter-Goodrich method. This involves the addition of a fixed base bias operating in the reverse direction of the normal self bias. The fixed bias is introduced by resistor R_F and separate voltage supply E_F. To overcome this reversed fixed bias, the self bias resistor R_B must be decreased to maintain the same base bias current. The reduced value of R_B increases the available negative d-c feedback from the collector circuit, thus providing greater transistor stability.

Fig. 5-5. Hunter-Goodrich bias method.

As in the preceding cases, the effect of the base and emitter circuit resistances (r_e + r_b) can be neglected in the calculations. The values of R_F and E_F depend upon the value of fixed bias desired. For example, assume that a fixed bias value I_b2 of 300 μa will provide the additional stability needed, and a battery E_F = 10 volts is available. Then

.

The current through the self bias resistor R_B is I_b1 = I_b + I_b2 = 400 + 300 = 700 μa; then

In comparison, R_B = 25,000 ohms in the simple self bias case. Since the input resistance of the transistor is small compared to R_F, practically all of the stabilizing current flows into the base-emitter circuit.

The Hunter-Goodrich bias method is extremely useful when a high degree of circuit stability is needed. Its particular disadvantage is that it requires two separate battery supplies.