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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 RF and separate voltage supply EF. To overcome this reversed fixed bias, the self bias resistor RB must be decreased to maintain the same base bias current. The reduced value of RB 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 (re + rb) can be neglected in the calculations. The values of RF and EF depend upon the value of fixed bias desired. For example, assume that a fixed bias value Ib2 of 300 μa will provide the additional stability needed, and a battery EF = 10 volts is available. Then

transistor_basics_05-11.giftransistor_basics_05-12.gif .

The current through the self bias resistor RB is Ib1 = Ib + Ib2 = 400 + 300 = 700 μa; then


In comparison, RB = 25,000 ohms in the simple self bias case. Since the input resistance of the transistor is small compared to RF, 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.

Last Update: 2010-11-17