Self Bias Plus Fixed Bias

Author: Leonard Krugman

One method of obtaining additional stabilization with only one battery is shown in Fig. 5-6 (A), the basic features of which are often used in transistor power stages. The fundamental differences between this circuit and the preceding fixed plus self bias method are the interchange of R_L and E_bb, and the connection of the reverse bias resistor R_F into the collector circuit. Interchanging the supply battery and the load resistor provides two points at which variations in collector voltage will appear. However, this interchange does not affect the d-c operation of the circuit. Connecting R_F, as illustrated, produces essentially the same result as the Hunter-Goodrich arrangement, except that the reverse bias is no longer fixed. If the previous

circuit constants are desired:

All the other values remain the same.

Fig. 5-6. (A) Stabilization of d-c operating point with one battery. (B) Typical power output stage.

In power amplifier circuits, the load usually consists of a transformer plus an additional stabilizing resistor. Figure 5-6 (B) illustrates one possible form of this arrangement for use as a transistor power amplifier stage.

A disadvantage of this bias method is that the d-c degeneration feedback is reduced, due to the shunting effect of resistor R_F, thus reducing the stabilization. On the other hand, this method provides for greater stability than does the simple self-bias method. It provides less stability than the Hunter-Goodrich method, but requires only one battery supply.