Voltage Divider Bias:

If the load resistance R_C is very small, e.g. in a transformer coupled circuit, then there is no improvement in stabilization in the collector to base bias circuit over fixed bias circuit. A circuit which can be used even if there is no dc resistance in series with the collector, is the voltage divider bias or self bias. fig. 3.

The current in the resistance R_E in the emitter lead causes a voltage drop which is in the direction to reverse bias the emitter junction. Since this junction must be forward biased, the base voltage is obtained from the supply through R₁, R₂ network. If R_b = R₁ || R₂ equivalent resistance is very – very small, then V_BE voltage is independent of I_CO and ¶ I_C / ¶ I_CO ® 0. For best stability R₁ & R₂ must be kept small.

Fig. 3

If I_C tends to increase, because of I_CO, then the current in R_C increases, hence base current is decreased because of more reverse biasing and it reduces I_C .

To analysis this circuit, the base circuit is replaced by its thevenin's equivalent as shown in fig. 4.

Fig. 4

Thevenin's voltage is

R_b is the effective resistance seen back from the base terminal.

If V_BE is considered to be independent of I_C, then

The smaller the value of R_b, the better is the stabilization but S cannot be reduced be unity.

Hence I_C always increases more than I_CO. If R_b is reduced, then current drawn from the supply increases. Also if R_E is increased then to operate at same Q-point, the magnitude of V_CC must be increased. In both the cases the power loss increased and reduced h.

In order to avoid the loss of ac signal because of the feedback caused by R_E, this resistance is often by passed by a large capacitance (> 10 m F) so that its reactance at the frequency under consideration is very small.

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