Voltage Divider Bias:

If the load resistance R_C is quite small, for instance in a transformer coupled circuit, then there is no improvement in stabilization in collector to base bias circuit over fixed bias circuit. The circuit which can be used even if there is no DC resistance in series with collector is the voltage divider bias figure 1.

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

Figure 1

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

To analysis this circuit, the base circuit is replaced by the thevenin's equivalent of it as shown in the figure 2.

Figure 2

Thevenin's voltage is

R_b is effective resistance seen back from the base terminal.

If V_BE is considered as independent of I_C, then

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

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

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

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