**Collector to Base Bias Circuit:**

The Collector to Base Bias Circuit shown in Fig. 5-17(a) has the base resistor (R_{B}) connected between the transistor collector and base terminals. As will be demonstrated, this circuit has significantly improved bias stability for h_{FE} changes compared to base bias.

Refer to Fig. 5-17(b) and note that the voltage across R_{B} is dependent on V_{CE},

Also, V_{CE}Â is dependent on the level of I_{C} and I_{B},

If I_{C}Â increases above the design level, there is an increased voltage drop across R_{C}, resulting in a reduction in V_{CE}. The reduced V_{CE} level causes I_{B} to be lower than its design level, and because I_{C}Â = h_{EE} I_{B}, the collector current is also reduced. Thus, an increase in I_{C}Â produces a feedback effect that tends to return I_{C}Â toward its original level. Similarly, a reduction in I_{C}Â produces an increase in V_{CE}Â which increases I_{B}, thus tending to increase I_{C}Â back to its original level.

Analysis of this circuit is a little more complicated than base bias analysis. To simplify the process, an equation is first derived for the base current. Substituting for V_{CE}Â from Eq. 5-4 into Eq. 5-5,

SubstitutingÂ I_{C}Â =h_{FE}Â I_{B} (fromÂ Eq. 5-3) into the above equation

This gives,

**Effect ofÂ h**_{FE(max)Â }and h_{FE(min)}

_{FE(max)Â }and h

_{FE(min)}

As discussed for the base bias circuit, transistors of a given type number have a wide range of h_{FE} values, (h_{FE} spread). This affects the current and voltage levels in all bias circuits. In the Collector to Base Bias Circuit Theory, the feedback from the collector to the base reduces the effects of h_{FE} spread.

It is important to note that, unlike the situation in a base bias circuit, the base current in a collector to base bias circuit does NOT remain constant when the transistor h_{FE} value is changed.

**Collector to Base Bias Using a PNP Transistor:**

A Collector to Base Bias Circuit Theory using a pnp transistor is illustrated in Fig. 5-21. Note that the voltage polarities and current directions are reversed compared to npn transistor collector to base bias circuit. This circuit can be analysed in exactly the same way as the npn transistor circuit.