**Voltage Divider Bias Circuit:**

**Circuit Operation –** Voltage Divider Bias Circuit, also known as **emitter current bias**, is the most stable of the three basic transistor bias circuits. A voltage divider bias circuit is shown in Fig. 5-22(a), and the current and voltage conditions throughout the circuit are illustrated in Fig. 5-22(b). It is seen that, as well as the collector resistor (R_{C}), there is an emitter resistor (R_{E}) connected in series with the transistor. As discussed already, the total dc load in series with the transistor is (R_{C} + R_{E}), and this total resistance must be used when drawing the dc load line for the circuit. Resistors R_{1} and R_{2} constitute a voltage divider that divides the supply voltage to produce the base bias voltage (V_{B}).

Voltage Divider Bias Circuit are normally designed to have the voltage divider current (I_{2}) very much larger than the transistor base current (I_{B}). In this circumstance, V_{B} is largely unaffected by I_{B}, so V_{B} can be assumed to remain constant.

Referring to Fig. 5-22(b),

With V_{B} constant, the voltage across the emitter resistor is also a constant quantity,

This means that the emitter current is constant,

The collector current is approximately equal to the emitter current, so I_{C}Â is held at a constant level.

Again referring to Fig. 5-22(b), the transistor collector voltage is,

The collector-emitter voltage is,

V_{CE} can also be determined as,

Clearly, with I_{C} and I_{B}Â constant, the transistor collector-emitter voltage remains at a constant level.

It should be noted that the transistor h_{FE} value is not involved in any of the above equations.

**Precise Circuit Analysis:**

To precisely analyse a Voltage Divider Bias Circuit, the voltage divider must be replaced with its **Thevenin equivalent circuit** (V_{T} in series with R_{T}) as illustrated in Fig. 5-25. From Figure 5-25(a),

and R_{T} is calculated as R_{1} in parallel with R_{2},

Referring to Fig. 5-25(b), an equation may be written for the voltage drops around the base-emitter circuit;

SubstitutingÂ I_{C}Â = h_{FE}Â I_{B},

Once I_{B} has been determined, I_{C} can be calculated using the appropriate h_{FE}Â value, and the transistor terminal voltages can then be calculated. The effects of the maximum and minimum h_{FE} values can also be investigated.

**Voltage Divider Circuit using Transistor:**

Voltage Divider Circuit using Transistor is shown in Fig. 5-29. Note that the positions of the collector and emitter resistors are reversed compared to the npn transistor circuit. Also, note that the base voltage (V_{B}) in Fig. 5-29 is the voltage drop across resistor R_{1}, not that across R_{2}. As in the case of other circuits using pnp transistors, the current directions and voltage polarities are the reverse of those in npn transistor circuits. Apart from these differences, a pnp transistor Voltage Divider Bias Circuit is analysed in exactly the same way as an npn transistor circuit.