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 R₁ and R₂ 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 (I₂) 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 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 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 R₁ in parallel with R₂,

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

Substituting I_C = h_FE I_B,

Once I_B has been determined, I_C can be calculated using the appropriate h_FE 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 R₁, not that across R₂. 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.