Transistor Voltage Divider Bias

A method of biasing a transistor for linear operation using a single-source resistive voltage divider. This is the most widely used biasing method.

 

Up to this point a separate dc source, V_BB, was used to bias the base-emitter junction because it could be varied independently of V_CC and it helped to illustrate transistor operation. A more practical bias method is to use V_CC as the single bias source, as shown in the above Figure.

To simplify the schematic, the battery symbol is omitted and replaced by a line termination circle with a voltage indicator (V_CC) as shown. A dc bias voltage at the base of the transistor can be developed by a resistive voltage-divider that consists of R1 and R2, as shown in Figure. V_CC is the dc collector supply voltage. Two current paths are between point A and ground: one through R2 and the other through the base-emitter junction of the transistor and R_E. Generally, voltage-divider bias circuits are designed so that the base current is much smaller than the current (I2) through R2 in Figure.

In this case, the voltage-divider circuit is very straightforward to analyze because the loading effect of the base current can be ignored. A voltage divider in which the base current is small compared to the current in R2 is said to be a stiff voltage divider because the base voltage is relatively independent of different transistors and temperature effects.

To analyze a voltage-divider circuit in which I_B is small compared to I2, first calculate the voltage on the base using the unloaded voltage-divider rule:

 

Once you know the base voltage, you can find the voltages and currents in the circuit, as follows:

 

Once you know V_C and V_E, you can determine V_CE.

V_CE = V_C – V_E