VOLTAGE DIVIDER BIAS

The voltage divider is formed using external resistors R₁ and R₂. The voltage across R₂ forward biases the emitter junction. By proper selection of resistors R₁ and R₂, the operating point of the transistor can be made independent of β. In this circuit, the voltage divider holds the base voltage fixed independent of base current provided the divider current is large compared to the base current. However, even with a fixed base voltage, collector current varies with temperature (for example) so an emitter resistor is added to stabilize the Q-point, similar to the above circuits with emitter resistor.

In this circuit the base voltage is given by:

Merits:

• Unlike above circuits, only one dc supply is necessary.
• Operating point is almost independent of β variation.
• Operating point stabilized against shift in temperature.

where R₁ || R₂ denotes the equivalent resistance of R₁ and R₂ connected in parallel.

• As β-value is fixed for a given transistor, this relation can be satisfied either by keeping R_E fairly large, or making R₁||R₂ very low.

• If R_E is of large value, high V_CC is necessary. This increases cost as well as precautions necessary while handling.
• If R₁ || R₂ is low, either R₁ is low, or R₂ is low, or both are low. A low R₁ raises V_B closer to V_C, reducing the available swing in collector voltage, and limiting how large R_C can be made without driving the transistor out of active mode. A low R₂ lowers V_be, reducing the allowed collector current. Lowering both resistor values draws more current from the power supply and lowers the input resistance of the amplifier as seen from the base.

• AC as well as DC feedback is caused by R_E, which reduces the AC voltage gain of the amplifier. A method to avoid AC feedback while retaining DC feedback is discussed below.

Usage:

The circuit's stability and merits as above make it widely used for linear circuits.

Voltage divider with AC bypass capacitor

Voltage divider with capacitor

The standard voltage divider circuit discussed above faces a drawback - AC feedback caused by resistor R_E reduces the gain. This can be avoided by placing a capacitor (C_E) in parallel with R_E, as shown in circuit diagram.

This capacitor is usually chosen to have a low enough reactance at the signal frequencies of interest such that R_E is essentially shorted at AC, thus grounding the emitter. Feedback is therefore only present at DC to stabilize the operating point, in which case any AC advantages of feedback are lost.

Of course, this idea can be used to shunt only a portion of R_E, thereby retaining some AC feedback.