Transistor biasing - Voltage divider bias

Transistor biasing

In order to amplify the input signal using a transistor, the signal is to be applied at an operating point called Q point in the active region. Once the operating point is established, its position should not change. If the Q point shifts near the saturation line or near cut off region of the output characteristics, the signal will be distorted after amplification.

The proper selection of operating point of a transistor and maintenance of proper emitter voltage during the passage of the signal is known as transistor biasing.

The most commonly used methods of obtaining transistor biasing are (i) base bias, (ii) base bias with emitter feedback, (iii) base bias with collector feedback and (iv) voltage divider bias.

The principle involved in all these types is to obtain the required base current corresponding to the operating point under zero signal conditions.

In all the bias circuits except voltage divider bias, the collector current depends on the current gain (β) of the transistor. But β of a transistor is very sensitive to temperature changes. For this reason, it is desirable to have a bias circuit whose action is independent of β. The requirement is met by the voltage divider bias circuit.

Voltage divider bias

This is the most widely used method of providing bias and stabilization to a transisitor. In this method, two resistances R₁ and R₂ are connected across the supply voltage V_CC (Fig) and provide biasing. The emitter resistance R_E provides stabilization. The voltage drop across R₂ forward biases the base emitter junction. This causes the base current and hence collector current to flow in zero signal conditions.

The stabilization provided by R_E can be explained as follows. Since β is very sensitive to temperature changes,

the collector current I_C increases with rise in temperature. Consequently, it can be seen that I_E increases. This will cause the voltage drop across emitter resistance R_E to increase. The voltage drop across R₂ = V_BE + V_RE. As voltage drop across R₂ is independent of I_C, V_BE decreases. This decreases I_B and the reduced value of I_B tends to bring back I_C to the original value. Hence any variation of β will have no effect on the operating point.