Biasing Method for BJT

BIASING METHOD FOR BJT

TRANSISTOR BIASING

Fixed Bias (or) Base Resistor Method

A common emitter amplifier using fixed bias circuit is shown in Fig. 2.6. The dc analysis of the circuit yields the following equation.

Since this equation is independent of current and the stability factor  is reduced to S = 1 + β

Since β is a large quantity, this is a very poor bias stable circuit. Therefore in practice, this circuit is not used for biasing the base.

Advantages

(i) Simplicity

(ii) Small number of components required

(iii) If the supply voltage is very large as compared to V_BE of the transistor, then the base current becomes largely independent of the voltage V_BE

Collector to Base Bias (or) Biasing with Feedback Resistor

A common emitter amplifier using collector to base bias circuit is shown in Fig.2.7. This circuit is the simplest way to provide some degree of stabilization to the amplifier operating point.

If the collector current I_C tends to increase due to either increase in temperature or the transistor has been replaced by the one with a higher β, the voltage drop across R_e. increases, thereby reducing the value of V_CE. Therefore, I_B decreases which in turn, compensates the increase in I_e. Thus greater stability is obtained

The loop equation for this circuit is

This value of the stability factor is smaller than the value obtained by fixed bias circuit. Also S can be made small and the stability can be improved by making R_B small or R_C large.

If R_C is very small, then S = (β + 1), i.e., stability is very poor. Hence the value of R_C must be quite large for good stabilization. Thus collector-to-base bias arrangement is not satisfactory for the amplifier circuits like transformer coupled amplifier where the dc load resistance in collector circuit is very small. For such amplifiers, emitter bias or self bias will be the most satisfactory transistor biasing for stabilization.

Self Bias or Emitter Bias

A simple circuit used to establish a stable operating point is the self-biasing configuration. The self bias, also called as emitter bias, or emitter resistor and potential divider circuit, that can be used for low collector resistance, is shown in Fig.2.8.

The current in the emitter resistor R_E causes a voltage drop which is in the direction to reverse bias the emitter junction. For the transistor to remain in the active region the base emitter junction has to be forward biased. The required base bias is obtained from the power supply through the potential divider network of the resistance R₁ and R₂.

Use of Self Bias Circuit as a Constant Current Circuit

If I_C tends to increase, say due to increase in I_CO with temperature, the current in R_E increases. Hence the voltage drop across R_E increases there by decreasing the base current. As a result I_C is maintained almost constant.

To Determine Stability Factor S

Applying Thevenin's theorem to the circuit of Fig. 2.8 for finding the base current, we have

The loop equation around the base circuit can be written as

Differentiating this equation with respect to Ic, we get

The value of S is equal to one if the ratio R_B/R_E is very small as compared to 1. As this ratio becomes comparable to unity, and beyond towards infinity, the value of the stability factor goes on increasing till S = 1 + β.

This improvement in the stability upto a factor equal to 1 is achieved as the cost of power dissipation. To improve the stability, the equivalent resistance R_B must be decreased, forcing more current in the voltage divider network of R₁ and R₂.

Often to prevent the loss of gain due to the negative feedback, R_E is shunted by a capacitor C_E. The capacitive reactance X_CE must be equal to about one-tenth of the value of the resistance R_E at the lowest operating frequency.