Biasing Stability

Biasing Stability

• Ideally, the Q point should be stable; it should not shift up and down on the dc load line. However, it is quite unstable.

• Two important factors are responsible for shifting the operating point are :

■ Temperature

■ Variation of hrE(P) within manufacturers tolerance

■ The process of stabilization of Q-point using biasing circuit is called bias stabilization.

Temperature

• The change in temperature affect the following parameters of the transistor :

■ I_CO  ■ V_BE ■ β _dc

1. I_CO : The minority carriers are temperature dependent. They increase with the temperature. The increase in the minority carriers increases the leakage current I_CEO,

I_CEO = (1 + β ) I_CO = (1 + β ) I_CBO

• Specifically, I_CBO doubles for every 10 ° C rise in temperature. Increase in ICEO in turn increases the collector current, I_C = β I_B + I_CEO and hence there is a change in Q-point.

2. VBE : Base to emitter voltage VBE decreases with increase in temperature at the rate of 2.5mV/° C. Reduction in VBE increases IB and hence IC. Thus, there is a change in Q-point.

3. β : β of the transistor is also temperature dependent. As P varies, IC also varies, since I_C = β I_B. The change in collector current changes the operating point.

• The biasing circuits should be designed to overcome the problem of temperature instability.

Transistor current gain hFE/ β

• Eventhough there is tremendous advancement in semiconductor technology, there are changes in the transistor parameters among different units of the same type, same number.

• This means if we take two transistor units of same type (i.e. same number, construction, parameter specified etc.) and use them in the circuit, there is change in the P value in actual practice.

• The biasing circuit is designed according to the required P value. But due to change in P from unit to unit, the operating point may shift.

Requirements of biasing circuits

1. The base-emitter junction must be forward biased and base-emitter junction must be reverse biased i.e. the transistor should be operated in the middle of the active region or operating point (Q point) should be fixed at the center of the active region.

2. The circuit design should provide thermal stability.

3. The operating point should be made independent of the transistor parameters (such as P).

• To maintain the operating point stable by keeping IC and VCE constant so that the transistor will always work in active region, the following techniques are normally used :

• Stabilization techniques

• Compensation techniques

• Stabilization techniques : Stabilization techniques refer to the use of resistive biasing circuits which allow IB to vary so as to keep IC relatively constant with variations in I_CO, β, and V_BE.

• Compensation techniques : Compensation techniques refer to the use of temperature-sensitive devices such as diodes, transistors, thermistors, etc., which provide compensating voltages and currents to maintain the operating point stable.

Review Questions

1. What is meant by bias stability ? What factors affect BJT biasing ?

2. Define stabilization techniques.

3. What is meant by compensation techniques ?

4. Explain thermal stabilization.