Bias Compensation Techniques

Bias Compensation Techniques

• As mentioned earlier, compensation techniques use temperature sensitive devices such as diodes, transistors, thermistors, etc. to maintain operating point constant.

AU : May-03, 10, 13, Dec.-04, 09, 10, 12

1. Diode Compensation for V_BE

• Fig. 2.9.1 shows the voltage divider bias with bias compensation technique.

• Here, separate supply VDD is used to keep diode in forward biased condition.

• The diode used in the circuit is of same material and type as the transistor. Thus, the voltage across the diode will have the same temperature coefficient (-2.5 mV/ ° C as the base to emitter voltage VBE-

• So when VBE changes by 3 VBE with change in temperature, VD changes by 3 VD and 3 VD = 3 VBE.

• These changes tend to cancel each other.

• Applying KVL to the base circuit of Fig. 2.9.1 we have

Considering leakage current we have,

Substituting the value of IB in equation (2.9.1) we have

Since V_D tracks V_BE wiA respect to temperatae, it is dear from equation (2.9.2) that I_C will be insensitive to variations in V_BE

2. Diode Compensation for I_CO

• In case of germanium transistors, changes in Ico with temperature are comparatively larger than silicon transistor.

• Thus, in germanium transistor changes in Ico with temperature play the more important role in collector current stability than the changes in the V_BE

• The Fig. 2.9.2 shows diode compensation technique commonly used for stabilizing germanium transistors. It offers stabilization against variation in I_CO.

• In this circuit diode is kept in reverse biased condition. In reverse biased condition the current flowing through diode is only the leakage current.

• The diode and the transistor are of the same type and material, hence the leakage current IO of the diode will increase with temperature at the same rate as the collector leakage current I_CO.

From Fig. 2.9.2 we have

I = V_CC – V_BE / R1 and I = I_B + I_O

I_B = I – I_O

• For germanium transistor V_BE = 0.2 V, which is very small and neglecting change in V_BE with temperature we can write,

I ≅ V_CC / R₁ ≅ constant

'We know, I_C = β I_B +(1 + β)I_CO, Substituting value of IB in above equation we get,

• As I is constant, Ic remains fairly constant. In other words we can say that changes by I_CO with temperature are compensated by diode and thus collector current remains fairly constant.

3. Thermistor Compensation Technique

This method of transistor compensation uses temperature sensitive resistive elements, thermistors rather than diodes or transistors. It has a Negative Temperature Coefficient (NTC), its resistance decreases exponentially with increasing temperature as shown in the Fig. 2.9.3.

• Fig. 2.9.3  (a) shows thermistor compensation technique. As shown in Fig. 2.9.3 (a), R2 is replaced by thermistor RT in self bias circuit.

• With increase in temperature, RT decreases. Hence voltage drop across it also decreases.

• This voltage drop is nothing but the voltage at the base with respect to ground. Hence, V_BE decreases which reduces IB. This behaviour will tend to offset the increase in collector current with temperature.

I_C = βI_B + (1 + β) I_CO

• In this equation, there is increase in I_CO and decrease in I_B which keeps I_C almost constant.

Review Questions

1. What is bias compensation using thermistor ?

AU : ECE : May-03, Marks 2

2. Draw a circuit that minimizes change in VBE due to temperature variation.

3. Explain the method of stabilizing the Q point.

AU : ECE : Dec.-09, 12, Hay-13, Harks 16

4. Explain the circuit which uses a diode to compensate for changes in V_BE and in I_CO

AU ; ECE : May-10, Marks 8

5. Discuss the operation of thermistor compensation.

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6. How can collector current be stabilized with respect to lco variations ?

7. Why is temperature compensation required ?

AU : ECE : Dec.-12, Marks 2

8. Explain bias compensation.

AU : ECE : Dec.-12, May-13, Marks 8