Diode and Thermistor Compensation technique

Compensation technique:

It refers to the use of temperature sensitive devices such as diodes, transistors, thermistors which provide compensating voltage and current to maintain Q point stable.

1.     Diode Compensation Techniques

Compensation for V_BE:

a) Diode in Emitter Circuit

Diagram shows the voltage divider bias with bias compensation technique.Here, separate supply V_DD is used to keep diode in forward If biased condition. If the diode used in the circuit is of same material and type as the transistor, the voltage across the diode will have the same temperature coefficient as the base to emitter voltage V_BE . So when V_BE changes by ∂ V_BE with change in temperature, V_D changes by V_D and ∂ V_D~=~∂ V_BE, the changes tend to cancel each other. Apply*g KVL to the base circuit of Fig. ,we have

Figure: Stabilization by means of voltage divider bias and diode Compensation Technique 

As V_D tracks V_BE with respect to temperature it is clear that I_C will be insensitive to variations in VBE .

Diode in voltage divider circuit

Diode is connected in series with resistance R₂ in the voltage divider circuit and it is forward biased condition.

For voltage divider bias,

When V_BE changes with temperature, I_C also changes

To cancel the changes in I_C , one diode is used in the circuit for compensation

The voltage at the base V_B is give as

 Substituting this value in equation I_C, we get,

The changes cancel each other , so the collector current is given as

The changes in V_BE. Due to temperature are compensated by changes in the diode voltage which keeps I_C stable at Q point.

Compensation for I_CO

_*         In germanium transistor changes in I_CO with temperature plays an important role collector current stability

*        The diode is kept at reverse bias condition ,so only leakage current flows

*        I_o  increases then I_CO also increases

As I is constant , I_C also remains constant. We can say that changes by I_CO with temperature are compensated by diode and collector current remains constant

2. Thermistor Compensation

With increase of temperature ,R_T decreases. Hence the voltage drop across it also decreases. That is V_BE decreases which reduces I_B .this will offset the increased collector current with temperature.

The equation shows if there is increase in I_CO and decrease in I_B keeps I_C almost constant.

Fig (b) shows another thermistor compensation technique . Here, thermistor is connected between emitter and Vcc to minimize the increase in collector current due to changes in ICO, VBE, or beta with temperature .I_C increases with temperature and R_T decreases with increase in temperature. Therefore, current flowing through RE increases, which increases the voltage drop across it. E - B junction is forward biased. But due to increase in voltage drop across R_E, emitter is made more positive, which reduces the forward bias voltage V_BE. Hence, bias current reduces.

As Ico increases with temperature, I_B decreases and hence. IC remains constant

Sensistor Compensation technique

This method of transistor compensation uses temperature sensitive resistive element, sensistors rather than diodes or transistors. It has a positive temperature coefficient, its resistance increases exponentially with increasing temperature as shown in the Fig

Slope of this curve = 

 is the temperature coefficient for thermistor and the slope is positive So we can say that sensistor has positive temperature coefficient of resistance (PTC).

Fig. shows sensistor compensation R₁ is replaced by sensistor R_T in self bias circuit. Now, R_T and R₂ resistors of the potential divider. As temperature increases, RT increases which decreases the current flowing through it. Hence current through R2 decreases which reduces the voltages drop across it. Voltage drop across R2 is the voltage between base and ground. So V_BE reduces which decreases 16. It means, when I_CBO increases with increase in temperature, I_B reduces due to reduction in V_BE, maintaining I_C fairly constant.