As the temperature increases, the resistance R T of thermistor decreases, which increases the current through it and the resistor R E. Thermistor is employed to minimize the increase in collector current. In an amplifier circuit, the changes that occur in I CO, V BE and β with temperature, increases the collector current. The below figure shows a self-bias amplifier with thermistor compensation. The resistance of a thermistor increases when the temperature decreases and it decreases when the temperature increases. Thermistor is a temperature sensitive device. There are two popular types of circuits in this method, one using a thermistor and the other using a Sensistor. to compensate for the variation in currents. There are other compensation techniques which refer to the use of temperature sensitive devices such as diodes, transistors, thermistors, Sensistors, etc. I is almost constant and if I O of diode and I CO of transistor track each other over the operating temperature range, then I C remains constant. So, the reverse saturation current I O of the diode will increase with temperature at the same rate as the transistor collector saturation current I CO. The following figure shows the circuit diagram of a transistor amplifier with diode D used for compensation of variation in I CO. Diode Compensation for Instability due to I CO Variation So the current I C remains constant in spite of the variation in V BE. The variation in V BE with temperature is same as the variation in V D with temperature, hence the quantity (V BE – V D) remains constant. The diode D is forward biased by the source V DD and the resistor R D. The following figure shows self-bias with stabilization and compensation. As the diode and transistor used are of same material, the voltage V D across the diode has same temperature coefficient as V BE of the transistor. A diode can be employed in the emitter circuit in order to compensate the variations in V BE or I CO. In a Silicon transistor, the changes in the value of V BE results in the changes in I C. Diode Compensation for Instability due to V BE Variation Let us understand these two compensation methods in detail.
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