Bias Compensation Using Diode & Transistor | Analog and Digital Electronics - Electrical Engineering (EE) PDF Download

BIAS COMPENSATION USING DIODE AND TRANSISTOR:

DIODE COMPENSATION:

The following fig 4.8 shows a transistor amplifier with a diode D connected across the base-emitter junction for compensation of change in collector saturation current I_CO. The diode is of the same material as the transistor and it is reverse biased by e the emitter-base junction voltage V_BE, allowing the diode reverse saturation current I_O to flow through diode D. The base current I_B=I-I_O.

As long as temperature is constant, diode D operates as a resistor. As the temperature increases, I_CO of the transistor increases. Hence, to compensate for this, the base current I_B should be decreased.

The increase in temperature will also cause the leakage current I_O through D to increase and thereby decrease the base current I_B. This is the required action to keep Ic constant.

This type of bias compensation does not need a change in Ic to effect the change in I_C, as both I_O and I_CO can track almost equally according to the change in temperature.        

THERMISTOR COMPENSATION:

The following fig 4.9 a thermistor R_T, having a negative temperature coefficient is connected in parallel with R₂. The resistance of thermistor decreases exponentially with increase of temperature. An increase of temperature will decrease the base voltage V_BE, reducing I_B and I_C.

SENSISTOR COMPENSATION:

In the following fig 4.10 shown a sensistor Rs having a positive temperature coefficient is connected across R₁ or R_E. Rs increases with temperature. As the temperature increases, the equivalent resistance of the parallel combination of R1 and Rs also increases and hence V_BE decreases, reducing I_B and Ic. This reduced Ic compensates for increased Ic caused by the increase in V_BE, I_CO and β due to temperature.