Bias Compensation

The biasing techniques offer stability to the operating point (I_CQ, V_CEQ) by virtue of negative feedback but they also reduce the gain of the circuit. In applications where the reduction in gain is not acceptable, compensation techniques are used to arrest the variation in the operating point.

Bias compensation techniques make use of temperature-sensitive devices such as diodes, thermistors, transistors, sensitors etc. to compensate for the changes in the operating point caused due to changes in temperature. Commonly used compensation circuits include the following

• Diode Compensation for Base-Emitter Voltage (V_BE)
• Diode Compensation for Leakage Current (I_CO)
• Thermistor Compensation

Figure below shows the circuit employing diode compensation for base-emitter voltage (V_BE) technique. The diode is kept in forward-bias mode through voltage (V_DD) and resistance (R_D). It is made of the same material as the transistor. Hence, there is same variation in the transistor’s base–emitter voltage (V_BE) and diode’s forward voltage (V_D) due to temperature.

Diode compensation for base–emitter voltage (V_BE)

Applying Kirchhoff’s voltage law to the base–emitter loop, we get

Substituting

in the above equation and solving for base current (I_b), we get

As collector current (I_C) is given by

Substituting the value of I_B in the expression for collector current, we get

If we observe the above equation carefully, we see that the change in the base–emitter voltage (V_BE) of the transistor due to the change in temperature is nullified by a similar change in the forward voltage (V_D) of the diode.

Refer to figure below. The diode placed across the base–emitter terminals of the transistor, compensates for the variation in the leakage current (I_CO). Since the diode is reverse-biased, therefore reverse saturation current (I_D) flows through it. The diode is of the same material as the transistor, therefore the reverse saturation current of the diode increases with temperature at the same rate as the leakage current of the transistor.

Diode compensation for leakage current (ICO)

The base current (I_B) is equal to

Therefore, collector current (I_C) is given by

As βand (β + 1) are nearly equal, the expression for I_C can be written as

Rearranging the terms, we get

The diode reverse current (I_D) tracks the transistor leakage current (I_CO) over the desired temperature range.

Figure below shows a thermistor-based compensation circuit. A NTC thermistor (negative temperature coefficient of resistance) is placed in parallel with resistor RB2. As the temperature increases, the value of the thermistor resistance decreases which in turn reduces the forward bias base-emitter voltage. Therefore, the base and collector currents of the transistor decrease. Therefore, the thermistor compensates for the increase in collector current due to increase in temperature

When the temperature decreases, the thermistor resistance increases. This results in an increase in the value of base–emitter voltage which in turn increases the base and the collector currents. Therefore, the thermistor compensates for the decrease in collector current due to decrease in temperature.

Thermistor compensation