Diode compensation uses a diode connected across the base-emitter junction of a transistor amplifier to stabilize the operating point against variations in the transistor's current gain and leakage current due to temperature changes. As temperature increases, the diode's leakage current also increases, decreasing the base current and keeping the collector current constant. Thermistor compensation uses a thermistor in parallel with a resistor; as temperature rises, the thermistor's resistance decreases, increasing the current through it and another resistor to compensate for the increased collector current. Sensistor compensation similarly uses a sensistor with a positive temperature coefficient in series with a resistor such that increased resistance at higher temperatures decreases base current to offset increased collector current.

1. Bias Compensation
 Stabilization occur due to negative feedback action.
 Improves the stability of operating point, it reduces
gain of amplifier.
Diode Compensation Fig : 1
 Resistive biasing circuits which permit IB to vary so as to keep
IC relatively constant.
 Two types - Diode compensation for instability due to VBE variation
Diode compensation for instability due to ICO variation
 Circuit – Transistor amplifier with D connected across the BE
Junction to compensate the change in Ico
 Reversed bias by VBE gives reverse diode Current Io. Base current
IB = I-Io
 Diode temperature increases, Ico of transistor also increases. To
compensate base current IB should decrease.
 Increase of temperature cause leakage current Io. Hence IC be
constant
 I is almost constant and if IO of diode and ICO of transistor track each
other over the operating temperature range, then IC remains
constant.
Fig : 1

2. Thermistor Compensation Fig : 2
 Temperature sensitive device.
 RT, negative temperature coefficient connected in parallel with R2
 Resistance of a thermistor decreases when the temperature
increases.
 Increases when the temperature decreases the VBE, reducing IB ,
IC
 Bias Stabilization provided by RE, CE.
 As the temperature increases, the resistance RT of thermistor
decreases, which increases the current through it and resistor RE.
 Thus the temperature sensitivity of thermistor compensates the
increase in collector current, occurred due to temperature.
Sensistor Compensation Fig : 3
 Heavily doped semiconductor that has positive temperature
coefficient RS connected across R1.
 Temperature increases , resistance of R1, RS also increases – VBE
decreases, Reducing IB, IC
 Reduced IC, compensates for increased IC,
 Due to the increase in ICO, VBE and β due to temperature rise.
Fig : 2
Fig : 3