BIAS STABILITY

BIAS STABILITY
Suppose the transistor in a circuit is BC 108, and we replace this transistor by
another BC 108 transistor.
The characteristics of these two transistors will not be exactly the same.
There will be slight difference.
It is because of the limitations in temperature and the fabrication
technology; the doping concentration, diffusion length etc.
Because of these reasons, transistors of the same type may not have exactly
the same characteristics. Hence if one AC 128 transistor is replaced by another
transistor, the operating point will change, since the characteristics of the
transistor will be slightly different.
Hence for the same circuit when one transistor is replaced by the other,
operating point
changes. In some cases, because of the change in the operating point, the
transistor may be cut off or become saturated.

• If IB

is fixed, operating point will shift with changes in the values of 𝛽

with temperature.

• lCO gets doubled for every 10°C rise in temperature. IC also increases

with temperature.

• In order to keep operating point fixed, IC and VCE should be kept

constant. There are two methods to keep IC constant.

1. Stabilization Technique: Here resistive biasing circuits are used to

allow IB

to vary to keep IC

relatively constant, with variation in lCO 𝛽

and VBE i.e. IB decreases if lCO increase, to keep IC constant.

2. Compensation Technique: In this method, temperature sensitive

devices such as diodes, transistor, thermistors etc., which provide

compensating voltage and currents to maintain the operating point

constant are used.