Question

explain the need of stabilization​

Answer 1

Answer:

Bias stabilization is done to keep the operating point unaltered by the changes in the device parameters like Ico(the reverse saturation current),Vbe(base-emitter voltage) and beta(current gain). So to ensure proper stability of the operating point, bias stabilization against these variations is required.

Answer 2

Answer:

A transistor can work as amplifier, only if the dc/ac voltages and currents in the circuit are suitably fixed. The operating point or bias point or quiescent point(or simply Q-point) is the voltage or current which, when applied to a device, causes it to operate in a certain desired fashion.

Need for BIAS STABILIZATION

After fixing the operating point suitably, it should remains there only. But there are two reasons for the operating point to shift.

The transistor parameters such as VBE & β changes from device to device

Transistor parameters are also temperature dependent. Since the collector current is Ic = β IB + (1+ β) Ico (β, IB and Ico are temperature dependent)

β of a transistor is strongly dependent on temperature. As temp. increases, β increases

The junction voltage VBE is also a function of temperature. As temperature increases, VBE decreases at the rate of 2.5mV/C

IB also increases

The Ico is a strong function of temperature that doubles for every 100C rise in temperature

The net result of these increase in β, IB and Ico is that, Ic increases, shifting the operating point in the upward direction. This causes distortion in the output i.e. Q-point is not stable against temperature variation.

As temperature increases, the leakage current Ico increases. This increases the collector current. The power dissipated at the collector junction also increases. This further rises the temperature of the collector junction. This increases the leakage current & thus in turn increases the Ic. Thus the whole cycle repeats again. Such cumulative increase in Ic will ultimately shift the operating point into the saturation region. This situation is called ‘Thermal Runway.’ The extent to which the stabilization of transistor is successful is measured by stability factor.

Method of biasing

It must establish the operating point at the centre of the load line

It must stabilize the operating point against temperature variations

It must provide the operating point independent of β of transistor

-that can be achieved by Fixed bias or Collector to base bias or Self bias/Emitter bias or potential Divider bias though all biases have some drawback.

Hope it helps you...