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**Q Point Instability due to Temperature:**

The junction temperature of a transistor is dependent on the amount of current flowing through the transistor. Due to increase in temperature the following parameters of a transistor will change:

1 $V_{BE}$: The base to emitter voltage decreases at a rate of 2.5 mV/? with increase in temperature. The base current $I_{B}$ will therefore increase and it will force $I_{C}$ to change, and hence the Q point.

- Current gain $\beta$dc: The current gain $\beta de$ of a transistor is temperature dependent. As $l_{C}$ =$\beta dc I_{B}$, changes in $\beta dc$ will change the collector current $I_{C}$

3.Reverse saturation current (l)

.The reverse saturation current of the reverse biased CB junction flows due to the minority carriers hence it is depend on temperature

- We know that,

IC= $\beta dc I_{B} + (1+ \beta dc)I_{CBO}$

Therefore change in ICBO due to temperature will force the collector current $I_{C}$ and hence the Q point to change.

To overcome this problem the biasing circuit must in kind of “temperature compensation", or "temperature stability”so that the changes in the values of these parameters can be kept under control.

**Q Point Instability due to Changes in $\beta dc$:**

The current gain $\beta$dc for a given type of transistor normally has a very wide tolerance. The value of $\beta$dc may typically range from 50 to 150 or more depending on the value of $I_{C}$. The wide tolerance can seriously affect the transistor bias conditions

**Q Point Instability due to Variation in Parameters from One Device to Other:**

Two transistors of identical number (e.g. BC 147) do not have exactly the same characteristics. Important parameters such as $\beta$dc will differ in value from one transistor to the other.

Hence if we replace one transistor by the other of same number, the Q point is going to get shifted

The characteristics shown in Fig, (a) explains the variation in the Q point due to device variation.

These are the output characteristics of two transistors of same number. The dashed characteristics are for the transistor having a $\beta$dc much larger than the other one.

Initially when the first transistor with small value of $\beta$dc being used. $Q_{1}$ represents the Q point. But if this transistor becomes faulty and hence replaced by the other transistor, then the Q point gets shifted to $Q_{2}$ as shown in Fig. (a)