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.

1. 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)