Test: Self Bias - UPSC MCQ

The collector current is analysed by the DC analysis of a transistor. It involves the DC equivalent circuit of a transistor. The base current is first found and the collector current is obtained from the relation, I_C=I_Bβ.

The self destruction of a transistor due to increase temperature is called thermal run away. It is avoided by the negative feedback produced by the emitter resistor in a self bias. The I_C which is responsible for the damage is reduced by decreased output signal.

The base current cannot be obtained directly from the KVL or KCL applications. The V_CC and V_BE cannot come under a single equation. So, the circuit is changed with a Thevenin’s voltage (V_TH) and Thevenin’s resistance.

The base current cannot be obtained directly from the KVL or KCL applications. A potential divider network is formed by R₁ and R₂.The V_CC and V_BE cannot come under a single equation. So, the circuit is changed with a Thevenin’s resistance.

The stability of the circuit is inversely proportional to the stability factor. The emitter resistor is very large when compared to the Thevenin’s resistance. When β is not that large, then S=(1+ β)( R_TH+ R_E)/ (1+ β)R_E+ R_TH.

We know, I_C=V_TH-V_BE/R_E
=9*3/9=3V. I_C=3-0.7/2.3=1mA.

 For silicon, V_BE=0.8V, V_CE=0.2V. I_C=V_TH-V_BE/R_E. By pitting the values, we have V_TH=1.3V. R₂ can be found from, V_CCR₂/R₁+R₂. We get R₂=40KΩ.

For all practical purposes, αac=αdc=α and practical values in commercial transistors range from 0.9-0.99. It is the measure of the quality of a transistor. Higher is the value of α, better is the transistor in the sense that collector current approaches the emitter current.

H_J-C is thermal resistance between junction and case and H_C-A is thermal resistance between case and ambient. The circuit designer has no control over H_J-C. So, a proper approach to dissipate heat from case to ambient is through heat sink.

The Self-bias circuit is also known as voltage divider bias.

The Self-bias circuit can be simplified by using the Thevenin theorem:

The stability factor for the self-bias circuit is:

So if R_E increases or R_B decreases,

then the circuit becomes more stable.

Hence the Assertion given is false.

The base resistance R_B or R_TH provides the required voltage to the base terminal and should be selected such that the emitter junction is forward biased.

Resistor R_E results in negative feedback action which helps in keeping Ic stable,

Thus R_E is responsible for stable Ic.

Hence, the Reason given is true.

So, option (d) is correct.