Test: Self Bias - Electrical Engineering (EE) 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 collector to emitter voltage is obtained in order to find the operating point of a transistor. It is taken when there is no signal applied to the transistor. The point thus obtained lies in the cut off region when the transistor is used as a switch.

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.

Before the feedback is applied, when the temperature is increased, the reverse saturation increases. The collector current also increases. When the feedback is applied, the drop across the emitter resistor increases with decreasing collector current and the thermal runway too.

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.