Electronic Devices and Circuits Multiple Choice Questions on “Self-Bias”.
1. The collector current (IC) that is obtained in a self biased transistor is_________
A. (VTH – VBE)/RE
B. (VTH + VBE)/RE
C. (VTH – VBE)/RE
D. (VTH + VBE)/RE
Answer: A
Clarification: 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, IC=IBβ.
2. The collector to emitter voltage (VCE) is obtained by_________
A. VCC – RCIC+RBIB
B. VCC – RCIC-REIE
C. VCC + RCIC
D. VCC + RCIB
Answer: B
Clarification: 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.
3. The thermal runway is avoided in a self bias because_________
A. of its independence of β
B. of the positive feedback produced by the emitter resistor
C. of the negative feedback produced by the emitter resistor
D. of its dependence of β
Answer: C
Clarification: 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 IC which is responsible for the damage is reduced by decreased output signal.
4. When the temperature is increased, what happens to the collector current after a feedback is given?
A. it remains same
B. it increases
C. it cannot be predicted
D. it decreases
Answer: D
Clarification: 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.
5. What is the Thevenin’s voltage (VTH) in a self bias shown below?
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A. VCCR2/R1+R2
B. VCCR1/R1+R2
C. VCCR2/R1-R2
D. VCCR2/R1-R2
Answer: A
Clarification: The base current cannot be obtained directly from the KVL or KCL applications. The VCC and VBE cannot come under a single equation. So, the circuit is changed with a Thevenin’s voltage (VTH) and Thevenin’s resistance.
6. What is the Thevenin’s resistance (RTH) in a self bias shown below?
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A. R1R2/R1+R2
B. R2/R1+R2
C. R1R2/R1-R2
D. R1/R1-R2
Answer: A
Clarification: The base current cannot be obtained directly from the KVL or KCL applications. A potential divider network is formed by R1 and R2.The VCC and VBE cannot come under a single equation. So, the circuit is changed with a Thevenin’s resistance.
7. The stability factor for a self biased transistor is_________
A. 1 – RTH/RE
B. 1 + RTH/RE
C. 1 + RE/RTH
D. 1 – RE/RTH
Answer: B
Clarification: 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)( RTH+ RE)/ (1+ β)RE+ RTH.
8. In the circuit, the transistor has a large β value (VBE=0.7V). Find the current through RC.
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A. 0.5mA
B. 2mA
C. 1mA
D. 1.6mA