# Electronic Devices and Circuits Questions and Answers – Stabilization against Variations in VBE and Beta for Self Bias Circuit

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Stabilization against Variations in VBE and Beta for Self Bias Circuit”.

1. What is the function of a bias circuit?
a) To simplify the circuit
b) To provide a non – linear output
c) To optimize the power
d) To provide steady current or voltage

Explanation: The Q – point of a device is the direct current or voltage of a device when no input is applied. The bias circuit is a part of the device with provides the steady current or voltage. It is designed by determining the necessary voltage and current levels across each resistor.

2. How many main types of bias circuits are there in bipolar transistors?
a) 3
b) 4
c) 5
d) 6

Explanation: There are five main type of biasing circuits. They are Fixed Bias, Collector – to – Base Bias, Voltage Divider Bias, Fixed Bias with Emitter Resistor and Emitter Bias.

3. Why do we require RE for a good stable bias circuit?
a) To obtain a current IE sensitive to β and VBE
b) To obtain a current IB sensitive to β and VBE
c) To obtain a current IE insensitive to β and VBE
d) To obtain a current IB insensitive to β and VBE

Explanation: The aim of having a bias circuit is to maintain a stable current or voltage, regardless of other parameters changing. Hence, we need RE to get a stable IE current, without RE the value of IE changes drastically with a small change in β.

4. In the circuit given below, assume VCC = 4V, VBB = 5V, VBE = 0.7V, RE = 3.3kΩ, RC = 10kΩ, RB = 40kΩ and β = 100. What is the current IE?

a) 1.388mA
b) 0.012mA
c) 10.47mA
d) 1.165mA

Explanation: Given the values VCC = 4V, VBB = 5V, VBE = 0.7V, RE = 3.3kΩ, RC = 10kΩ, RB = 40kΩ and β = 100. To find the value of IE, we substitute the values in this equation:
IE = (VBB – VBE ) / (RE + (RB / β + 1))
IE = (5 – 0.7) / (3.3 + (40 / 100 + 1))
IE = 4.3 / (3.3 + 0.396) = 1.388mA.

5. Why is it essential to stabilize the operating point of a circuit?
a) Thermal runaway, power efficiency, individual variations
b) Temperature dependency of IB, individual variations, power efficiency
c) Thermal runaway, Temperature dependency of IC, individual variations
d) Temperature dependency of IC, thermal runaway, cost efficiency

Explanation: It is essential to stabilize the operating point of a circuit to prevent thermal runaway, temperature dependency of IC and individual variations. Thermal runaway occurs because of the self – destruction of an unstable transistor. Temperature dependency on IC causes major variations in IE. Individual variations of parameters such as β and VBE cause huge fluctuations in IE.
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6. In an ideal stable self – bias circuit, what should be the current IB with respect to IC?
a) IB should be 20% of IC
b) IB should be 5% of IC
c) IB should be 10% of IC
d) IB should be 50% of IC

Explanation: Ideally, the resistor values are designed in a way that the voltage drops across RE is approximately 10% of Vcc and IB should be 10% of Ic. Hence, stable self – bias circuits work best at low power supply voltages.

7. What values of VBE and β would provide maximum stability?
a) VBE = 5V, β = 50
b) VBE = 4V, β = 100
c) VBE = 4.5V, β = 50
d) It is irrelevant

Explanation: In a stable bias circuit, the variations in the value of VBE and β do not affect the stability of the system. The current through the emitter remains unchanged.

8. What is the stability factor if RE = 8kΩ and RTH = 11kΩ?
a) 1.335
b) 2.375
c) 1.727
d) 0.272

Explanation: Stability factor is calculated by the following equation. On plugging in the given values RE = 8kΩ and RTH = 11kΩ, we get:
S = 1 + RTH / RE
S = 1 + 11 / 8 = 2.375.

9. Collector base feedback and emitter feedback combined together provide stability for self – bias circuits.
a) True
b) False

Explanation: Collector base feedback and emitter feedback combined provide stability for self – bias circuits. This is because the emitter base junction is forward biased due to the voltage drop across RE.

10. Where does degeneration take place in a self – bias circuit?
a) Across RB
b) Across RE
c) No degeneration occurs
d) Across RC

Explanation: Degeneration or negative feedback occurs across RE which in turn stabilizes the fluctuations of current IE due to temperature changes and variations in VBE and β.

Sanfoundry Global Education & Learning Series – Electronic Devices and Circuits.

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