# Electronic Devices and Circuits Questions and Answers – Input Bias and Offset Currents of the Bipolar Differential Amplifier

This set of Electronic Devices and Circuits Question Paper focuses on “Input Bias and Offset Currents of the Bipolar Differential Amplifier”.

1. Which transistor bias circuit provides good Q-point stability with a single-polarity supply voltage?
a) Base bias
b) Collector feedback bias
c) Voltage divider bias
d) Emitter bias

Explanation: When the transistor starts operating, temperature at the junction increases. Hence IC increases. As a result of which, Ie increases. Due to this increase in Ie, voltage drop across Re increases. This reduces the forward voltage across the emitter. Thus Ib reduces.

We know that, for any value of ICO, IC = β * Ib +(1+ β) ICO

where ICO =reverse saturation current (increases with temperature)
β = gain

Therefore, β and ICO increases and at the same time there is decrease in Ib. Hence above equation confirms that IC can be maintained within limits. Thus the circuit is more thermally stable and the operating point is more stable.

2. Ideally, for non linear operation, a transistor should be biased so that the Q-point is ________
a) near saturation
b) near cut off
c) where Ic is maximum
d) halfway between cut off and saturation

Explanation: If Q-point is near to saturation then positive clipping of input signal, and to cutoff then negative clipping of input signal, if IC is maximum then Q-point is in saturation region. Linear operation means output varies according to input without any distortion.

3. The most stable biasing technique used is the ____________
a) voltage-divider bias.
b) base bias
c) emitter bias
d) collector bias

Explanation: Voltage divider biasing is commonly used because of the main reason that the transistor under this biasing always remains in the active region.
In voltage divider biasing, the voltages at the transistor’s base, emitter and collector all depend upon the external circuit i.e. the biasing resistors R1 and R2 whose valued are fixed thus variation with beta is not present here.
However, voltage divider biasing has the advantage that its stability factor is greater than that of collector feedback biasing, that’s why it is used.

4. What is the Q-point for a fixed-bias transistor with IB = 75 µ A , βDC = 100, VCC = 20 V, and RC = 1.5 K Ohm?
a) VC = 0 V
b) VC =12.25V
c) VC = 8.75 V
d) VC = 20V

Explanation: VCE=Vcc-IC Rc (VCE = VC , β = IC/IB => IC= β *IB)

Vc = Vcc – β * IB * Rc
=20-100x75x10-6 x1.5x 10-3
=8.75V.

5. Emitter bias requires _______
a) only a positive supply voltage
b) only a negative supply voltage
c) no supply voltage
d) both positive and negative supply voltage

Explanation: If a dual power supply is there, then it is the most useful, and provides zero bias voltage at the emitter or collector for load. The negative supply is used to forward-bias the emitter junction through emitter resistor. The positive supply is used to reverse-bias the collector junction. Also has good stability.

6. Which transistor bias circuit arrangement has poor stability because its Q-point varies widely with β DC?
a) base bias
b) voltage-divider bias
c) emitter bias
d) collector bias

Explanation: In base bias IB, RB and IC are fixed therefore it is also called fixed bias transistor. When the temperature is vary then the Q point is also varied. Therefore base bias has no stabilization.

7. What is the most common bias circuit?
a) Base
b) Collector
c) Emitter
d) Voltage-divider

Explanation: Due to the best stabilization, voltage divider circuit is commonly used. Under this biasing technique, the transistor always remains in the active region. In voltage divider biasing, the voltages at the transistor’s base, emitter and collector all depend upon the external circuit i.e. the biasing resistors R1 and R2 whose valued are fixed thus variation with beta is not present here.

8. Voltage-divider bias has a relatively stable Q-point, as does_________
a) base bias
b) collector-feedback bias
c) emitter bias
d) both base and emitter bias

Explanation: The collector feedback biasing is also beta-independent. It has the same circuit as the voltage divider biasing except there is only one resistor used other than load resistor Rc. However, voltage divider biasing has the advantage that its stability factor is greater than that of collector feedback biasing.

9. The linear (active) operating region of a transistor lies along the load line below ________ and above ___________
a) cut off, saturation
b) saturation, cut off
c) active, saturation
d) cut off, active

Explanation: Q-point is generally taken to be the intersection point of load line with the output characteristics of the transistor. That’s why; the linear operating region of a transistor must lie along the load line below saturation and above the cut off.

10. The input resistance of the base of a voltage-divider biased transistor can be neglected _________
a) at all times
b) only if the base current is much smaller than the current through R2 (the lower bias resistor)
c) at no time
d) only if the base current is much larger than the current through R2 (the lower bias resistor

Explanation: The input resistance can be neglected if the values of R1 and R2 are very large. Therefore the base current becomes very small.

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

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