# Electronic Devices and Circuits Questions and Answers – Comparison of Transistor Amplifier

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Comparison of Transistor Amplifier”.

1. Which type of amplifiers exhibits the current gain approximately equal to unity without any current amplification?
a) CE
b) CB
c) CC

Explanation: In common base amplifier, input signal is applied at emitter terminal while the amplified output signal is obtained at the collector terminal with respect to ground.
For the AC signals, the base terminal is specifically connected to ground through the capacitor.
Even, the output resistance is very high & hence, the current gain is approximately equal to unity. Due to this, there is no possibility of current amplification. Consequently, the CB amplifier exhibits high voltage gain.

2. Why is the Darlington configuration not suitable for more than two transistors?
a) Because leakage current increases and voltage gain decreases with multiple numbers of transistors
b) Because leakage current decreases and voltage gain increases with multiple numbers of transistors
c) Because leakage current as well as voltage gain increases with multiple numbers of transistors
d) Because leakage current as well as voltage gain decreases with multiple numbers of transistors

Explanation: As the number of transistors increases, the leakage current also increases. The leakage current gets multiplied by the current gain of Darlington configuration.
Generally, the voltage gain of CC configuration is nearly equal to ‘1’ but the voltage gain of Darlington configuration is very less than ‘1’.
Therefore, if we increase the number of transistors in Darlington configuration the voltage gain will ultimately reduce. But, in order to prevent these likely undesirable conditions, Darlington configuration is completely inappropriate for more than two transistors.

3. What should be the level of input resistance to allow the occurrence of source loading in common base amplifier configuration?
a) low
b) high
c) moderate
d) stale

Explanation: As per the configuration of CB amplifier, it is evident that its input resistance is very low but its output resistance is enormously high.
However, the lower value of input resistance allows the stipulation of source loading in common base amplifier circuit.
Thus, there is no current amplification because of unity current gain. These all reasons eventually add to high level of voltage gain.

4. Which among the below assertions is not a relevant property of CE amplifier?
a) High voltage gain
b) High current gain
c) High input resistance
d) High output resistance

Explanation: The voltage gain, current gain and input resistance CE amplifiers are utterly high but it has low output resistance.
The collector resistor (Rc) performs the purpose of controlling the collector current. Input and emitter resistors are adopted for biasing of transistor in an active region so that it becomes possible for the transistor to function as an amplifier.
Due to high current gain at the output of RC coupled CE amplifier, the resistance level at the output is exceedingly low.

5. What is the phase-shift between input and output voltages of CE amplifier?
a) 90°
b) 120°
c) 180°
d) 270°

Explanation: During the amplification method of RC coupled CE amplifier, there is a phase shift of about 180o between input and output. Basically, the output is said to be reversed version of input.
The magnitude of output voltage becomes higher as compared to that of input signal but the shape is correctly similar to that of an input signal.
But, the input ac signal gets amplified along with the phase-shift of 180o between input and output.
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6. Which capacitor is used to block DC portion by allowing to pass only AC portion of the amplified signal to load?
a) Input Coupling Capacitor
b) Bypass Capacitor
c) Output Coupling Capacitor
d) Both coupling and bypass capacitor

Explanation: In RC coupled CE amplifier, the transistor is connected in common-emitter (CE) configuration and capacitors C1 & C2 are coupling capacitors.
Input coupling capacitor is used for coupling the ac input voltage to the transistor base. Inversely, the output coupling capacitor (C2) is used for coupling an output of an amplifier to the load resistance or to the next stage of an amplifier.
Besides these, input coupling capacitor blocks any DC element present in AC input voltage & couples only AC component of input signal whereas bypass capacitor offers a low reactance to the amplified AC signal.

7. The configuration in which voltage gain of transistor amplifier is lowest is ____________
a) common collector
b) common emitter
c) common base
d) common emitter & base

Explanation: In common collector configuration (also known as the emitter follower) because the emitter voltage follows that of the base. Offering a high input impedance and a low output impedance it is extensively used as a buffer. The voltage gain is unity, even though current gain is high. The input and output signals are in phase.

8. The configuration in which current gain of transistor amplifier is lowest is ___________
a) common collector
b) common base
c) common emitter
d) common emitter & base

Explanation: In Common base configuration, the input impedance is very low; While offering a high output impedance. Although the voltage is high, the current gain is low and the overall power gain is also low when compared to the other transistor configurations available. Thus, there is no current amplification because of unity current gain.

9. The configuration in which input impedance of transistor amplifier is lowest is ___________
a) common collector
b) common emitter
c) common base
d) common emitter & base

Explanation: In Common base configuration, the input impedance is very low; While offering a high output impedance. Although the voltage is high, the current gain is low and the overall power gain is also low when compared to the other transistor configurations available.

10. The configuration in which output impedance of transistor amplifier is highest is ___________
a) common collector
b) common base
c) common emitter
d) common collector and base

Explanation: In Common base configuration, the input impedance is very low; While offering a high output impedance. Although the voltage is high, the current gain is low and the overall power gain is also low when compared to the other transistor configurations available.

11. What should be the gain of an amplifier at 20 kHz if the half power frequencies are fL = 20 Hz and fH = 15 kHz along with mid band gain = 80?
a) 22.76
b) 45.09
c) 40.08
d) 48.07

Explanation:
fL = 20 Hz
fH = 15 kHz
Av(mid) = 80

To determine: Voltage gain at 20 kHz
Formula : Av = Av(mid) / √1 + (f/fH)2

To determine the voltage gain at 20 kHz, we recognize that,

Av = Av(mid) / √1 + (f/fH)2
= 80 / √1 + (20 k/15k)2
= 48.07.

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