Electronic Devices and Circuits Questions and Answers – Voltage Amplifiers

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

1. The emitter of a swamped amplifier __________
a) is grounded
b) has no dc voltage
c) has an ac voltage
d) has no ac voltage
View Answer

Answer: c
Explanation: The resistance of the emitter diode r’e equals 25mV / IE and depends on the temperature. Any change in r’e will change the voltage gain in CE amplifier. In some applications, a change in voltage is acceptable. But in many applications a stable voltage gain is required.
To make it stable, a resistance rE is inserted in series with the emitter and therefore emitter is no longer ac grounded.
Because of this the ac emitter current flows through rE and produces an ac voltage at the emitter. If rE is much greater than r’e almost the entire ac input signal appears at the emitter, and the emitter is bootstrapped to the base for ac as well as for dc.
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2. A swamped amplifier has ________ input impedance as compared to CE stage amplifier.
a) smaller
b) equal
c) larger
d) zero
View Answer

Answer: c
Explanation: A swamped amplifier reduces variations in voltage gain by increasing the ac resistance of the emitter circuit to get a stable voltage gain. It is also referred as gain-stabilized amplifier. That’s why the input impedance is larger.

3. We can increase _______ to reduce distortion of an amplified signal.
a) Collector resistance
b) Emitter feedback resistance
c) Generator resistance
d) Load resistance
View Answer

Answer: b
Explanation: In a negative feedback amplifier, when emitter feedback resistance is increased, the magnitude of the gain of the amplifier reduces; but remains stable. Since the gain reduces the distortion and the noise in the signal is also reduced; as the overall signal we get at output is the input signal multiplied by the gain of the amplifier.

4. A swamped amplifier uses________
a) Base bias
b) Positive feedback
c) Negative feedback
d) A grounded emitter
View Answer

Answer: c
Explanation: A swamped amplifier reduces variations in voltage gain by increasing the ac resistance of the emitter circuit to get a stable voltage gain. As it involves reduction in gain, it is a negative feedback.

5. In a swamped amplifier, the effects of the emitter diode become _________
a) Important to the voltage gain
b) Critical to input impedance
c) Significant to analysis
d) Unimportant
View Answer

Answer: d
Explanation: To make the voltage gain stable, a resistance rE is inserted in series with the emitter and therefore emitter is no longer ac grounded.
Because of this the ac emitter current flows through rE and produces an ac voltage at the emitter. If rE is much greater than r’e (emitter diode resistance) almost the entire ac input signal appears at the emitter, and the emitter is bootstrapped to the base for ac as well as for dc.
Thus the effects of the emitter diode become unimportant in swamping amplifier.
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6. What is the function of feedback resistor?
a) To increase voltage gain
b) Reduce distortion
c) Decrease collector resistance
d) Decrease input impedance
View Answer

Answer: b
Explanation: In a negative feedback amplifier, when emitter feedback resistance is increased, the magnitude of the gain of the amplifier reduces; but remains stable. Since the gain reduces the distortion and the noise in the signal is also reduced; as the overall signal we get at output is the input signal multiplied by the gain of the amplifier. Therefore, feedback in an amplifier helps in reducing distortion.

7. If the emitter bypass capacitor is open , the ac output voltage will ____________
a) decrease
b) increase
c) remains the same
d) equals zero
View Answer

Answer: a
Explanation: The emitter bypass capacitor is connected in parallel to the emitter resistance. The use of this connection is to provide an ac ground at the emitter terminal of the transistor. This has the effect of increasing the circuit voltage gain.
The bypass capacitor is normally a high-value component that provides little reactance at the lowest circuit operating frequency. Therefore, in the absence of this bypass capacitor, the circuit voltage gain will reduce.

8. If any capacitor is open, the ac output voltage wills ____________
a) Decrease
b) Increase
c) Remains the same
d) Equals zero
View Answer

Answer: a
Explanation: Coupling capacitors (or dc blocking capacitors) are use to decouple ac and dc signals so as not to disturb the Q-point of the circuit when ac signals are provided at the input. Bypass capacitors are used to force signal currents around elements by providing a low impedance path at the frequency. The voltage drop across the resistor provides a negative bias to the emitter terminal, thereby reducing its DC gain, while the AC gain will also reduce as the bypass capacitor which was providing low impedance path is left open.

9. If the emitter resistor is open, the ac input voltage will __________
a) Decrease
b) Increase
c) Remains the same
d) Equals zero
View Answer

Answer: b
Explanation: An AC signal amplifier circuit is mainly used to stabilize the DC biased input voltage to the amplifier and thus only amplify the required AC signal. This stabilization is achieved by the use of an Emitter Resistance which provides the required amount of automatic biasing needed for a common emitter amplifier. If the emitter resistor is open, then the input ac voltage will eventually increase.
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10. The input impedance of the base increases when ______
a) Beta increases
b) Supply voltage increases
c) Beta decreases
d) AC collector resistance increases.
View Answer

Answer: a
Explanation: Input impedance of the base is given as = beta * rE.
Therefore, beta(gain ratio) is directly proportional input impedance of the base.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn