Amplifiers
Amplifiers
Learning Objectives:
- Understand the fundamental role of amplifiers in electronics, which is to increase the amplitude or power of electrical signals.
- Learn about different types of amplifiers, including voltage amplifiers, current amplifiers, power amplifiers, and operational amplifiers (op-amps).
- Recognize the key characteristics of amplifiers, including gain, bandwidth, input and output impedance, linearity, and efficiency.
- Understand common amplifier configurations, including common-emitter, common-base, common-collector, and common-source, and their applications.
An Amplifier is an electronic device or circuit which is used to increase the magnitude of the signal applied to its input.
Gain of an Amplifier:- Voltage gain (A$_v$) = $\dfrac{\mathrm{Output\ voltage}}{\mathrm{Input\ voltage}}$
- Current gain: (A$_i$) = $\dfrac{\mathrm{Output\ current}}{\mathrm{Input\ current}}$
- Power gain = A$_v$ $\times$ A$_i$
Yves-Laurent Allaert, CC BY-SA 3.0, via Wikimedia Commons
Solved Example: 9912-01
As compared to FET, BJT has ________ input impedance and ________ output impedance.
A. low, low
B. low, high
C. high, high
D. high, low
Correct Answer: B
Solved Example: 9912-02
An Amplifier circuit of voltage gain 100 gives 10V output, the value of input voltage is:
A. 1000 V
B. 100 mV
C. 10 V
D. 10 mV
Correct Answer: B
Solved Example: 9912-03
The output voltage of a voltage amplifier is found to decrease by 20% when a load resistance of 1 K$\Omega$ is connected. What is the output resistance of the amplifier?
A. 200 $\Omega$
B. 250 $\Omega$
C. 1000 $\Omega$
D. 5000 $\Omega$
Correct Answer: B
Solved Example: 9912-04
The power input to an amplifier is 2 $\mu$W. The power gain of the amplifier is 40 dB. The output power of amplifier is:
A. 80$\mu$W
B. 200$\mu$W
C. 80 mW
D. 20 mW
Correct Answer: D
Solved Example: 9912-05
The efficiency of a class B push-pull amplifier is:
A. 90-95%
B. 95-100%
C. 60-65%
D. 75-80%
Correct Answer: D
Solved Example: 9912-06
In an amplifier, the signal handling capacity will be high when the operating point is selected:
A. At the extremities of the active region
B. Close to cut-off region
C. Close to saturation
D. In the middle of the active region
Correct Answer: D
Solved Example: 9912-07
Negative feedback in an amplifier:
A. Reduce gain
B. Increases frequency and phase distortions
C. Reduces bandwidth
D. Increases noise
Correct Answer: A
Solved Example: 9912-08
Find P$_{in}$ for an amplifier with a power gain of 20 and an output power of 1 W.
A. 50 mW
B. 5 W
C. 50 W
D. 0.5 W
Correct Answer: A
Solved Example: 9912-09
Overmodulation of transmitter signal is corrected by the adjustment of the:
A. Speech amplifier
B. Power amplifier
C. RF oscillator
D. Frequency doubler
Correct Answer: A
Solved Example: 9912-10
When amplifiers are cascaded:
A. The gain of each amplifier is increased
B. A lower supply voltage is required
C. The overall gain is increased
D. Each amplifier has to work less
Correct Answer: C