amplifiers - wifistudy.com...depending upon the method of coupling one stage to the other, there are...

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Amplifiers

22.04.2019

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Types of Amplifiers

Based on number of stages:-

Depending upon the number of stages of Amplification, there are Single-stage amplifiers

and Multi-stage amplifiers.

Single-stage Amplifiers− This has only one transistor circuit, which is a single-stage

amplification.

Multi-stage Amplifiers− This has multiple transistor circuit, which provides multi-

stage amplification.

Based on the Coupling method:-

Depending upon the method of coupling one stage to the other, there are RC coupled,

Transformer coupled and direct coupled amplifier

RC Coupled amplifier− A Multi-stage amplifier circuit that is coupled to the next

stage using resistor and capacitor (RC) combination can be called as a RC coupled

amplifier.

Transformer Coupled amplifier− A Multi-stage amplifier circuit that is coupled to

the next stage, with the help of a transformer, can be called as a Transformer coupled

amplifier.

Direct Coupled amplifier− A Multi-stage amplifier circuit that is coupled to the next

stage directly, can be called as a direct coupled amplifier.

Based on Biasing Conditions:-

Depending upon their mode of operation, there are class A, class B and class C

amplifiers.

Class A amplifier− The biasing conditions in class A power amplifier are such that

the collector current flows for the entire AC signal applied.

Class B amplifier− The biasing conditions in class B power amplifier are such that

the collector current flows for half-cycle of input AC signal applied.

Class C amplifier− The biasing conditions in class C power amplifier are such that

the collector current flows for less than half cycle of input AC signal applied.

Class AB amplifier− The class AB power amplifier is one which is created by

combining both class A and class B in order to have all the advantages of both the

classes and to minimize the problems they have.

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Based on the input signals:-

Depending upon the magnitude of the input signal applied, they can be categorized as

Small signal and large signal amplifiers.

Small signal Amplifiers− When the input signal is so weak so as to produce small

fluctuations in the collector current compared to its quiescent value, the amplifier is

known as Small signal amplifier.

Large signal amplifiers− When the fluctuations in collector current are large i.e.

beyond the linear portion of the characteristics, the amplifier is known as large signal

amplifier

Based on Types of Feedbacks:-

The process of injecting a fraction of output energy of some device back to the input

is known as Feedback.

It has been found that feedback is very useful in reducing noise and making the

amplifier operation stable.

Depending upon whether the feedback signal aids or opposes the input signal, there

are two types of feedbacks used.

Positive Feedback

The feedback in which the feedback energy i.e., either voltage or current is in phase

with the input signal and thus aids it is called as Positive feedback

Though the positive feedback increases the gain of the amplifier, it has the

disadvantages such as

o Increasing distortion

o Instability

It is because of these disadvantages the positive feedback is not recommended for the

amplifiers.

If the positive feedback is sufficiently large, it leads to oscillations, by which

oscillator circuits are formed.

Comparison between Positive and

Negative feedback

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Parameter Positive Feedback Negative Feedback

Overall phase shift 0 or 360 degrees 180 degrees

Input and output

voltage, noise increases due to feedback

Decreases due to

feedback

Feedback signal and

input signal In phase Out of phase

Gain increases decreases

Stability poor better

applications or use oscillators amplifiers

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Comparison between different type of

coupling method

Parameter R-C Coupling Transformer

Coupling Direct Coupling

Coupling Element R & C Transformer None

Type of coupling

signal Only AC Only AC DC & AC

Ability to Block

DC YES YES NO

Impedance

Matching No YES NO

DC amplification No No Yes

Frequency

Response Drop Off at low

frequency Not Uniform Improved at low

frequency

Application AF amplifiers Power Amplifiers OP-AMP

A VOLTAGE AMPLIFIER is an amplifier in which the output signal voltage is

larger than the input signal voltage. In other words, a voltage amplifier amplifies the

voltage of the input signal.

A POWER AMPLIFIER is an amplifier in which the output signal power is greater

than the input signal power.

Most power amplifiers are used as the final amplifier (stage of amplification) and

control (or drive) the output device. The output device could be a speaker, an antenna,

or the heads on a tape recorder.

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Voltage amplifier Power Amplifier

In voltage amplifier the amplitude of

input A.C signal is small.

In power amplifier the amplitude of

input A.C signal is large.

In voltage amplifier the collector

current is low, about 1 mA.

In power amplifier the collector

current is very high above greater than

100mA.

The transistor used can dissipate less

heat produced during its operation.

The transistor used can dissipate more

heat produced as compared to voltage

amplifier during its operation.

RC coupling is used in voltage

amplifier.

In power amplifier invariably

transformer coupling is used.

The transistor used has thin base to

handle low current.

The transistor used has thick base to

handle large current.

In voltage amplifier the A.C power

output is low.

In power amplifier the A.C power

output is high.

The physical size of transistor used is

usually small and is known as low or

medium power transistor.

The physical size of transistor used is

usually large and is known as power

transistor.

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Class Class- A Class- B Class-C Class- AB

Operating

Cycle 360◦ 180◦ Less than

180◦ 180◦ to 360◦

Position Of

“Q” Centre On X-axis Below X-

axis Above X-

axis

Efficiency 25% or 50% 78.5% High(Almost

100%) 50% to

78.5%

Distortion Absent Present

More than

Class-A

Highest Present less

than Class-B

Comparison of Amplifier

Classes

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1.

In a trans-conductance amplifier, the device output

a) Voltage depends upon the input voltage.

b) Voltage depends upon the input current.

c) Current depends upon the input voltage.

d) Current depends upon the input current.

2.

A trans-conductance amplifier has

a) High input impedance and low output impedance.

b) Low input impedance and high output impedance.

c) High input and output impedances.

d) Low input and output impedances.

3.

In a trans-conductance amplifier it is desirable to have

a) A large input resistance and a large output resistance

b) A large input resistance and a small output resistance

c) A small input résistance and a large output resistance

d) A small input resistance and a small output resistance

4.

For a trans-conductance amplifier, input and output resistances are respectively

a) ∞ and 0 b) 0 and ∞

c) 0 and 0 d) ∞ and ∞

5.

The input resistance Ri , and output resistance R0 of an ideal current amplifier, in ohms are

a) 0 and 0 b) 0 and ∞

c) ∞ and 0 d) ∞ and ∞

6.

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Feedback in amplifier always helps in

a) Controlling its output.

b) Increasing its gain.

c) Reducing its input impedance.

d) Stabilizes its gain.

7.

Negative feedback in an amplifier

a) Reduces gain.

b) Increases frequency and phase distortions.

c) Reduces bandwidth.

d) Increases noise.

8.

Negative feedback is used to

a) Increase the gain.

b) Increase stability

c) Design as oscillator

d) All of the above.

9.

Negative feedback in amplifiers

a) Improves the signal to noise ratio at the input.

b) Improves the signal to noise ratio at the output.

c) Does not affect the signal to noise ratio at the output

d) Reduces distortion.

10.

The voltage gains of an amplifier has an open loop gain of -100. If 4% of the output is feed

back in a degenerative loop, what is the closed loop gain of the amplifier?

a) -33.3

b) -25

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c) -20

d) +25

11.

An amplifier without feedback has a gain of 1000. What is the gain with a negative

feedback of 0.009?

a) 900

b) 125

c) 100

d) 10

12.

In a negative feedback amplifier using voltage – series (,i.e., voltage sampling, series

mixing ) feedback.

a) Ri decreases and R0 decreases

b) Ri decreases and R0 increases

c) Ri increases and R0 decreases

d) Ri increase and R0 increases

(Ri and R0 denote the input and output resistances respectively )

13.

In the shunt series feedback amplifier, the basic amplifier and the feedback network are

parallel connected at the input and series connected at the input and series connected at the

output. The signal sampled and the summing down will be respectively

a) Current and current

b) Current and voltage

c) Voltage and current

d) Voltage and voltage

14.

Voltage series feedback (also called series – shunt feedback) results in

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a) Increases in both input and output impedances.

b) Decreases in both input and output impedances.

c) Increases in input impedance and decrease in output impedance.

d) Decrease in input impedance and increase in output impedance.

15.

An amplifier has an open-loop gain of 100. And input impedance of 1 kΩ, and an output

impedance of 100Ω. A feedback network with a feedback factor of 0.99 is connected to the

amplifier in a voltage series feedback mode. The new input and output impedances.

Respectively, are

a) 10Ω and 1Ω

b) 10Ω and 10 Ω

c) 100KΩ and 1Ω

d) 100 kΩ and 1 kΩ

16.

In a shunt – shunt negative feedback amplifier, as compared to the basic amplifier

a) Both, input and output impedances, decrease.

b) Input impedance decrease but output impedance increase.

c) Input impedance Increase but output impedance increase.

d) Both input and output impedances increase.

17.

The effect of current shunt feedback in an amplifier is to

a) Increase the input resistance and decrease the output resistance.

b) Increase both input and output resistance.

c) Decrease both input and output resistance.

d) Decrease the input resistance and increase the output resistance.

18.

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An amplifier without feedback has a voltage gain of 50, input resistance of 1 k Ω and

output resistance of 2.5 kΩ. The input resistance of the current - shunt negative feedback

amplifier using the above amplifier with a feedback factor of 0.2 is.

a) 1/11KΩ

b) 1/5Ω

c) 5 kΩ

d) 11 kΩ

20.

To obtain very high input and output impedances in a feedback amplifier, the topology used

is

a) Voltage – series

b) Current – series

c) Voltage – shunt

d) Current – shunt

21.

Which one of the following type of negative feedback increases the input resistance and

decrease the output resistance of amplifier?

a) Current series feedback.

b) Voltage series feedback.

c) Current shunt feedback

d) Voltage shunt feedback

22.

The upper or lower cut off frequency is also called ………….. frequency

a) Resonant

b) Sideband

c) 3 db

d) None of the above

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23.

The bandwidth of a single stage amplifier is …………. that of a multistage amplifier

a) More than

b) The same as

c) Less than

d) Data insufficient

24.

The purpose of RC or transformer coupling is to …………

a) Block A.C.

b) Separate bias of one stage from another

c) Increase thermal stability


25.

The number of stages that can be directly coupled is limited because ……..

a) Changes in temperature cause thermal instability

b) Circuit becomes heavy and costly

c) It becomes difficult to bias the circuit


26.

RC coupling is generally confined to low power applications because of ………

a) Large value of coupling capacitor

b) Low efficiency

c) Large number of components


27.

If the power level of an amplifier reduces to half, the db gain will fall by …….

a) 5 db

b) 2 db

c) 10 db

d) 3 db

28.

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The total gain of a multistage amplifier is less than the product of the gains of individual

stages due to …………

a) Power loss in the coupling device

b) Loading effect of the next stage

c) The use of many transistors

d) The use of many capacitors

29.

The frequency response of transformer coupling is ………….

a) Good

b) Very good

c) Excellent

d) Poor

30.

A gain of 1,000,000 times in power is expressed by …………….

a) 30 db

b) 60 db

c) 120 db

d) 600 db

31.

A gain of 1000 times in voltage is expressed by …………..

a) 60 db

1. 30 db

2. 120 db

3. 600 db

32.

The lower and upper cut off frequencies are also called ………………… frequencies

a) Sideband

b) Resonant

c) Half-resonant

d) Half-power

33.

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In transistor amplifiers, we use ……………. transformer for impedance matching

a) Step up

b) Step down

c) Same turn ratio


34.

The final stage of a multistage amplifier uses ………………

a) RC coupling

b) Transformer coupling

c) Direct coupling

d) Impedance coupling

35.

If a three-stage amplifier has individual stage gains of 10 db, 5 db and 12 db, then total gain

in db is ……….

a) 600 db

b) 24 db

c) 14 db

d) 27 db

36.

Transformer coupling is generally employed when load resistance is ………

a) Large

b) Very large

c) Small


37.

When a multistage amplifier is to amplify d.c. signal, then one must use …….. coupling

a) RC

b) Transformer

c) Direct


38.

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The noise factor of an ideal amplifier expressed in db is …………..

a) 0

b) 1

c) 1

d) 10

39.

Transformer coupling is used for ………….. amplification

a) Power

b) Voltage

c) Current


40.

In an RC coupled amplifier, the voltage gain over mid-frequency range …………….

a) Changes abruptly with frequency

b) Is constant

c) Changes uniformly with frequency


41.

Which of the following is an advantage of RC coupling scheme?

a) Good impedance matching

b) Economy

c) High efficiency

d) Frequency response

42.

In RC amplifier stage has a bandwidth of 500 khz. What will be the rise time of this

amplifier stage?

a) 0.35 micro sec.

b) 0.7 micro sec.

c) 1.0 micro sec.

d) 2.0 micro sec.

43.

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As compared to the impedance of the signal source, the input impedance of a voltage

amplifier should be-

a) Small

b) Large

c) Equal

d) None

44.

What is the purpose of impedance matching between the output of previous stage and the

input of next stage in cascaded amplifiers?

a) To achieve high efficiency

b) To achieve maximum power transfer

c) To achieve reduce distortion

d) None

45.

The best frequency response is of ………. coupling

a) RC

b) Transformer

c) Direct


46.

………….. coupling provides the maximum voltage gain

a) RC

b) Transformer

c) Direct

d) Impedance

47.

The gain of an amplifier is expressed in db because ………..

a) It is a simple unit

b) Calculations become easy

c) Human ear response is logarithmic


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48.

The use of amplifier in a circuit is to _____________ for input signal

a) Provide a phase shift

b) Provide strength

c) Provide frequency enhancement

d) Make circuit compatible

49.

The unwanted characteristics of amplifier output apart from desired output is collectively

termed as

a) Inefficiency

b) damage

c) Fault

d) Distortion

50.

Unit of power rating of a transistor is expressed in

a) Watts

b) KWh

c) W/s

d) Wh

51.

Power amplifier directly amplifies

a) Voltage of signal

b) Current of the signal

c) Power of the signal

d) All of the mentioned

52.

Which of the following amplifier class have highest linearity and lowest distortion?

a) Class A

b) Class B

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c) Class C

d) Class B push-pull

53.

Which of the following letter is not used to represent a class?

a) D

b) E

c) C

d) K

54.

Which of the following class has poorest linearity

a) Class A

b) Class B

c) Class C

d) Class AB

55.

Which of the following amplifier cannot be used for audio frequency amplification?

a) Class A

b) Class C

c) Class AB

d) Class B push-pull

56.

Which of the following amplifier is less efficient than others?

a) Class C

b) Class B

c) Class A

d) Class AB

57.

Which of the following class have a theoretical efficiency of 50%?

a) Class A

b) Class C

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c) Class AB

d) Class D

58.

Which of the following class have a theoretical efficiency of 78.5%?

a) Class A

b) Class D

c) Class C

d) Class B

59.

The power rating of amplifier is 100watts then the transistor can only operate at

a) Power higher than 100w

b) Power lower than 100w

c) Power lower than 50w

d) None

60.

Amplitude distortion is due to ___________

a) Shift in Q-point

b) Change in input

c) Linear amplification

d) Small input signal

61.

If output of amplifier exceeds maximum allowable value, ___________ is occurs in output

waveform.

a) Clipping

b) Clamping

c) Rectifying

d) Rounding

62.

The distortion caused by multiple frequencies in output is called _________

a) Amplifier distortion

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b) Harmonic distortion

c) Phase distortion

d) None of the mentioned

63.

Which of the following components in a transistor circuit is really responsible for harmonic

distortion?

a) Capacitor

b) Resistor

c) Transistor

d) Inductance

64.

Which of the following is (are) power amplifiers?

a. Class A

b. Class B or AB

c. Class C or D

d. All of the above

65.

By how much does the output signal vary for a class AB power amplifier?

a. 360º

b. 180º

c. Between 180º and 360º

d. Less than 180º

66.

Which type of power amplifier is biased for operation at less than 180º of the cycle?

a. Class A

b. Class B or AB

c. Class C

d. Class D

67.

[22 ]

Which type of amplifier uses pulse (digital) signals in its operation?

a. Class A

b. Class B or AB

c. Class C

d. Class D

68.

Which of the following describe(s) a power amplifier?

a. It can handle large power.

b. It can handle large current.

c. It does not provide much voltage gain.

d. All of the above

69.

The maximum efficiency of a transformer-coupled class A amplifier is _____.

a. 25%

b. 50%

c. 78.5%

d. 63.6%

70.

How many transistors must be used in a class B power amplifier to obtain the output for the

full cycle of the signal?

a. 0

b. 1

c. 2

d. 3

71.

nMOS and pMOS transistors can be used for class B.

a. True

b. False

72.

An amplifier has open loop gain of 200 and cut-off frequencies 20Hz and 40Hz. If 2% of

the output is returned to the input then closed loop gain will be-

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a) 40

b) 60

c) 80

d) 20

73.


the output is returned to the input then cut-off frequency will be-

a) 4Hz & 100Hz

b) 4Hz & 200Hz

c) 6Hz & 60Hz

d) 8Hz & 150Hz

74.


the output is returned to the input then reverse transmission factor will be-

a) 0.03

b) 0.02

c) 0.05

d) 0.06

75.

A class B Push-Pull amplifier using Transistor with power rating 20 W. Maximum AC

power which can be supplied to load-

a) 100

b) 200

c) 50

d) 150

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