Download Multi Stage Amplifiers

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Chapter 3 : Multistage Amplifiers
Q 1. What do you understand by multi-stage transistor amplifier?
Mention its need.
OR
What are multi-stage amplifiers?
Ans.
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A transistor circuit in which a number of amplifier stages are connected
one after another is called as multistage transistor amplifier.
When the output of one amplifier acts as an input to the next amplifier
with suitable coupling device and so on then it is called as cascaded
amplifier.
The voltage gain of a multistage amplifier is equal to the product of the
gains of the individual stages. If and A1 ar&thjrhaLt guins,stLpzi
iaiagaini1inei then overall voltage gain is given by:
Thus if we want to increase the voltage gain., current gain or power gain we
need multistage transistor amplifiers.
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Q 2. A single stage amplifier has a gain of 60. The collector load
and the input impedance is
Calculate the over all gain
when two such stages are cascaded through R-C coupling.
Ans.
When two such amplifiers are cascaded, the loading effect of input impedance
of the second stage will reduce the gain of first amplifier but the gain of second
stage will not be affected.
Q 3. Define the unit decibel for expressing:
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(i) voltage
(ii) current and
(iii) power.
Ans.
(i) Decibel for expressing voltage : It is 20 times the common logarithm of
voltage gain
(ii) Decibel for expressing current : It is 20 time.s the common logarithm of
current gain
(iii) Decibel for power: It is 10 times of the common logarithm of power gain
Q 4. Explain in brief the function of a tank circuit in a tuned-voltage
amplifier.
Ans.
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Tuned voltage amplifier amplify a seleted frequency signal.
In this tank circuit very high impedance at resonant frequency (i.e. XL =
Xc) and very small impedance at er frequencies.
The resonant frequency of tank circuit can be selected by adjusting the of
variable capacitor connected in the circuit.
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Q 5. State what will happen to the voltage gain of an amplifier if the by
passitor is open circuited.
Ans.
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If the by pass capacitor is not put in the circuit, a.c. emitter current will
flow through using an ac voltage drop across it.
This voltage produce a feedback effect and there by tie output voltage.
Thus for proper amplification, an emitter bypass capacitor is essential.
Q 6. A multistage amplifier consists of two stages. The voltage gain of
the s are 30 and 80. Calculate the overall voltage gain in dB.
Ans.
Voltage gain of one stage = 20 log10 30
and Voltage gain of seôond stage = 20 log10 80
Overall voltage gain is 201og10 30 + 20 tog10 80,
Q 7. Which configuration of bipolar transistor is called as emitter
follower r why for what purpose is it used?
Ans.
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Common collector amplifier configuration is called as emitter follower 1
this circuit the output voltage developed across the emitter resistor just
follows voltage .
Hence the name emitter follower and it can be redrawn to form collector
as terminal.
Hence the circuit i.also called a common collector amplifier.
The voltage gain of this circuit is one, but input impedance is very high
impedance is very low.
Therefore, this circuit is basically employed for imJeud1I(. IIIdL
Q 8. What are the different types of configurations used in multistage
am circuits?
Ans.
Different types of configurations used in multistage amplitude circuits are
1.Class-A
2.Class-B
3. Class-C
4.Class-D
Q 9. What is the advantages of slagger tuner amplifier?
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Ans.
Stagger tuner amplifier have high selectivity, high gain and desirable band
Q 10. Explain the statement : The cut-off frequencies of single stage
amp are influenced by R-C combinations. .
Ans.
The cut-off frequecies of single stage amplifiers are influnced by R-C
combinatic because :
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The first cut off frequency lies in low frequency range, in which the
impedance by coupling capacitor Cc is comparable to the collector
resistance Rc. Thus it largely the current amplification.
The second cut-off frequency lies in the high frequency range where the
area offered by coupling capacitor C is very small and it may be
considered as a short circui Thus with increase in input frequency the
magnitude of voltage gain, falls off.
Q 11. State the factors to be considered while designing a biasing
circuit t good transistor voltage amplifier
Ans.
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(i) It should ensure proper zero signal collector current and VCE.
(ii) It should ensure that .VCE does not fall ihe.required limits i.e. 1V for Si and
0.5 V.
(iii) It should ensure the stabilisation of collector current against temperature
variatic
(iv) It should ensure to make the operating point independent of transistor
parameters
Q. 12. What are half-power frequencies?
OR
What is meant by frequency response of an ‘amplifier? What are halfpower frequencies?
Ans.
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As the signal frequency varies the voltage gain of an amplifier also varies
due to capacitive reactances of the circuit.
The curve drawn between the signal frequency and voltage gain of an
amplifier is called• as frequency response of an amplifier. The low
frequency and high frequency at which the gain falls by 3 db are known
as half‘r frequencies or 3-db points.
It is termed as half power because the power output is 3db from its
normal level.
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Q 13. Why is coupling capacitor provided in RC coupled amplifier also
called locking capacitor?
OR
Explain the effect of coupling capacitor on low frequency response of
BJT amplifiers.
Ans.
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The first stage of RC coupled amplifies is coupled to the base of the
second stage rough the coupling-capacitor Cc.
The coupling capacitor C blocks thç dc voltage and aows the ac signal to
reach the base of the second stage.
Thus, the coupling capacitor Cc is also known as blocking capacitor.
Q 14. Discuss briefly the choice of transistor configuration in a cascade
amplifier ?
Ans.
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In a cascade amplifier the most commonly used configuration is common
emitter (CE) because the voltage gain of common emitter amplifier is
greater than unity while common collector (CC) configuration has voltage
cain less than unity as well as the voltage gain of common base (CB) is
also less than unity.
Q 15. What are the requirements for efficient coupling in a multistage
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amplifier?
Ans.
The requirements for efficient coupling in a multistage amplifier is
(i) There should be proper impedance matching.
(ii) Distortion should be removedduring coupling.
Q 16. Give applications for transformer coupled amplifier.
Ans.
(i) It provide impedance matching due to which maximum power can be
transferred.
(ii) It keeps the dc power small.
Due to following reasons transformer coupled amplifier isused in public address
systems to drive speakers and to drive other power devices.
Q 17. What is the function of tank circuit in tuned voltage amplifier?
Ans.
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The tank circuit is parallel combination of L and C. It is used to select a
particular audio frequency for amplification.
The selected frequency depends upon the resonance frequency of the
tank circuit In this case the impedance Z, is high and thus current 1r is S
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which reduces the power loss,
Q 18. Draw and label the frequency response of an RC coupled
amplifier?
Ans.
The frequency response of an RC coupled amplifier is as shown:-
Q 19. What are the limitations of single tuned voltage aplifIers?
Ans
Limitations of single tuned voltaged amplifiers:
(i) Single tuned voltage amplifier reauces the bandwidth to large extent to get
higher 10 factor.
(ii) This results in poor reproduction.
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Q 20. A tuned circuit has a resonant frequency of 1600 kHz and
bandwidth.What is the value of Q factor ?
Ans.
Q 21. A transformer coupling is used in the final stage of a multistage
amplifier. If the output impedance of transistor is
and the speaker
has a resistance of
, find the turn ratio of the transformer so that
maximUm power is transferred to the load.
Ans.
According to maximum power transfer theorem the source impedance should
be equal to load impedance.
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Q 22. State and explain atleast one typical application of each type of
coupling.
Ans.
(i) RC coupling transistor amplifier : AC coupled amplifiers provide
excellent audio fidelity over a wide range of frequencies. Therefore, they are
Widely used as voltage amplifiers.
The function of voltage amplifier is to raise the voltage level of the input signal.
As R-C coupling has smaller size, smaller weight, lower cost and occupy less
space therefore, it is preferred to couple various stages of voltage amplifiers by
this coupling method as shown fig. below,
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(ii) Transformer Coupling: Because of poor frequency response at lower
frequencies (i e audio frequency -ange 50 Hz to 20 KHz) These amplifiers are
not used for amplification of audio frequencies However these are widely used
for amplification of radio frequency signals By putting suitable shunting
capacitors across each winding of transformer it get resonance at any desired
frequency where the gain is maximum Such amplifiers are called tuned voltage
amplifiers fig shows below is a tuned amplifier
(iii) Direct coupled amplifiers: When extremely low frequency (< 10 Hz)
signals (such as photo electric current in film projector, thermo couple currents
for temperature measurements etc) are required to be amplified, direct
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coupling is used
Q 23. Derive the equation for the overall voltage gain of multistage
amplifier in terms of the individual voltage gains?
Ans.
Let us consider the block diagram for multistage amplifier as shown:
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Q 24. A multistage amplifier consists of three stages. The voltage gain
of stages are 60, 100 and 160. Calculate the overall gain in db.
Ans.
Voltage gain in db for 1st stage 20 log10 60 = 20 X 1.778 = 35.56db
Voltage gain in db for 2nd stage = 20 log10 100-= 20 x 2 = 40db
Voltage gain in 1b for 3rd stage = 20 log10 160 = 20 x 2.04 44.08 db
The overall voltage gain in db = 35.56+ 40 + 44.08
Av(overat() = 119.64 db.
Q 25. What is stagger tuning? Explain in brief
OR
Write note on Stagger tuned amplifier?
Ans.
Stagger tuned amplifier:
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It has been observed that if two or more tuned circuits, which are
synchronously tuned are cascaded, the overall bandwidth decreases.
However, if the different tuned circuits are cascaded, are tuned to slightly
different frequencies, it is possible to obtain an increased bandwidth with
a flat pass-band with steep sides.
This technique is known as stagger-tuning Following fig shows a two
stage tuned voltage amplifier
The stagger tunning in this circuit, may be achieved by resonating the tuned
circuits L1 C1 and L2 C2 to slightly different frequencies.
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Q 29. Draw the circuit diagram of a tuned amplifier Discuss its
operation How are current gain and voltage gain given in this cases
Name various types of tuned amplifiers
Ans.
Tuned amplifiers are those amplifiers which amplify a selected frequency or a
selected band of frequencies Circuit diagram of a single tuned amplifier is as
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shown
Operation:
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When high frequency signal is applied at the input terminals the resonant
frequency of parallel tuned circuit select and amplifiers the resonant
frequency by changing the value of C.
Only a particular frequency is selected and others are rejected by the
tuned
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Q 30. Find gain in dB for voltage gain = 40.
Ans.
Voltage gain in dB
= 20 log10 40
= 20 x 1.602
= 32.04
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Q 31. A multi stage amplifier consists of three stages as voltage gain in
dB 33, 36,40 dB respectively.
Then calculate the overall voltage gain in dB.
Ans.
Overall voltage, gain in dB
= 33 + 36 + 40 = 109 dB.
falls
Q. 32. In an amplifier the maximum voltage gain is 2000 and occurs at
10 KHz. It to 1414 at 20 KHz and 30 Hz. Determine
(i) lower cut off frequency
(ii) upper cut-off frequency.
Ans.
The frequency response curve is shown in figure.
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Thus, lower cut off frequency = 30 Hz
and upper cut off frequency = 20 KHz.
Q 33. From 032 find bandwidth
Ans.
Bandwidth = 30 Hz to 20 KHz = (30 - 20) KHz = 10 KHz
Q 34. Calculate voltage gain of single stage if it having
and
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Q 35. In two stage RC coupled amplifier and for each stage
Find
(i) voltage gain of second stage
(ii) Overall voltage gain.
Ans.
(i) As no load is for, second stage
Voltage gain of second stage
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Q 36. Two stage amplifier stages coupled by coupling transformer. f
output impedance of first stage is
and input impedance of second
stage is
.Calculate the inductance of primary and secondary of
transformer for perfect matching at frequency =
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Q 38. Draw cascode connection as tuned amplifier.
Ans.
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Q 39. Explain the features of class B push-pull operation
Ans
1. Greater power output
2. Higher power efficiency
3. No power loss at zero signal
4. With signal the DC current increases
Q 40. Use of cascading amplifiers.
Ans.
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Cascading amplifiers means the output of one amplifier acts as an input
to the similar stage and so on.
The use of cascading amplifiers is to increase the strength of the signal
by a large value.
If input to an amplifier is 5V and it gives output 5 times, then the output
of cascading amplifiers having two stages is given by
Thus, larger outputs are obtaining if number of amplifier stages increases.
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Q 41. Differentiate between tuned and untuned amplifiers.
Ans.
The main difference between tuned and untuned amplifiers is that the,
tuned amplifiers are designed for specific, usually narrow bandwidth but
untuned are not designed by such specifications.
Q 42. Why R-C coupling is the most widely used coupling between the
two stages cascaded amplifier?
Ans.
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R-C coupling gives excellent frequency response i.e. a constant gain over
the audio-frequency range.
Thus, are useful for speech and music.
Also R-C coupling is cheaper in cost because of low cost of resistors and
capacitors used in the circuit.
By using R-C coupling with circuit becomes very compact because of its
components are very light and small.
Q 43. Compare the characteristics of single, double and stagger tuned
amplifiers. Give typical circuits and list their important applications.
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Ans.
1. Single tuned voltage amplifier:
2. Double tuned voltage amplifier:
3. Stagger tuned voltage amplifier
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Application of tuned voltage amplifiers
1. Tuned voitage amplifiers are used in the wireless communication systems.
2. These are used in communication transmitters to transmit the signals.
3 These are used in communication receivers to: receive the desired signals.
Q 44. Write shrt note on: Single turned amplifier.
Ans.
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Single tuned amplifier is consists of ransistor amplifier with a parallel
tuned circuit as the collector load. The values of capacitance and
inductance- ot the tuned circuit are so selected that the frequency to be
amplified is equal to its resonant frequency.
It is as shown in fig.
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The output of the single tuned amplifier can be obtained by using either a
coupling capacitor C or by using a secondary coil at the output stage.
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