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Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
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Amplifier and Multistage Amplifier
Q. What is an amplifier.
Ans. An Amplifier is the electronic device which is used for raising the strength of a
weak signal is called an amplifier.
Q.Classification of amplifier on the basis of different factors.
Ans. Linear amplifiers may be classified according to their mode of operations i.e. the
way they operate or the predetermined set of values. Their descriptions are based on
the following factors
1. According to input signal
(a) Small signal amplifier,
(b) Large signal amplifier.
2. According to output:
(a) Voltage amplifier,
(b) Power amplifier.
3. According to transistor configuration
(a) Common-emitter (CE) amplifier,
(b) Common-base (CB) amplifier,
(c) Common-collector (CC) amplifier.
4. According to biasing conditions
(a) Class-A – amplify complete Cycle ,
(b) Class - B, - amplify Half cycle
(c) Class – AB – amplify More than half but less than Full cycle
(d) Class – C – amplify less than half cycle
5 According to frequency response :
(a)direct current (DC) amplifier,
(b) Audio frequency (AF) amplifier,
(c) Radio frequency (RF) amplifier,
(d) Ultra-high frequency (UHF) and micro-wave frequency amplifier.
6.According to number of stages:
(a) Single-stage amplifier
(b) Multi-stage amplifier,
7.According to coupling methods:
(a) Direct coupled (DC) amplifier,
(b) Resistance-capacitance (AC) coupled amplifier,
(c) Transformer coupled (TC) amplifier.
Q. Draw the single stage RC coupled amplifier. Explain the working and state
function of each component?
Ans : When only one transistor with associated circuitry is used for amplifying a
weak signal the circuit is known as single stage transistor amplifier.
A single stage amplifier has one transistor, bias
circuit and other auxiliary components.
Since shown fig. has RC at input and output
for coupling signal it is called as single stage RC
coupled amplifier. Above shown circuit is practical RC
coupled amplifier (single stage)
Function of different components:Biasing circuit :
R1, R2 and RE forms the biasing circuit and
stabilization circuit
Voltage across R2 keeps the BE
junction forward bias.
RE stabilizes the Q -point against variation in
temperature.
Collector Resistor RC :
It is used to limit collector and act as load resistor for amplifier and output is
taken across Rc
Input capacitor (CCi)
It is electrolytic capacitor (10f)is used to couple the signal from Signal source
to base of transistor. This capacitor allows only AC signal to flow and also isolates
Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
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signal source from R2 i.e. it does not allow internal resistance of source to come in
parallel with R2 and hence biasing is not affected.
Coupling capacitor (CC0):
Coupling capacitor couples one stage to next stage. This capacitor also block
DC and only passes AC signal to next stage of this capacitor is not used then RC
come in parallel with R1 and hence biasing condition may change drastically.
Emitter bypass capacitor (CE):
An emitter bypass capacitor (100f) is used in parallel with RE to provide low
reactance path to the amplified AC signal. If it is not used then the AC signal will
flow through RE and will cause more voltage drop across it thereby reducing the
output because of negative feedback. i.e. it reduces AC degeneration
NPN transistor:- It is to amplify weak signals.
Working of single stage RC coupled amplifier
The weak signal to be amplified is fed (applied) between the base and emitter
terminal and output is taken from collector and Emitter terminal. The same we get
across the load resistance RL . Hence at any instance the Output is given by
VCE = Vcc -ICRC.
When input is positive half cycle the base current due to signal increases hence
total base current increases resulting the change in collector i.e. hence voltage across
Rc increases.
As Vcc is constant the total output as per above equation i.e. VCE decreases
and reverse operation takes place when Input is negative.
Fig : Shows the input and output waveform
In other words the signal voltage is increasing in the positive half cycle, The
Output voltage decreases as the Input voltage decreases the Output voltage increases
i.e. output is 1800 out of phase with Input or provides phase reversal.
Q. What is multistage amplifier?
Ans. When a number of amplifier stages are used in succession in order to achieve
greater voltage and power gain, it is called a multistage amplifier or a cascaded
amplifier.
Q.State various methods of coupling used in amplifiers. and Explain
Ans. Methods of coupling
1. Resistance capacitance coupling (AC)
2. Transformer coupling (TC)
3. Direct coupling (DC)
Resistance Capacitance (RC) Coupling
It is the most important method of coupling the signal from one stage to the
next and is shown in Figure. In this method, the signal developed across the collector
resistor of each stage is coupled through capacitor into the base of the next stage.
The cascaded stages amplify the signal and the overall gain is equal to the product of
individual stage gains. The amplifiers, using this coupling scheme, are called RCcoupled amplifier
Q.State the function of coupling used in amplifiers.
Ans.
1. It tranfers the a.c. output of one stage to the input of the next stage.
2. It isolates the d.c. conditions of one stae to the next.
3. It prevents the shifting of operating point.
Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
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MULTISTAGE AMPLIFIER
Q. Explain the need of coupling?
Ans. The purpose of coupling devices is
1> To transfer signal output of one stage to the input of next stage.
2> To isolate DC condition of one stage from next stage.
3> To properly match the resistance of both stage so maximum power
takes place .
First
Stage
Mic
RC Coupled
Second
Stage
transfer
Third
Stage
Transformer Coupled
Speaker
Fig : Block Diagram of Coupled Amplifier
Each stage of transistor and associated circuitry is coupled to the next
stage through a coupling device. Difference coupling devices used are Resistor,
capacitor and transformer.
Q. Explain the necessity of Multistage amplifier.
Ans. The output of single stage amplifier is usually insufficient to drive the output
device gain of amplifier depends on:1.Device parameter.
2.Circuit components.
Hence voltage, power gain and frequency response obtained from single stage of
amplification are insufficient to meet the requirement of either a composite electronic
circuit or a load device. Hence two or more single of amplifier are frequently used to
meet above criterions.
Hence, a transistor circuit more than one stage of amplification is known as
Multistage amplifier.
Each stage consist of one transistor and associated circuitry and is coupled to
the next stage through a coupling device. The name of the amplifier is usually after
the type of coupling eg.
Name of coupling
Name of Multistage amplifier
RC coupling
RC coupled amplifier.
Transformer coupling
Transformer coupled amplifier.
Direct coupling
Direct coupled amplifier
First
Stage
Input Stage
Coupling
Device
Second
Stage
Coupling
Device
Intermediate Stage
Third
Stage
Output Stage
Fig Block Diagram of Multistage Amplifier
Q: Explain with neat circuit diagram, stating the function of each component
operation of two stage RC coupled amplifier ?
Ans: In RC coupling a capacitor is used as coupling device. The capacitor connects
the output of one stage to the
input of next stage in order to AC
signal and block DC condition.
Fig :Show two stage RC coupled Amplifier
The function of components are mentioned in RC couple single stage amplifier.
Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
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Operation:When AC signal is applied to the base of the first transistor. It appears in the
amplified form at output of 1st stage with 180 degree out of phase with input (i.e.
phase reversal ).
The output of 1st stage is coupled to input of 2nd stage by RC & CC . The signal
at base of T2 is amplified and further phase is reversed and output appears across
RC' This output is coupled through Cc' to load resistor RL .
The output signal Vo is the twice amplified of input signal Vi. Hence, output is
in phase with input .
The total gain Av is equal to,
Av = Av1 * Av2
where
Av1= gain of 1st stage.
Av2= gain of 2nd stage.
Q: List advantages and disadvantages of RC coupled amplifier?
Ans: Advantages
1. Excellent frequency response.
2. Cheap or low cost since it uses R&C.
3. Compact circuit because modern R&C are small in size.
Disadvantages:1. RC coupled amplifier has low voltage and power gain.
2. Noisy operation with aging and moist atmosphere.
3. Impedance matching is poor.
Q: List application of RC coupled amplifier?
Ans: The coupled amplifier have an excellent audio fidelity over a wide range of
frequency. Therefore, they are used as voltage amplifier i.e. initial stage of public
address system (PAS).
Q: Explain with neat graph frequency response of RC coupled amplifier?
Ans:
The voltage gain (Av) of amplifier change
with frequency of signal because of
reactance of capacitor in circuit changes
with signal frequency and hence effect
the output.
The curve between voltage gain (Av)
and signal frequency of an amplifier is
called as frequency response .
Fig : Frequency response of RC Coupled Amplifier
The curve is plotted to study performance of an amplifier.
From the frequency response it is clear that gain drops at low frequency
(<50Hz) and at high frequencies (>20KHz) whereas it is uniform over mid frequency
range (50Hz to 20KHz) i.e, audio frequency range. Due to this RC coupled amplifier is
called as audio frequency amplifier .
Gain falling at low frequency [<20Hz]:The reactance of coupling capacitor is high (XC= 1/2FC ) and hence small part of
signal will pass from one stage to another stage also input signal is not coupled to
base of transistor.
Also due to CE, because CE cannot bypass AC signal due to large reactance
(XE = 1/2FCE) due to which large negative feedback occurs and hence gain is
reduced.
Gain constant at mid frequency (50-20KHz):The voltage gain of amplifier remains constant in this range of frequencies the
reactance of Cc and CE is moderate hence loading effect due to capacitor is reduced.
Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
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Hence maximum signal is coupled from one to another stage and negative feedback
due to CE is reduced.
Fall of gain at high frequency (>20KHz):The reactance of Cc and CE is very small and it behaves short Circuit This
increases the loading effect of next stage and Hence reduces the voltage gain. Also at
high frequency capacitive reactance of base emitter junction is low which increases
the base current. This reduces current amplification factor B. Hence due to above
two reason voltage gain drop at High frequency.
Q. What is Bandwidth of amplifier and give importance of gain and bandwidth
product?
Ans.
Fig. Frequency Response of RC coupled Amplifier
The range of frequency over which the gain is equal to greater than 70.7% of
the maximum gain is known as Bandwidth.
B.W= F2-F1 Hz
Where
F1=lower cutoff frequency
F2=upper cutoff frequency
For distortion less amplification it is important that signal frequency range must be
within the bandwidth of amplifier.
The bandwidth of an amplifier can also be defined in terms of db(decibel)The
bandwidth of amplifier is the range of frequency at the limit the range of frequency at
the limit of which its voltage gain falls by 3db from the maximum gain.
The frequency F1 and F2 are called as 3db frequency or half power frequency
The name 3db is given because the voltage gain at these frequency is 3db below
maximum value.
The term half power is used because when voltage is down to 0.707 of its
maximum value the power is down to (0.707) or one half of its maximum
value.
From the frequency response it is seen that the BW increases gain decreases
and vice versa. Hence, product of bandwidth and gain is taken for comparing the
performance of the amplifier. For any amplifier the product of gain and bandwidth is
constant.
Q. What all advantage of expressing gain in db(decibel)
Ans: The common logarithm (log to the base 10) of power gain is known as bel power
gain
power gain =log10. P(out)
P(in)
bel
1bel = 10decibel
power gain =10 log10 P(out)
P(in)
decibel(db)
Advantages of using dB
1. The unit db is logarithmic unit and also ear response is also logarithmic Hence
unit tallies natural response of ear
2. For multistage amplifier the overall gain is product of individual gain in db The
overall gain is sum of individual db gain hence calculation is easier
3. While calculating B.W the calculation of 0.707 *AV(max) can not be easily done in
db by just subtracting 3db from the calculated db gain
4. Logarithmic scale helps in denoting very small and very high value by convenient
figure.
Eg. 0.0001 to 1000 can be represented in db as -3 to +3 db.
Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
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Q Explain Transformer coupled amplifier with neat labeled diagram.
Ans:- In transformer coupling, transformer is used as the coupling device the
Transformer provides two functions :
1) To pass AC signal and block DC
2) Impedance matching
Transformer coupling is used for power amplification and also where load is very
small like speaker
Function Of Components:Function of all components other than transformer is same as RC coupled .
X1 and X2 :
Transformer :- are used to provide impedance matching and also to coupled
the signal from one stage to another stage
Fig. Two Stage Transformer coupled Amplifier
Working :
The weak AC signal is applied to the base of transformer T1 the signal appears
in the amplified form across the primary of coupling capacitor. This voltage is
induced in the secondary of the same transformer with 1800 phase shift this signal
across secondary of X1 is fed to the base of transformer T2 where further
amplification takes place as in case of T1. Hence due to two stage the phase shift
between input and output is 3600 or 00 i.e. same phase
Q:- List advantage and disadvantage and application of transformer coupled
amplifier?
Ans:Advantage:
1. Efficient operation because of low DC resistance of primary connected in collector
circuit.
2. Provides high voltage gain and power gain
3. Good impedance matching hence maximum power transfer from one stage
to another provide high power gain
Disadvantage:
1. The coupling transformer is costly & bulky particularly when operated at audio
frequencies because of it's heavy iron core
2. At radio frequency the inductance and winding capacitance present lot of problem
3. Due to sensitivity of transformer for frequency it's frequency is poor
4. It introduced hum in the output
Application:Transformer coupling is mostly used for impedance matching. Generally it
used where maximum. power is to be transferred from one stage to another. By
perfect impedance matching. transformer coupled amplifier are used as last stage of
public address system (PAS)
Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
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Q. Explain the operation of direct coupled amplifier?
Ans. In direct coupled or dc coupling the individual amplifier stage bias condition are
so designed that two stage may be directly connected without the necessity of d.c.
isolation.
Fig : Direct Coupled two Stage Amplifier
There are many applications in which extremely low frequency (<10Hz) signals are
to be amplified eg. amplifying photo electric current thermo coupled current also to
measure change in small dc current direct coupled amplifier are used.
Functions of components:Function of R1,R2,RE1,RC2,RE2 is same as single stage amplifier.
Working:The weak signal to be amplified is applied to the input of the transistor T, the
amplified output is available at collector of T1,which is connected to base of second
transistor T2.T2 provides further amplification and amplified signal is available at
collector of T2.Hence there is no phase shift between input and amplified output.
Q. List advantages, disadvantages and application of direct coupled amplifier
Ans:
Advantages:
1) Simple circuit because less number of components.
2) It is inexpensive since no coupling device is used.
3) It has no coupling or bypass capacitor which reduce gain at low frequency.
4) Compact in size.
5) It has outstanding ability to amplify direct current(DC amplifier) and low
frequency signals.
Disadvantages:
1) Cannot amplify high frequency signal.
2) Poor temperature stability i.e. operation point is shifted due to temperature
change.
Applications:
1) Used to amplifying low frequency signal.
2) Regulator Circuit of electronic power supply.
3) Pulse amplifier.
4) Computer circuitry.
5) Differential amplifier.
6) Electronic instrument.
7)They can amplify dc current also.
Q. Compare different types of coupling.
Ans.
Particular
RC coupling
Transformer coupling
Frequency response
good
poor
Cost
less
more
space and weight
less
more
Impedance matching
Not good
Excellent
low frequency
Direct coupling
best
least
least
good for
Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
voltage gain
Distortion
7) Use
less
less
voltage
amplifier
initial stage
in multi stage
more
more
Power amplifer
as final stage in
Multistage amplifier
8
less
less
For
amplifying
extremely
Low freq
Q. Describe the frequency response of the Transformer Coupled amplifier ?
Ans. Fig. shows the frequency response curve of a transformer coupled CE amplifier.
It is clear that the voltage gain rolls of at low as well as at high frequencies, whereas
it remains constant in mid-frequency range.
Another noticeable feature is that at one particular frequency (f the voltage
gain increases and then rolls off continuously.
Response at Low Frequency The output voltage of a transformer coupled amplifier is the product of collector
current and the reactance of primary winding of coupling transformer (T1 At low
frequencies, the reactance of primary winding (XL = 2FL) begins to decrease and the
voltage gain reduces.
Response at High Frequency At high frequencies, the effect of
leakage inductance and distributed
capacitance
(i.e.
the
capacitance
between the turns of the winding forms
bypass
capacitor)
becomes
more
significant and acts as a shunt capacitor
to reduce the output voltage and hence
the voltage gain reduces. The peak gain results due to the resonance effect of
inductor and distributed capacitor which forms a resonant circuit or tuned circuit.
Mid Frequency The flat part of frequency response of transformer coupled amplifiers is small
as compared to that of RC coupled amplifiers. As a result of this, these amplifiers
cannot be used over a wide range of frequencies.
There is frequency distortion i.e. all frequencies are not amplified equally. In
fact, the flat response part of the curve is small as compared to RC coupling.
However, transformer-coupled amplifiers can be designed to have a flat frequency
response curve and excellent fidelity over the entire audio frequency range.
Q.State advantages , disadvantages and application of transformer coupled
amplifier
Ans. This Circuit is called as Transformer Coupled amplifier because if uses
transformer as coupling device.
Advantages- The advantages of transformer—coupled amplifiers are
1. There is loss of signal power in the collector or base resistors because the d.c.
resistance of the primary winding (P) is very low
2. It provides an excellent impedance matching between the stages.
3. It provides a higher voltage gain than that of R.C. coupled amplifier.
Disadvantages
The disadvantages of transformer-coupled amplifiers are
1. The coupling transformer is expensive, bulky and large when used at audio
frequencies.
2. It introduces frequency distortion when used at radio frequencies.
3. It has poor frequency response.
4. It tends to produce ‘hum’ in the output.
5. The transformer introduces loses in the form of copper loss, eddy current loss
and hysteresis loss at high frequencies.
Because of the drawbacks, the transformer coupling method is not used for
amplifying low (an audio frequency signals.)
Applications :
The applications of a transformer coupled amplifier are as follows
1. It is mostly used for impedance matching between the individual stages.
Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
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2. It is widely used as a voltage amplifier in the final stage of multistage amplifier
3. It is widely used for amplification of radio—frequency (RF) signal.
4. It is used to transfer power to the low impedance load such as loudspeaker,
Q. Describe the frequency response of the Direct coupled amplifier
Ans.
Fig. shows the frequency response
graph of a direct coupled CE
amplifier. It may be noted that the
voltage gain is uniform upto a certain
frequency denoted by f1 Beyond this
frequency, the voltage gain rolls off
slowly. The gain rolls off at high
frequencies due to the increased
base-to-emitter junction capacitance
and stray wiring capacitance.
Q. Describe the different coupling used in the amplifier
Ans.
In fact, amplifiers are classified according to the coupling network used. The three
basic methods of coupling are
Resistance-Capacitance (RC) Coupling
It is also known as capacitive coupling. Amplifiers using this coupling are
known as RC-coupled amplifiers. Here, RC coupling network consists of resistors Rc
and one capacitor C. The connecting link between the two stages is C. The function
of RC-coupling network is two-fold:
(a) to pass ac signal from one stage to the next
(b) to block the passage of dc voltages from one stage to the next.
Transformer Coupling
Since secondary of the coupling transformer conveys the ac component of the
signal directly to the base of the second stage, there is no need for a coupling
capacitor. Moreover, the secondary winding also provides a base return path, hence
there is no need for a base resistance. Amplifiers using this coupling are called
transformer-coupled amplifiers
Direct Coupling
This coupling is used where it is desirable to connect the load directly in series
with the output terminal of the active circuit element. The examples of such load
devices are (i) headphones (ii) loud-speakers (iii) dc meters (iv) dc relays and (v) input
circuit of a transistor etc. Of course, direct coupling is permissible only when
1. dc component of the output does not disturb the normal operation of the load
device.
2. device resistance is so low that it does not appreciably reduce the voltage at
the electrodes.
Important Terms :
In the study of multistage amplifier, we shall frequently across the terms
gain, frequency response, decibel gain and bandwidth.
Gain (A) :
The ratio of the output electrical quantity (i.e. voltage, current or power) to the
input of the amplifier is called its gain. The gain of a multistage amplifier is equal to
the product of gains of individual stages. If A1, A2, A3 are the individual gains of a
three stage amplifier, then overall gain is given by,
A = A1 x A2 xA3
Voltage Gain (Av)
The ratio of the output voltage to the input voltage of the amplifier is called
voltage gain. It is denoted by a letter, Av The
voltage gain is given by,
Av = Output voltage
=
Vo
Input voltage
Vi
Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
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The voltage gain of a multistage amplifier is equal to the product of voltage
gains of individual stages. If A1, A2 and A3 are the individual voltage gains of a three
stage amplifier, then the overall voltage gain is given by,
Av= Av1 X Av2 X Av3
In practice, the overall voltage gain is less than the product Av1 X Av2 X Av3, due to
the loading effect of the next stage.
Current gain (Ai) :
The ratio of the output current to the input current of the amplifier is called
current gain. It is denoted by a letter A. The current gain is given by,
Ai = output current
=
Io
Input current
Ii
The current gain of a multistage amplifier is equal to the product of current
gains of individual stages. If Ai1, Ai2 and Ai3 are the individual current gains of
a three stage amplifier, then the overall current gain is given by, Ai = Ai1 X Ai2 X Ai3
Power Gain (Ap) : The ratio of the output power to the input power of the amplifier is
called power gain. It is denoted by a letter A and it is given by,
Ap = Output power
= Po
Input power
Pi
The power gain of a multistage amplifier is equal to the product of power gains
of individual stages. If Ap1, Ap2 and Ap3 are the individual power gains of a three
stage amplifier, then the overall current gain is given by, Ap = Ap1 X Ap2 X Ap3
Decibel Gain (dB)
In many problems it is found very convenient to compare two powers on a
logarithmic scale rather than on a linear scale. The telephone industry proposed
logarithmic unit, named bel after Alexander Grahm Bell. The number of bels by
which a power P exceeds a power P is defined as:
bel = log10 Po /Pi
Ap = log10 Po /Pi
bel ,
Thus the common logarithmic (log to the base 10) of power gain is called
bel power gain.
For practical purposes, it has been found that the unit bel is quite large. Another
unit, one tenth as large, in more convenient. This smaller unit is called the decibel
(abbreviated as dB),
1 bel = 10dB
A = 10 log10 ( Po /Pi )
dB
Note that the unit dB denotes a power ratio.
As P  V2  I2, therefore, the voltage or current gains are expressed in dB
Av = 10 Log
Av = 20 Log
10(
Vo/Vi)2
10( Vo/Vi)
dB
dB
Av = 10 Log
Av = 20 Log
10(Io/Ii)2
dB
dB
10( Io/Ii)
The overall gain of a multistage amplifier can be easily computed if the gains
of the individual stages are known in dB. If we take the logarithmic (to the base 10)
of Eq. and then multiply each term by 20, we get
20 log10 A = 20 log10 A1 + 20 log10 A2+ 20 log10 A3... + 20 log10 An
In the above equation, the term on the left is the overall gain of the multistage
amplifier expressed in dB. The term on the right denote the gains of the individual
stages expressed in dB. Thus, the overall gain in dB if a multistage amplifier is the
sum of the decibel gains of the individual stages. That is,
AdB = AdB1 + AdB1 + AdB1 + AdB1 +……..AdBn
Frequency Response
The voltage gain of the amplifier varies with signal frequency. The
characteristic curve between the voltage gain and frequency of an amplifier is known
as frequency response.
Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
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3dB Bandwidth
The range Of frequency over which the voltage gain of the amplifier is equal or
greater than 70.7% of the maximum is known as 3 dB bandwidth. In other words, it
is the range of frequency at the limits of which the voltage gain of an amplifier falls
by 3dB from its maximum voltage gain.
The voltage gain of an amplifier changes with the
frequency. The frequency of an amplifier is shown
in Fig. It shows that f1 and f2 are the limiting
frequencies. The former (f1 is called the lower
cut-off frequency and the latter (f2 is called the
upper cut-off frequency. Therefore, f2 — f1 is the
3dB bandwidth. It is important that the signal
frequency range must be within the bandwidth of
the amplifier for distortion amplification.
Suppose the maximum voltage gain is 100. Then 70.7% of it is 70.7.
Fall in the voltage gain = 20 log(100/70.7)dB = 2Olog101.4142
= 3dB
Q.Why the limit is set to 70.7% of the maximum gain
Ans. At the cut-off frequencies the voltage gain is
= 0.707 Avm = ( 1/2 ) Avm
where Avm is the maximum gain or the mid-frequency gain of the amplifier. It means
at these frequencies, the output voltage is 1/2 times the maximum voltage. Since
the power is proportional to the square of the-voltage, the output power at these cutoff frequencies becomes one-half of the power at mid-frequencies. On the dB scale
this is equal to a reduction in power by 3 dB. For this reason, these frequencies are
also called 3 dB frequencies.
We have taken difference of 3 dB in power to define the cut-off frequencies,
because this represents an audio-power difference that can just be detected by the
human ear. For the frequencies below f1 and above f2, the output power will reduce
by more than 3 dB.
f1 and f2 are also called as half power points because at this frequencies the
total power of the amplifier reduces to 50% of the total power.
Hence band width is also defined as the range of frequencies between two half
power points f1 and f2 ie
BW = f2 – f1
Assignment from Chapter of Transistor
Assignment on transistors & transistor biasing;
(short answers)
1} define alpha & beta for transistor?
2} draw symbol of pnp transistor?
3} state relation between alpha & beta?
4} draw circuit of common collector amplifier?
5} draw commonly used Biasing circuit for BJT?
6} list specifications of transistors?
7} draw structure of NPN transistor & show different terminals & junctions?
8} explain effect of leakage current on alpha & beta?
9} draw common base configuration of transistor?
(brief answers)
1. draw the circuit of base bias with emitter feedback for transistor amplifier. how
the circuit provides stability?
2. Compare CB & CE configurations?
3. Draw self biased circuit for transistor?
4. Define:
(1) thermal runaway
(2) thermal stability
5. explain transistor as an amplifier with circuit & load line analysis?
Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
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6. why is CE configuration is ost popular?
7. Draw i/p & o/p characteristics of CE configurations with the help of circuit
diagram & explain them briefly?
8. Explain working on NPN transistor in active region with diagram?
9. justify that current gain alpha in common base transistor configuration is less
than early equal to 1?
10. what is the significance of transistor?
11. compare CB & CE configurations on the basis of current gain & i/p
impedence?
12. What is Quiescent point?why it should be at the center of the active region for
the transistor to work as an amplifier?
13. if alpha is given: how that beta=alpha/1-alpha.
14. draw o/p characteristics of CB configuration & write the working in three
different regions?
15. what are the different current components of PNP transistor?
Assignment on Seniconductor theory & PN junction diode:
(short answers)
1}
2}
3}
4}
5}
6}
what is the difference between ionic bond & covalent bond?
what type of impurity is antimony? when added to germanium what happens?
define intrinsic & extrinsic semiconductors with examples?
compare silicon & germanium diode?
define forward current & reverse current?
define static resistance of diode?
(brief answers)
1. state two semiconductor materials used for converting electrical energy to light
energy?
2. state static & dynamic resistance with reference to pn junction diode?
3. give significance if Forbidden energy gap.
4. three materials & their Forbidden energy gap is given below, identify the
materials;
A-1ev
B-10ev
C-10-4ev
5. draw forward bias characteristics of germanium & silicon diode?
6. explain the formation of junction in p-n diode & list of specification of diode?
7. Compare intrinsic & extrinsic semiconductors? what is N type semiconductor?
8. explain different energy bands in solids?
9. list applications of semiconductor diode & explain 1 application?
Assignment on Rectifiers & filters:
(short answers)
1} state the function of filter circuit?
2} define ripple factor?
3} draw circuit diagram of half wave rectifier?
4} classify filters & state the advantage of pi filter over other filters?
5} draw o/p wave forms of half wave,full wave,& bridge rectifier rectifier?
(brief answers)
1. define ripple factor.with its expression,show that the ripple factor of a half
wave
rectifier is 1.21?
2. Compare capacitor filter & inductor filter?
3. compare half wave,full wave center tapped rectifier with respect to;
1} no of diodes
2} ripple factor
3} efficiency
4} PIV
4. state merits & demerits of bridge rectifier with circuit?
Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
13
5. compare half wave & full wave rectifier?
6. what is the significance of ripple factor & PIV of rectifiers?
7. what should be the values of PIV of diodes used in center tapped,full wave &
bridge rectifiers?
8. explain operation of half wave rectifier with waveform?
9. draw the circuit of bridge rectifier with choke i/p filter & explain its working
with waveform?
10. compare filter circuits?
Assignment on capacitors,inductors,switches,zener diode ,&colourcode
(short answers)
1} list specifications of capacitor?
2} list types of fixed resistors?
3} list specifications of zener diode?
4} what is capacitor?state its units?
5} name the different types of switches?
6} draw the characteristics of zener diode?
6} draw symbol of LED?
7} define dynamic resistance of diode?
8} what are passive components give examples?
(brief
1.
2.
3.
4.
5.
6.
7.
8.
9.
answers)
explain avalanche breakdown & zener break down?
explain how zener diode can be used as a regulator?
compare zener & avalanche break down?
In what respect is an LED different from an ordinary P N junction diode?state
applications for LED?
draw symbol of varacter diode & state its applications, structure &
characteristics?
differentiate between active & passive components?
what are different types of inductors & capacitors? give 4 applications in
electronics devices?
explain the operation of zener diode as shunt voltage regulator?
draw VI characteristics of zener diode mark the part of characteristics usrd for
voltage regulation & hence define voltage regulation. draw the circuit diagram
of zener regulator?
Assignment on JFET & MOSFET.
(short answers)
1} draw symbol of enhancement mosfet?
2} define:(1) dynamic drain resistance with respect to JFET.
(2) pinch off voltage.
3} draw the structure of depletion type MOSFET?
4} draw the structure of n chanel JFET?
5} list advantages of FET over BJT?
6} define thermal stabality?
7} why i/p impedence of JFET is high?
8} draw the symbol of JFET(n-channel)?
(brief answers)
1. if a p-channel JFET is replaced by an n-channel JFET,what will be the
change in supply connections?
2. define transconductance of FET?
3. define & state the formula for amplification factor, transconductance, drain
resistance
with respect to JFET & give relation between them?
4. sketch the structure of JFET & explain mutual characteristics?
5. with a neat diagram explain the construction & working of JFET & define
the parameters?
6. draw the o/p characteristics of JFET & explain how it works as a voltage
controlled resistor?
Multi-Stage Amplifier – Mr.Manoj S. Kavedia – EDC/EBE – FYCM/ IF/ ET
14
7. draw the drain characteristics & transfer characteristics of JFET, show all
regions?
8. explain with suitable diagram the construction of p channel depletion
MOSFET. draw the characteristics if the same type?
9. state the advantages of MOSFET over JFET?
10. draw the circuit diagram of JFET in common source configuration, draw
the circuit using common transistor with common emitter. give their input
impedance &explain why it is so?