Download 2nd Year 1st Term Lecture Material_01

Tuned Amplifiers
Dr. Monir Hossen
ECE, KUET
Department of Electronics and Communication Engineering, KUET
Introduction of Tuned Amplifiers (1/2)
 Amplifiers which amplify a specific frequency or
narrow band of frequencies are called tuned
amplifiers.
 It used for the amplification of high or radio
frequencies:
 It is not used for the amplification of audio
frequencies as they are mixture of frequencies
from 20 Hz to 20 KHz and not single.
 It is widely used for radio and television ckts.
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Introduction of Tuned Amplifiers (2/2)
 A tuned amplifier ckt. consist of:
 Transistor
 LC tuner ckt.
 Coupling capacitors
 Resistors
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Parallel Resonance (1/2)
 A parallel tuned circuit consists of
 A capacitor and
 An inductor in parallel
 Resonance is occurred when circuit power
factor is unity, (i.e., applied voltage and the
current are in phase)
 If we draw the phasor diagram of the
above parallel circuit then we find:
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Parallel Resonance (2/2)
 The coil has its own resistance R and the coil current
IL has two rectangular components:
 Active component ILcosφL
 Reactive component ILsinφL
 The power factor will be unity only when the net
reactive component of the circuit current is zero, i.e.,
I C  I L sin  L  0 or I C  I L sin  L
Resonance in parallel circuit can be obtained by
changing the supply frequency.
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Resonant Frequency (1/2)
 The frequency at which parallel resonance
occurs is called the resonant frequency fr
 At parallel resonance we have
I C  I L sin  L
xL
V
Now, I L 
; sin  L 
and
ZL
ZL
So

V
V xL


xc Z L Z L
xC xL  Z L2
L

 Z L2  R 2  xL2
C

L
2
 R 2  2f r L 
C
IC 
V
xc
xL
ZL
R
ZL= R+jxL
ZL2= R2+xL2
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Resonant Frequency (2/2)
L
 2f r L    R 2
C
L
 2f r L 
 R2
C
2

1
L
fr 
 R2
2L C

1
fr 
2
1 R2
 2
LC L
If coil resistance R is small then

fr 
1
2 LC
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Characteristics of Parallel Resonant Ckt.
(i) Impedance of tuned circuit:
 The parallel tuned circuit is used to select the resonant
frequency and reject all others.
 At resonant condition I C  I L sin  L
 So the line current at resonant condition is:
I  I L cos L
V
V
R



Zr ZL ZL
1
R

 2
Zr ZL
1
R
CR



Zr L
L
C
L
 Zr 
CR
I C  I L sin  L
xL
ZL
R
V
xc

V xL

ZL ZL
Z L2  xC xL
Z L2 
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L L

C C
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Characteristics of Parallel Resonant Ckt.
(ii) Circuit current:
V
 At parallel resonance line current is: I 
Zr
(iii) Quality factor Q:
 The ratio of inductive reactance of the coil at resonance to
its resistance is known as quality factor Q.
xL 2f r L

R
R
The Q of a parallel tuned circuit is very
important because:
 The sharpness of the resonance curve
and selectivity of the circuit depends on
it.
 Higher the value of Q the tuned circuit is
more selective
Q
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Advantages of Tuned Amplifier
1) Small power loss: Due to small resistance in the
tuned circuit power loss is quite low.
2) High Selectivity: It can select the desire
frequency for amplification out of a large no. of
frequencies simultaneously impressed upon it.
3) Smaller collector supply voltage: Because of
little resistance in the parallel tuned circuit, it
requires small collector supply voltage Vcc.
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Relation Between Q and Bandwidth
The quality factor Q of a tuned amplifier is equal to
the ratio of resonant frequency ‘fr’ to the bandwidth
(BW).
fr
Q
BW
Again the Q of an amplifier is determined by the
circuit component values.
Generally the Q of a tuned amplifier is greater than
10. when this condition met the resonance frequency
1
is
f 
r
2 LC
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Why Tuned Amplifiers are not Used for
Low Frequency Amplification?
1) Low frequencies are never single: Tuned amplifiers
are used to amplify a narrow or a single frequency
but available practical low frequencies are not single,
e.g., 20 Hz to 20 KHz.
2) High Values of L and C: The resonance frequency of a
parallel tuned amplifiers is
fr 
1
2 LC
If fr is low then L and C should be large. This will
make the tuned circuit bulky and expensive.
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Example
The Q of a tuned amplifier is 60. If the resonant
frequency for the amplifier is 1200 KHz, find (i) BW,
and (ii) cut-off frequencies.
Soln:
f r 1200
 20 KHz
(i) BW  
Q
60
(ii) Lower cut-off frequency, f1=1200-BW/2=1200-10=1190
Upper cut-off frequency, f1=1200+BW/2=1200+10=1210
Self study:
Example: 15.1, 15.2, 15.3 (V.K. Mehta)
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Single Tuned Amplifier
 A single tuned amplifier consists of a transistor
amplifier containing a parallel tuned circuit as the
collector load.
 The values of capacitance and inductance of the tuned
circuit are so selected that its resonant frequency is
equal to the frequency to be amplified.
 Operation?
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AC Equivalent Circuit of Tuned Amplifier(1/3)
Ac equivalent circuit of the tuned amplifier. Note
the tank circuit components are not shorted
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AC Equivalent Circuit of Tuned Amplifier(2/3)
 In order to completely understand the operation of this
circuit. We should consider the three frequency conditions
(a) fin = fr , (b) fin < fr , and (c) fin > fr
(a) When fin = fr:
• At fr, the parallel circuit offers a very high impedance,
i.e., act as open circuit.
• All the ac collector current flows through RL.
• So, voltage drop across RL is maximum, i.e., voltage gain
is maximum.
(b) When fin < fr :
 At this condition, the circuit is effectively inductive.
 As the frequency decreases from fr a point is reached
when xc-xL = RL the gain falls by 3db.
 So, lower cut-off frequency f1 for the circuit occurs when
xc-xL = RL
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AC Equivalent Circuit of Tuned Amplifier(3/3)
(c) When fin > fr:
 In this condition, the circuit is effectively
capacitive.
 As fin is increased beyond fr a point is reached
when xL-xC = RL the voltage gain of the amplifier
will again falls by 3db.
 So, upper cut-off frequency for the circuit will
occur when xL-xC = RL
Self-study, Example 15.7 VK Mehta
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Double Tuned Amplifier (DTA)
 It consists of a transistor amplifier containing two
tuned circuits one (L1C1) in the collector and the
other (L2C2) in the output.
 The resonant frequency of
tuned circuit L1C1 is made
equal
to
the
signal
frequency.
 The output from this tuned
circuit is transferred to the
second tuned circuit L2C2
through mutual induction.
 Double tuned circuits are used for coupling the
various circuits of radio and television receivers.
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Frequency Response of DTA
The frequency response of double tuned circuits
depends upon the degree of coupling, i.e., upon the
amount of mutual inductance between the two circuits.
When L2 is coupled to coil L1 a portion of load resistance
is coupled into primary tank circuit L1C1 and affects the
primary circuit in exactly the same manner as a resistor
had been added in series with primary coil L1 .
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Types of Coupling in DTA (1/2)
 There are two types of coupling:
(i) Loose coupling
(ii) Tight coupling
 (i) Loose coupling:
 When the coils are spaced apart all the fluxes of
primary coils ‘L1’ will not link the secondary coil
‘L2’.
 In this condition, resistance reflected from the
secondary circuit is small.
 So the resonance curve will be sharp and Q will
be high
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Types of Coupling in DTA (1/2)
 (ii) Tight coupling:
 When the coils are very close together all the
fluxes of primary coils ‘L1’ will link the secondary
coil ‘L2’.
 This coupling is called tight coupling
 In this condition, resistance reflected from the
secondary circuit is large.
 So the resonance curve will not sharp and Q will
be lower
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Bandwidth of DTA
 In DTA, bandwidth (BW) increases with the degree of
coupling
 If coupling is tighter the bandwidth is greater
BWDTA = frK
here, fr is the resonance frequency and K is the
coefficient of coupling
Example 15.8: It is desired to obtain a bandwidth of 200 kHz at an
operating frequency of 10 MHz using a double tuned circuit. What value of coefficient of coupling should be used ?
Soln:
Here, BWDTA =200 kHz , fr =10 MHz
We know:
BWDTA = frK
BWDTA 200 103
k

 0.02
6
fr
10 10
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Advantages of DTA
1. Bandwidth is increased.
2. Sensitivity (i.e. ability to receive weak signals)
is increased.
3. Selectivity (i.e. ability to discriminate against
signals in adjacent bands) is increased.
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Thanks for Your Kind
Attention
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