unit 3 tuned amplifier
TRANSCRIPT
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TUNED AMPLIFIER
1
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The tuning machines (with spiral metalworm gears) are mounted on the back of
the headstock on the bass guitar neck
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INTRODUCTION
A tuned amplifier amplifies specific frequencyor narrow band of frequency and it uses a tunedcircuit i.e. circuit that selects that particular bandof frequencies.
It has a tuned or resonance circuitpasses onlya relatively narrow band of frequencies.
The center of this frequency is the resonance
frequency of the tuned circuit .
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APPLICATION
Tuned amplifiers are mostly used for the amplification of high or radio frequencies.
The signals are transmitted by modulation.
It has a relatively narrow band of frequencies centered around the carrier frequency.
When it reaches the antenna it induces a weak voltage in it.
It is not possible to extract the original audio signal from this weak signal.
It is thus first amplified and this is done at the front end of the radio receiver with the
help of a tuned amplifier.
This amplifier not only selects the desired radio frequency signal (corresponding to
a particular broadcasting station) ,but also amplifies this signal to a suitable level.
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Simple Transistor Tuned Amplifier
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Difference between Amplifier and Tuned
Amplifier
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ILLUSTRATION
Consider a tuned amplifier that is designed toamplify only those frequencies that are within
20 kHz of the central frequency of 1000 kHz.
f1 = 980 kHz
fr = 1000 kHz f2 = 1020 kHz
BW = 40 kHz
If input signal is within the range of 980 1020
kHz, it will be amplified. If the frequency of inputsignal goes out of this range, amplification will
be drastically reduced.8BY C GOMATHI, SVCE
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CLASSIFICATION
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Small signal tuned amplifier
To amplify low frequency signals.
Power output is low.
Operated in Class A
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LARGE SIGNAL TUNED AMPLIFIER
To amplify large RF signals.
Power output is more.
Operated in Class B,class C or class ABmodes.
Pushpull configuration - reduces harmonicdistortion.
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Single Tuned Capacitive Coupled Amplifier
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ANALYSIS
EQUIVALENT CIRCUT OF SINGLE TUNED AMPLIFIER USING HYBRID PI
MODEL
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SIMPLIFIED EQUIVALENT CIRCUT OF
SINGLE TUNED AMPLIFIER
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Ci input capacitance. Ceq - output capacitance
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gce output resistance of currentgenerator.
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Vid hi lVid hi l
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Condition for equivalence of Series
and Parallel circuit
Equating the admittance of series andparallel circuit.
Admittance of series combination of RL is
given as,
LjRY 1
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Vid thi lVid thi l
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Centre frequency
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Quality factor,Q
Unloaded Q
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Loaded Q
22
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Simplified output circuit
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Effective equivalent Q factor
RtresistanceShuntofeConductanc
CecapacitancorLinductanceofeSusceptancQeffective
teqr RCQeff 24BY C GOMATHI, SVCE
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Voltage gain
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3 dB Bandwidth
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Double Tuned Amplifier
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Double Tuned Amplifier
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COUPLING SECTION OF TRANSFORMERCOUPLED DOUBLE TUNED AMPLIFIER
ANALYSIS
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EQUIVALENT CIRCUIT
ANALYSIS
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TRANSISTOR- current source with R0.
C1 and L1 - Tank circuits of primary side.
C2 and L2 - Tank circuits of secondary side.
R1 is series resistance of inductance L1.
R2
is series resistance of inductance L2.
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Series (R) and parallel (Rp)elements are combinedinto series elements.
R
L
RP
22
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ANALYSIS
SIMPLIFIED EQUIVALENT CIRCUIT
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y p
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y p
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111
1
22
R
oR
Lo
R
212
2
22
R
i
R
Lo
R
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y p
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y p
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Current source is replaced by voltagesource.
Mutual inductance on primary and
secondary sides.
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y p
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y p
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Q factor for individualtank circuits are, Usually Q factor for bothcircuits are same.
Q1=Q2=Q
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y p
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y p
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From equivalent circuit, the output voltageis given as,
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Consider this circuit
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Transfer admittance
Where
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The simplified equ. Ckt of double tunedamplifier is similar to the previous ckt.
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On further simplification
Zf becomes
K- coefficient of coupling
For Zi - Multiplying numerator and denominator by
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Similarly for Zo+Z
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Then
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Substituting the value of I2 , i.e Vi X YT
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Taking magnitude,
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Universal response curve for double tuned amplifier
The Frequency deviation
at gain peaks can be
found by maximizing Av.
Equating to zero
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BY C GOMATHI, SVCE 47
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Inference from graph
The two gain peaks in the frequency response canbe given at frequencies,
At K 2 Q 2= 1,i.e
i. K= (1/Q),f1= f
2= f
r
This condition critical coupling
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ii. K < 1/QPoor coupling, peak gain is < max gain.
iii. K > 1/Q
Over coupling, peak gain is doubled.
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The ratio of peak gain and dip gain isdenoted as
It represents the magnitude of the ripple in
the gain curve.
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The bandwidth b/w the freqs at which the
gain is magnitude of dip gain and it is the
useful bandwidth of doubled tuned amp.
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3dB BW = 2fr/Q single tuned amp.
3dB BW = 3.1 fr/Q double tuned amp.
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Effect of cascading single tuned amplifier
on BW
Consider n stages of single tuned direct coupledamp. connected in cascade.
The overall gain is the product of voltage gains of
individual stages.
The relative gain of single tuned amp. w.r.t
resonance freq. is given as,
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The relative gain of n stage cascadedamplifier becomes
The 3 dB frequencies for the n stage
cascaded amplifier can be found by
equating,
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Substituting for frequency deviation
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Let f1 and f2 be lower 3dB and upper 3dB
frequencies.
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The BW on n stage identical amplifier isgiven as,
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Effect of cascading double tuned
amplifier on BW
= 3dB BW of single stage doubletuned amplifier
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Stagger tuned amplifier
Improves the overall frequency response.
Overall response exhibits maximal flatness
around the centre frequency.
It needs a number of tuned circuit operating inunion.
The overall frequency response of a Stagger
tuned amplifier is obtained by adding theindividual response together.
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Since the resonant Frequencies ofdifferent tuned circuits are displaced or
staggered, they are referred as STAGGER
TUNED AMPLIFIER.
INCREASED BANDWIDTH
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Circuit diagram
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The stagger tuning in this circuit isachieved by resonating the tuned circuits L1
C1, L2 C2 to slightly different Frequencies.
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Overall response of stagger tunedwww.Vidyarthiplus.comwww.Vidyarthiplus.com
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Overall response of stagger tuned
amplifier
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Analysis
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Single tuned amplifier with separateresonant frequencies are used in stagger
tuned amplifier.
The resonant frequencies are
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The gain of single tuned amplifier is
According to this tuned frequencies, the
selectivity function is given as,
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The overall gain is product of individualgain of the 2 stages.
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Substituting for X,
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Large single tuned amplifier
Efficiency of tuned amplifier increases asthe operation class A to class C.
Output power and efficiency are of great
concern in radio transmission.
Class C amplifiers are used at the output
stages in transmitters.
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Class C tuned amplifier
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The Q point is below the power supplyvalue.
The output is obtained for less than half
cycle for full input cycle.
Conduction Angle The total angle during
which current flows is less than 180degrees.
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Waveform representing class C Amplifier
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Maximum A.C Power
When no input signal
is applied,
Vce =Vcc.
When signal is
applied, it causes Vce
to swing above and
below this input
voltage.
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BY C GOMATHI, SVCE 75
Vce can have maximum of 2Vcc andminimum 0 value.
Output voltage has peak value of Vcc.
The maximum A.C output power is,
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ff
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Maximum Efficiency
The D.C input power from supply is
Pd power dissipation of transistor.
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In class C operation transistor remains offfor most of the input signal cycle.
So Power dissipation (Pd) for class C
amplifier is very less.
Pd can be neglected.
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A li ti
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Applications
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Schematic of mixer circuit
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Mixer with class C amplifier is used inradio receivers.
BY C GOMATHI, SVCE 79
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BY C GOMATHI, SVCE 80
Mixer circuits are used to translate signalfrequency to lower or higher frequencies.
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St bilit f t d lifi
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Stability of tuned amplifiers
When there exist a positive feedback dueto low reactance (collector to base), it
generates oscillation.
This causes instability.
To prevent oscillations,Reduced gain,
Detuning, detrimental effects on selectivity
Q reduction,
neutralization.
BY C GOMATHI, SVCE 81
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N t li ti
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Neutralization
BY C GOMATHI, SVCE 82
Neutralization
Hazeltine
Neutralization
Modified
Hazeltine
Neutralization
(Neutrodyne)
Neutralization
using coil.
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H lti N t li ti
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Hazeltine Neutralization
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Cn - neutralizing capacitor.
This capacitor provides signal of oppositepolarity to nullify the effects of collector to
base capacitor signal.
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Modified Ha eltine Ne trali ation
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Modified Hazeltine Neutralization
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Cn - connected from the lower end of the
base coil of the next stage to the base of
the transistor.
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Neutralization using coil
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Neutralization using coil.
BY C GOMATHI, SVCE 87
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L part of the tuned circuit at the base of
next stage is oriented for minimum
coupling to the other winding.
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Review
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Review
Single Tuned Amplifier
Double Tuned Amplifier
Stagger Tuned Amplifier
Class C Tuned Amplifier
Stability of Tuned Amplifier