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• The tuning machines (with spiral metal worm gears) are mounted on the back of the headstock on the bass guitar neck

• A tuned amplifier – amplifies specific frequency or narrow band of frequency and it uses a tuned circuit i.e. circuit that selects that particular band of frequencies.
• It has a tuned or resonance circuit passes only a relatively narrow band of frequencies.
• The center of this frequency is the resonance frequency of the tuned circuit .
BY C GOMATHI, SVCE 4

• 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
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• 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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Difference between Amplifier and Tuned
Amplifier
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• Consider a tuned amplifier that is designed to amplify 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 input signal goes out of this range, amplification will be drastically reduced.
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• To amplify low frequency signals.
• Power output is low.
• Operated in Class A
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• To amplify large RF signals.
• Power output is more.
• Operated in Class B,class C or class AB modes.
• Pushpull configuration - reduces harmonic distortion.
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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 current generator.
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Condition for equivalence of Series and Parallel circuit
• Equating the admittance of series and parallel circuit.
• Admittance of series combination of RL is given as,
Y  1 R  j  L
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• Unloaded Q
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• Loaded Q
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Qeffective 
Susceptanc e of inductance L or capacitanc e C
Conductanc e of Shunt resistance Rt
Qeff   r
C eq
R t
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Double Tuned Amplifier
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• COUPLING SECTION OF TRANSFORMER
COUPLED DOUBLE TUNED AMPLIFIER
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• EQUIVALENT CIRCUIT
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• TRANSISTOR- current source with R
0.
• C
1
and L
1
- Tank circuits of primary side.
• C
2 and L
2
- Tank circuits of secondary side.
• R
1
is series resistance of inductance L
1.
• R
2 is series resistance of inductance L
2.
BY C GOMATHI, SVCE 31

• Series (R) and parallel (R p into series elements.
)elements are combined
R
P 

2
R
L
2
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• SIMPLIFIED EQUIVALENT CIRCUIT
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R
11


2 o
R o
L
2
1
 R
1
R
12


2 o
R i
L
2
2
 R
2
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• Current source is replaced by voltage source.
• Mutual inductance on primary and secondary sides.
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• Q factor for individual tank circuits are,
Usually Q factor for both circuits are same.
Q
1
=Q
2
=Q
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• From equivalent circuit, the output voltage is given as,
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Where
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• The simplified equ. Ckt of double tuned amplifier is similar to the previous ckt.
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 Zf becomes
K- coefficient of coupling
 For Zi
- Multiplying numerator and denominator by
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Then
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Substituting the value of I
2 ,
i.e V i
X Y
T
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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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The two gain peaks in the frequency response can be given at frequencies,
At K 2 Q 2 = 1,i.e i. K= (1/Q),f
1
= f
2
= f r
This condition –
critical coupling
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ii. K < 1/Q
Poor 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 is denoted as
• It represents the magnitude of the ripple in the gain curve.
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• The bandwidth b/w the freq’s 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 coupled amp. 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 cascaded amplifier becomes
• The 3 dB frequencies for the n stage cascaded amplifier can
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 is given as,
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• = 3dB BW of single stage double tuned amplifier
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• Improves the overall frequency response.
• Overall response exhibits maximal flatness around the centre frequency.
• It needs a number of tuned circuit operating in union.
• The overall frequency response of a Stagger tuned amplifier is obtained by adding the individual response together .
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• Since the resonant Frequencies of different tuned circuits are displaced or staggered, they are referred as STAGGER
TUNED AMPLIFIER.
• INCREASED BANDWIDTH
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• The stagger tuning in this circuit is achieved by resonating the tuned circuits L1
C1, L2 C2 to slightly different Frequencies.
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• Single tuned amplifier with separate resonant 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 individual gain of the 2 stages.
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• Substituting for X,
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• Efficiency of tuned amplifier increases as the 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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• The Q point is below the power supply value.
• 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 180 degrees.
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Waveform representing class C Amplifier
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• When no input signal is applied,
Vce =Vcc.
• When signal is applied, it causes Vce to swing above and below voltage.
this input
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• Vce can have maximum of 2Vcc and minimum 0 value.
• Output voltage has peak value of Vcc.
• The maximum A.C output power is,
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• The D.C input power from supply is
• Pd – power dissipation of transistor.
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• In class C operation transistor remains off for 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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• Schematic of mixer circuit
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• Mixer with class C amplifier is used in radio receivers.
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• Mixer circuits are used to translate signal frequency to lower or higher frequencies.
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• When there exist a positive feedback due to low reactance (collector to base), it generates oscillation.
• This causes instability.
• To prevent oscillations,
 Reduced gain,
 Detuning, detrimental effects on selectivity
 Q reduction,
 neutralization.
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Neutralization
Hazeltine
Neutralization
Modified
Hazeltine
Neutralization
( Neutrodyne )
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Neutralization using coil.
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• Cn - neutralizing capacitor.
• This capacitor provides signal of opposite polarity to nullify the effects of collector to base capacitor signal.

• Cn - connected from the lower end of the base coil of the next stage to the base of the transistor.

BY C GOMATHI, SVCE 87

• L part of the tuned circuit at the base of next stage is oriented for minimum coupling to the other winding.

• Single Tuned Amplifier
• Double Tuned Amplifier
• Stagger Tuned Amplifier
• Class C Tuned Amplifier
• Stability of Tuned Amplifier