Unit 3 Tuned Amplifier

7/25/2019 Unit 3 Tuned Amplifier

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TUNED AMPLIFIER

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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

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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.






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






Vid thi lVid thi l


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Vid thi lVid thi l


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Centre frequency






Vid thi lVid thi l


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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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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



y p


y p



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111

1

22

R

oR

Lo

R

212

2

22

R

i

R

Lo

R



y p


y p



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Current source is replaced by voltagesource.

Mutual inductance on primary and

secondary sides.



y p


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


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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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.



Eff t f di i l t d lifiwww.Vidyarthiplus.comwww.Vidyarthiplus.com


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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,



Effect of cascading double tunedwww.Vidyarthiplus.comwww.Vidyarthiplus.com


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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.



Overall response of stagger tunedwww.Vidyarthiplus.comwww.Vidyarthiplus.com


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Overall response of stagger tuned

amplifier





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Analysis


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


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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Vce can have maximum of 2Vcc andminimum 0 value.

Output voltage has peak value of Vcc.

The maximum A.C output power is,


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.



A li ti



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Applications


Schematic of mixer circuit




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Mixer with class C amplifier is used inradio receivers.





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Mixer circuits are used to translate signalfrequency to lower or higher frequencies.


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.



N t li ti



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Neutralization


Neutralization

Hazeltine

Neutralization

Modified

Hazeltine

Neutralization

(Neutrodyne)

Neutralization

using coil.


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.


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.


Neutralization using coil



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Neutralization using coil.





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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.


Review



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Review

Single Tuned Amplifier


Stagger Tuned Amplifier

Class C Tuned Amplifier

Stability of Tuned Amplifier

Unit 3 Tuned Amplifier

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