BJT Tuned amplifiers

Tuned Amplifier fundamentals, single Tuned AMplifiers

1. Tuned AmplifiersTuned Amplifiers
C VenkataSudhakar
Assistant Professor, Department of ECE
SreeVidyanikethanEngineering college
mail ID:[email protected]

2. Contents:

Introduction to TunedAmplifier

Q-Factor

Small signal tunedamplifier

Capacitance single tuned amplifier

Double Tuned Amplifier

Double Tuned Amplifier

Effect of Cascading single TunedAmplifier

Staggered tunedAmplifiers

Stability of TunedAmplifiers

3. Tuned Amplifier
Tunedamplifiersareusedtofortheamplificationofaspecific
frequencysignaloranarrowbandfrequencysignal.Basically
highfrequencyorradiofrequencysignalsareamplifiedusing
tunedamplifiers.Tuning(i.e.,selecting)offrequencyisdoneby
usingatunedorresonantcircuitattheload.usingatunedorresonantcircuitattheload.

4. Types of Tuned Circuits
Series Tuned Circuit
Theinductorandcapacitorconnectedinseriesmakeaseries
tunedcircuit(Seriesresonantcircuit),asshowninthefollowing
circuitdiagram.
Atresonantfrequency,aseriesresonantcircuitofferslow
impedancewhichallowshighcurrentthroughit.Aseriesresonant
circuitoffersincreasinglyhighimpedancetothefrequenciesfar
fromtheresonantfrequency.

5. Parallel Tuned Circuit
The inductor and capacitor connected in parallel make a parallel
tuned circuit (parallel resonant circuit), as shown in the below
figure.
Atresonantfrequency,aparallel
resonantcircuitoffershighimpedance
whichdoesnotallowhighcurrent
throughit.Aparallelresonantcircuit
offersincreasinglylowimpedancetooffersincreasinglylowimpedanceto
thefrequenciesfarfromtheresonant
frequency.

6. Characteristics of a Parallel Tuned Circuit
Thefrequencyatwhichparallelresonanceoccurs(i.e.reactive
componentofcircuitcurrentbecomeszero)iscalledtheresonant
frequencyf
r
.
Atparallelresonance,thecircuitorlinecurrentIdecreaseswhile
theimpedanceZ
r
increases.
ImpedanceofthecircuitdecreasesforthevaluesaboveandImpedanceofthecircuitdecreasesforthevaluesaboveand
belowtheresonantfrequency f
r
.
https://www.tutorialspoint.com/amplifiers/tuned_amplifiers.htm

7. RESONANCECIRCUITS:
Atresonance,thetunedcircuitcanbeusedasaresistiveload
foramplifiersbecausethereactancepartbecomeszero,i.e.,the
inductiveandcapacitiveeffectscancelout.Hencethecircuitacts
asapureresistiveonewithbothVandIinphase.
ParallelResonance:Forfrequenciesabovefr,thecircuitactsas
acapacitiveoneandhencethecurrentleadstheappliedvoltage.
Atfrequenciesbelowfr,itactsasaninductiveone,inwhich
currentlagsbehindtheappliedvoltage.currentlagsbehindtheappliedvoltage.

8. Advantages of Tuned Amplifiers
The usage of reactive components like L and C, minimizes the
power loss, which makes the tuned amplifiers efficient.
The selectivity and amplification of desired frequency is high, by
providing higher impedance at resonant frequency.
A smaller collector supply VCC would do, because of its little
resistance in parallel tuned circuit.resistance in parallel tuned circuit.
Disadvantagesof Tuned amplifiers
The overall circuitry is costlyas well as bulky due to the
presence of inductors and capacitors in tuned circuits.
Amplification in the range of audio frequency cannot be
achieved.
Increase in bandwidth leads to complexityin the circuit.

9. Q-factor
The Q-factor as quality factor of an inductor at operating
frequency ‘ω’ is defined as the ratio of impedance of the coil to its
resistance and can be defined as
ThelowercoilResistance,givesthebetterQ-factorforan
inductorcoil.ThehigherthevalueofQ,thesharperresonance
curveandbetterselectivitywillbe
Q-factorofInductorisωL/R
Q-factorofCapacitoris1/ωCR

10. Frequency Response of Tuned
Amplifier
Foranamplifiertobeefficient,itsgainshouldbehigh.This
voltagegaindependsuponβ,inputimpedanceandcollector
load.Thecollectorloadinatunedamplifierisatunedcircuit.
Thevoltagegainofsuchanamplifierisgivenby
WhereZ
C
=effectivecollectorloadandZ
in
=inputimpedance
oftheamplifier.
ThevalueofZ
C
dependsuponthefrequencyofthetuned
amplifier.AsZ
C
ismaximumatresonantfrequency,thegain
oftheamplifierismaximumatthisresonantfrequency.

11. Bandwidth
Therangeoffrequenciesatwhichthevoltagegainofthetuned
amplifierfallsto70.7%ofthemaximumgainiscalledits
Bandwidth.
ThebandwidthofatunedamplifierdependsupontheQofthe
LCcircuiti.e.,uponthesharpnessofthefrequencyresponse.The
valueofQandthebandwidthareinverselyproportional.
FigureshowsthebandwidthandfrequencyresponseoftheFigureshowsthebandwidthandfrequencyresponseofthe
tunedamplifier.
BW=f
2
-f
1

12. Relation between Q and Bandwidth
The quality factor Q of the bandwidth is defined as the ratio of
resonant frequency to bandwidth, i.e.,
In general, a practical circuit has its Q value greater than 10.
Under this condition, the resonant frequency at parallel
resonance is given by

13. Q-factor of Inductor
The Q-factor of a inductor at the operating frequency ‘ω’ is the
ratio of reactance of the inductor to its series resistor (R).

14. Q-factor of Inductor

15. Q-factor of Capacitor
The Q-factor of a capacitor at the operating frequency
‘ω’ is the ratio of reactance of the capacitor to its series resistor (R).

16. Q of lossycapacitor

17. 17

18. 18
Q

19. DEFINITION-Tuned Amplifier
InanamplifiercircuitRc(loadcircuit)replacedwithatank
circuit/tunedcircuitiscalledtunedamplifier.
Tunedamplifieramplifiesthesignalswithinanarrow
frequencybandcenteredaboutafrequencyfo.Thetuned
amplifierisdesignedtorejectallfrequenciesbelowalower
cutofffrequency,f
L
andaboveauppercutofffrequency,f
H
.The
tunedamplifierisextensivelyusedincommunication
equipment,especiallyinbroadcastreceiver
.
equipment,especiallyinbroadcastreceiver
.

20. Cont’d….
Thegainofatransistoramplifierisdirectlyproportionaltothe
valueofitsloadimpedance.Aparalleltunedcircuithashigh
impedanceatitsfrequencyofresonanceandtheimpedancefalls
offsharplyasthefrequencydepartsfromthefrequencyof
resonance.Hence,thegainversusfrequencycurveofthe
tunedamplifierisalmostsimilartotheimpedanceversus
frequencycurveofthetunedamplifier.

21. Classificationof
TunedCircuits
Small signal amplifier,
low power, radio
frequency
Large signal amplifier,
largepower, radio
frequency
ClassA
Single Tuned
circuit(one
parallelcircuit
isemployed)
Double tuned
circuit(two
tuned circuit
areemployed)
Staggered
Tunedamplifier
ClassAB,B&C
Shuntpeaked
tuned with
higher band
width

22. CLASSIFICATION OF TUNED AMPLIFIERS

SmallSignalTuned Amplifiers :-They are used to amplify
the RF signals of smallmagnitude.
They arefurtherclassifiedas:
(a)
SingleTuned Amplifiers:-Single tuned amplifiers use
one parallel resonant circuit as the load impedance in
each stage and all the tuned circuits are tuned to the
same frequency.
(b)
DoubleTunedAmplifiers:-Inthisweusetwo
(b)
DoubleTunedAmplifiers:-Inthisweusetwo
mutuallycoupled tuned circuits foreverystagebothof
tuned circuits are tuned at samefreq.
(c)
Stagger Tuned Amplifiers:-Stagger tuned amplifiers use
a number of single tuned stages in cascade, the successive
tuned circuits being tuned to slightly different frequencies.
Single tuned amplifiers can be further classified as
(a) capacitance coupled single tuned amplifier and
(b) transformer coupled or inductively coupled single tuned
amplifier
.

23. Capacitive coupled single tuned Amplifier
An amplifier circuit with a single tuner section
being at the collector of the amplifier circuit is
called as Single tuner amplifier circuit.
Asimpletransistoramplifiercircuit
consistingofaparalleltunedcircuit
initscollectorload,makesasingle
tunedamplifiercircuit.Thevaluesoftunedamplifiercircuit.Thevaluesof
capacitanceandinductanceofthe
tunedcircuitareselectedsuchthatits
resonantfrequencyisequaltothe
frequencytobeamplified.
The tuned circuit offers high impedance at tuned frequency, which helps to
offer high output across the tuned circuit , all the other frequencies which get
lower impedance

24. AC Analysis
Figure gives the equivalent circuit for the tuned amplifier using
high frequency hybrid-πmodel for the transistor.
24

25. Here, A is the voltage gain of the amplifier. C
1
and C
2
are the stray wiring
capacitances in the input and output circuits, respectively.
The equivalent circuit shown in Figure can be simplified where all the
capacitances in the input circuit can be grouped together to form C
S
given by
Similarly, all the capacitances in the output circuit can be grouped together
to form C given by
Similarly Similarly
where R o is the output resistance of current generator gm Vb’e
.

26. The reactancesof the bypass capacitor C
E
and the coupling capacitor C
C
are
negligibly small at the operating frequency and these elements can be
neglected in the simplified equivalent circuit shown in Figure

27. 92

28. The output circuit of the amplifier can be modified as shown in Figure
Taking R
t
as the parallel combination of Ro, R
P
and R
i
, i.e.
28

29. Impedance Z
Z is the impedance of C, L and R
t
in parallel. The admittance Y is
given by

30. Impedance Z cont’d…
Let δindicate the fractional frequency variation, i.e., variation in
frequency expressed as a fraction of the resonant frequency.
Then
or

31. Impedance Z at resonancecont’d…

32. The output circuit can be modified as shown in Figure
From the output circuit shown in Fig.
From the simplified circuit neglecting Cs
Substituting in the above equation we get voltage gain

34. Bandwidth

35. 
SINGLETUNEDAMPLIFIERUSINGFET

36. LIMITATION:
•
This tuned amplifier are required to be highly selective.
But high selectivity required a tuned circuit with a high Q-
factor.
•
AhighQ-factorcircuitwillgiveahighA
v
butatthe
sametime,itwillgivemuchreducedbandwithbecause
bandwidthisinverselyproportionaltotheQ-factor.bandwidthisinverselyproportionaltotheQ-factor.
•
Itmeansthattunedamplifierwithreducebandwidthmay
notbeabletoamplifyequallythecompletebandof
signals&resultispoorreproduction.Thisiscalled
potential instability in tunedamplifier.

37. Double tunedAmplifier:

38. 38

39. 39

40. 40

41. 41

42. 42

43. 43

44. 44

45. 45

46. STAGGER TUNED AMPLIFIERS:

47. 47

48. 48

49. StabilityConsiderations
49

58. Video Amplifiers (ICMC1550)

60. APPLICATIONS OF TUNEDAMPLIFIER
Tuned amplifiers serve the best for twopurposes:
a)Selection of desiredfrequency.
b)Amplifying the signal to a desiredlevel.
USEDIN
:
USEDIN
:

Communication transmitters andreceivers.

Infilter design:--BandPass, lowpass, High passand
band reject filterdesign.

61. ADVANTAGES

It provides highselectivity.

It has small collectorvoltage.

Power loss is also less.

Signal to noiseratioofO/P isgood.

Signal to noiseratioofO/P isgood.

They are well suited for radio transmitters and
receivers.

62. DISADVANTAGES

They are not suitable to amplify audio
frequencies.

If the band of frequency is increase thendesign
becomescomplex.

Since they use inductors and capacitors as tuning
elements, the circuit is bulky andcostly.

63. References
•
•
•
“Integrated Electronics”, J. Milliman and C.C.
Halkias, Tata McGraw Hill, 1972
“Electronic Circuit Analysis-B.V. Rao, K.R.Rjeswari,
P.C.R.Pantulu-Pearsonpublication
“Electronic Devices and Circuits” –Salivahanan, Suresh•
•
•
•
“Electronic Devices and Circuits” –Salivahanan, Suresh
Kumar,
A.Vallavaraj, TATA McGraw
Hill www.nptel.ac.in
www.nesoacademy.org
www.tutorialspoint.com

64. ThankyouThankyou