Download - Tuned amplifire
TUNED AMPLIFIER
Rakesh Mandiyahttps://in.linkedin.com/in/[email protected]
TUNED AMPLIFIER
IntroductionBand Pass AmplifiersSeries & Parallel Resonant Circuits & their
BandwidthAnalysis of Single Tuned AmplifiersAnalysis of Double Tuned AmplifiersPrimary & Secondary Tuned Amplifiers with
BJT & FETMerits and de-merits of Tuned Amplifiers
DEFINITION:-
An amplifier circuit in which the load circuit is a tank circuit such that it can be tuned to pass or amplify selection of a desired frequency or a narrow band of frequencies, is known as Tuned Circuit Amplifier.
CHARACTERISTICS OF TUNED AMPLIFIER Tuned amplifier selects and amplifies a single frequency
from a mixture of frequencies in any frequency range.
A Tuned amplifier employs a tuned circuit. It uses the phenomena of resonance, the tank circuit which is
capable of selecting a particular or relative narrow band of frequencies.
The centre of this frequency band is the resonant frequency of the tuned circuit .
Both types consist of an inductance L and capacitance C with two element connected in series and parallel.
RESONANCE CIRCUITS: When at particular frequency the inductive reactance
became equal to capacitive reactance and the circuit then behaves as purely resistive circuit. This phenomenon is called the resonance and the corresponding frequency is called the resonant frequency.
C L
Tuned circuit
Resonance circuits
Series Parallel
Classification of Tuned Circuits
Small signal amplifier, low power, radio
frequency
Class A
Single Tuned
circuit(one parallel circuit is
employed)
Double tuned
circuit(two tuned
circuit are employed)
Staggered
Tuned amplifi
er
Large signal amplifier, low power, radio
frequency
Class B&C
Shunt peaked tuned with higher
band width
CLASSIFICATION OF TUNED AMPLIFIER
Tuned amplifier
Small Signal Amplifier
Single Tuned
Amplifier
Double Tuned
Amplifier
Stagger Tuned
Amplifier
Large signal Amplifier
CLASSIFICATION OF TUNED AMPLIFIERS
Small Signal Tuned Amplifiers :- They are used to amplify the RF signals of small magnitude.
They are further classified as: (a) Single Tuned Amplifiers:- In this we use one
parallel tuned circuit in each stage. (b) Double Tuned Amplifiers:- In this we use two
mutually coupled tuned circuits for every stage both of tuned circuits are tuned at same freq.
(c) Stagger Tuned Amplifiers:- It is a multistage amplifier which has one parallel tuned circuit for every stage but tuned frequency for all stages are slightly different from each other.
(2) Large signal tuned amplifiers:- They are meant for amplifying large signals
in which large RF power is involved & distortion level is also higher. But tuned circuit itself eliminates most of the harmonic distortion.
BAND PASS AMPLIFIER:An amplifier designated to pass a definite band
of frequencies with uniform response.
The new band pass amplifier perform both function of low noise amplifier (LNA) & band pass filter is proposed for application of 900Mhz RF Front – end in wireless receivers .
BAND PASS AMPLIFIER:
It is having two differential stage comprising two transistor.
Main function of band pass filter to remove the band noise ,which also contributes to the rejection of image signals.
Finally a band pass amplifier amplifies only a band of frequency which lie in bandwidth of amplifier & thus named as band pass amplifier .
BAND PASS AMPLIFIER
BAND PASS AMPLIFIER
BAND PASS FILTER
SERIES RESONANT CIRCUITIt is the circuit in which all the resistive and reactive components are in series.
SERIES RESONANT LC
SERIES RESONANT CIRCUIT Impedance Of The Circuit: -
Z = R2 + (XL – Xc)21/2
Z = R2 + (ωL – 1/ ωC)21/2
For resonant frequency:-(XL = XC )
XL = ωL = 2 π frL XC = 1/ ωC = 1 / 2 π frC
SERIES RESONANT CIRCUITSince at resonance, XL = Xc
2 π frL = 1 / 2ПfrC fr = 1 / 2 π √LC ωr = 1 / √LC
RESONANCE CURVE OF SERIES RESONANT CIRCUIT :
QUALITY FACTOR It is voltage magnification that circuit produces at
resonance is called the Q factor.
Voltage Magnification = Imax XL / Imin R
= XL/ R
At Resonance XL/R = XC/R ωrL / R = 1 / ωrRC
THUS
Qr = ωrL / R = 1/ ωrC R = 2 π fr L / R = (2 π L / R) * (1 / 2 π √LC ) = √(L/C) / R = tanФ tan Ф = power factor of coil
IMPORTANT POINTS (1) Net reactance , X = 0. (2) Impedance Z = R . (3) Power factor is unity. (4) Power expended = 6 watt. Current is so large & will produce large voltage
across inductance & capacitance will be equal in magnitude but opposite in phase.
Series resonance is called an acceptor circuit because such a circuit accepts current at one particular frequency but rejects current at other frequencies these circuit are used in Radio – receivers .
REACTANCE CURVE SERIES RESONANT CIRCUIT
XL = 2ΠfL
X = XL - XC XC = 1
2ΠfC
R
fr
curr
ent
PARALLEL OR CURRENT RESONANCE
PARALLEL OR CURRENT RESONANCE
When an inductive reactance and a capacitance are connected in parallel as shown in figure condition may reach under which current resonance (also known as parallel or anti- resonance ) will take palace. In practice, some resistance R is always present with the inductor.
Such circuit is said to be in electrical resonance when reactive(watt less) components of line current becomes zero. The frequency at which this happened is known as resonant frequency.
Current will be in resonance if reactive component of R-L branch IR-L sinФ R-L = Reactive component of capacitive branch, neglecting leakage reactance of capacitor C
FREQUENCY V/S IMPEDANCE CURVE FOR LCR CIRCUIT
CURRENT AT RESONANCECapacitive Current Ic = 2 π fr CV
Coil Current IR-L = V/Z
= V / √ R2+ (ωrL)2 ФR-L = Cos-1(R/Z)
ФR-L = Sin-1(2ПfrL/Z)
( V / √ R2+ (ωrL)2 ) * (ωrL / √ R2+ (ωrL)2 ) = ωrCV
C = L / (R2+ (ωrL )2
ωr = (1 / √LC) * (( 1- CR2/L) )1/2
ωr = √(1/LC – R2/L2)
fr = ωr /2 π = (1/2 π ) * ((1/LC)- (R2 / L2))
RESONANCE CURVE OF PARALLEL RESONANT CIRCUIT :
With low resistance
With high resistance
current
Res
onan
t fr
eque
ncy
fR
Active component of coil IA = IR- L cosФR-L
= (V/Z) * (R/Z)
= VR/Z2
Reactive component of coil IR = IR-L sinФ = (V/Z ) * (2 π frL/Z )
Since at resonance Reactive component of coil current = Capacitive current (V / Z ) * (2 π frL / Z) = 2 π frCV Z = √(L/C) ………..(1) Line current IL = Active component of coil current = IA = IR-L cosФR-L
= VR/Z2 [using (1)] = VR(C/L) IL = [ V / (L/RC) ] (L/RC) = Effective or equivalent dynamic impedance of
parallel circuit at resonance.
IMPORTANT POINTS FOR CURRENT OR PARALLEL RESONANCE:
(1) Net susceptance is zero (1 / XC ) = ( XL / Z2 )(2) Admittance = Conductance(3) Power factor is unity as reactive ( wattles)
components of the current is zero(4) Impedance is purely resistive ZMax = (L / CR)(5) ILine(Min) = V / ( L/CR ) ( in phase with applied
voltage)
(6) f = (1/2П) * ( √(1/LC) – (R2/ L2)) Hz The frequency at which the net susceptance curve
crosses the frequency axis is called the resonant frequency .
At this point impedance is maximum or admittance is minimum & is equal to G , consequently (I) Line is minimum .
Band with of parallel resonant circuit B.W. = (f2 – f1) Quality Factor Q = XL / R = 2ПfrL / R Quality factor determines sharpness of resonance curve
and selectivity of circuit. Higher the value of quality factor more selective the tuned
circuit is.
CHARACTERSTICS OF PARALLEL OR CURRENT RESONANCE
Admittance is equal to conductance. Reactive or watt less component of line current is zero
hence circuit power factor is unity. Impedance is purely resistive , maximum in
magnitude and is equal to L/CR. Line current is minimum and is equal to
V / (L/CR) in magnitude and is in phase with the applied voltage.
REACTANCE CURVE PARALLEL RESONANT CIRCUIT
XL = 2ΠfL
X = XL - XC
curr
ent
1
XC =
2ΠfC
R
(1) SINGLE TUNED AMPLIFIER
+
Vs
Cin R1
R2
CeRe
Cc RL
LC
Vcc
(1) SINGLE TUNED AMPLIFIER• O/P of this amplifier may be taken either with the
help of Capacitive.
• A parallel tuned circuits is connected in the collector circuit.Tuned voltage amplifier are usually employed in RF stage of wireless communication , where such circuits are assigned the work of selecting the desired carrier frequency and of amplifying the permitted pass-band around the selected carrier frequency.
SINGLE TUNED AMPLIFIER Tuned amplifier are required to be
R1, R2, & Re = For biasing & stabilization circuit. Ce = By pass capacitor L-C = Tuned circuit connected in collector, the impedance of which depend upon frequency, act as a collector
load. If i/p signal has same frequency as resonant frequency
of L-C circuit . Large amplification will be obtain because of high impedance of L-C ckt.
SINGLE TUNED AMPLIFIER USING FET
SINGLE TUNED AMPLIFIER USING FET In the shown figure the single tuned amplifier
is depicted using a field effect transistor. The value of L and C is selected as per the
desired frequency level. One of the components either L or C is
variable so as to adjust the variable frequency.
THE HIGH FREQUENCY SIGNAL TO BE APPLIED BETWEEN BASE & EMITTER. THE RESONANT FREQUENCY OF CIRCUIT IS MADE EQUAL TO FREQUENCY OF I/P SIGNAL BY VARYING L OR C .
NOW TUNED CKT WILL OFFER VERY HIGH IMPEDANCE TO THE SIGNAL FREQ. & THUS LARGE O/P APPEAR ACROSS IT. AV = ( Β RAC )/ RIN RAC = TUNED CIRCUIT IMPEDANCE = Β(L/CR)/ RIN
AV = ΒL / ( CRRIN ) BANDWIDTH = (F2- F1 )
THE AMPLIFIER WILL AMPLIFY ANY FREQ. WELL WITHIN THIS RANGE.
CIRCUIT OPERATION
LIMITATION This tuned amplifier are required to be highly selective. But
high selectivity required a tuned circuit with a high Q- factor .
A high Q- factor circuit will give a high Av but at the same time , it will give much reduced band with because bandwidth is inversely proportional to the Q- factor .
It means that tuned amplifier with reduce bandwidth may not be able to amplify equally the complete band of signals & result is poor reproduction . This is called potential instability in tuned amplifier.
DOUBLE TUNED CIRCUIT :
DOUBLE TUNED CIRCUITThe problem of potential instability with a single tuned
amplifiers overcome in a double tuned amplifier which consists of independently coupled two tuned circuit :
(1) L 1C1 in collector circuit (2) L2 C2 in output circuit
A change in the coupling of two tuned circuit results in change in the shape of frequency response . By proper adjustment of coupling between two coils of two tuned circuits, the required results are :
High selectivity High voltage gain Required bandwidth
CIRCUIT OPERATION The resonant freq. of tuned circuit connected in collector circuit is
made equal to signal freq. by varying the value of C1. Tuned circuit (L 1C1) Offer very high impedance to
signal frequency & this large o/p is developed across it. The o/p of (L1C1) is transferred to (L2C2) through
mutual inductance. Thus the freq. response of double tuned circuit depends upon
magnetic coupling of L1 & L2. Most suitable curve is when optimum coefficient of coupling exists
between two tuned circuit .The circuit is then highly selective & also provides sufficient amount of gain for a particular band of frequency.
Volta
ge
gain
AV
Frequency fr
K=2
K=1.5K=1
fr
Critical coupling
Loose coupling
Resonance curve of Parallel Resonant circuit:
SHUNT PEAKED CIRCUITS FOR INCREASED BANDWIDTH
For expanding bandwidth we use various combinations of BJT & FET(MOS) in series or shunt so that we can use Stagger tuned amplifiers.
Shunt Peaking If a coil is placed in parallel (shunt) with the output signal path,
the technique is called SHUNT PEAKING. R1 is the input-signal-developing resistor. R2 is used for bias and temperature stability. C1 is the bypass capacitor for R2. R3 is the load resistor for Q1 and develops the output signal. C2 is the coupling capacitor which couples the output signal to the next stage.
SHUNT PEAKED CIRCUITS FOR INCREASED BANDWIDTH :
STAGGER TUNED AMPLIFIERS It is a multistage amplifier which has one parallel resonant
circuit for every stage, while resonant frequency of every stage is slightly different from previous stages. From circuit diagram it is clear that first stage of this
amplifiers has a resonant circuit formed by L1 & C1 that f1 = 1 / (2Π √L1 C1)
The o/p of stage is applied to second stage which is tuned to slightly higher frequency.
f2 = 1 / (2Π √L2 C2)
Second stage amplifiers the signals of frequency f2 by maximum amplitude while other frequency signal are amplified by less quantity . Thus frequency response
Curve of second stage has a peak of f2 which is
slightly higher than f1.
STAGGER TUNED AMPLIFIERS :
Over all response
Freq. response of first stage
Freq. response of second stage
f1 f0f2
Volta
ge
Frequency
STAGGER TUNED AMPLIFIER
STAGGER TUNED AMPLIFIERS
Over all response of these two stage is obtained by combining individual response & it exhibits a maximum flatness around the center frequency f0 . Thus overall bandwidth is better than individual stage.
Since two stages are in parallel (shunt) & overall bandwidth is increased thus, it behaves like shunt circuits for the increased bandwidth.
LARGE SIGNAL (NARROW BAND AMPLIFIER) TUNED AMPLIFIER
Single & double stage amplifier are not suitable for applications involving larger RF power , because of lower of efficiency of class A operation (single double) such as for excitation of transmitting antenna.
For such application larger signal tuned amplifier are employed because they are operation in class C operation that has high efficiency & capable of delivering more power in comparison to that of class A operation .
CIRCUIT DIAGRAM OF LARGE SIGNAL (NARROW BAND AMPLIFIER) TUNED AMPLIFIER :
+
Vcc
RBVs
C L
Cc
RL
Cs
Tuned class C amplifier
The resonant tuned circuit is tuned to freq. of i/p signal . When circuit has a high Q- factor , parallel resonance occur approximate freq. :
f = 1 / (2 π √LC)
At resonant freq. the impedance of parallel circuit is very large & purely resistive.
LARGE SIGNAL (NARROW BAND AMPLIFIER) TUNED AMPLIFIER
LARGE SIGNAL (NARROW BAND AMPLIFIER) TUNED AMPLIFIER
Higher the Q of circuit faster gain drops on either side of resonance freq.
A large Q leads to small bandwidth equal top sharp tuning this amplifier has Q>> 10,This means Bandwidth is less than 10% of fr & for this reason , it is called as narrow band amplifier.
COMPARISON BETWEEN TUNED AND AF AMPLIFIER
It has to amplify narrow band of frequencies defined by the tuned load at the collector
They are bulky and costlier
Used in radio transmitters and receivers, and television receivers circuits .
Works with a complete audio frequency range
More compact Amplifies sound
signals and act as drive for loud speakers
Tuned Amplifier AF Amplifier
APPLICATIONS OF TUNED AMPLIFIERTuned amplifiers serve the best for two
purposes:a)Selection of desired frequency.b)Amplifying the signal to a desired level. USED IN: Communication transmitters and receivers. In filter design :--Band Pass, low pass, High
pass and band reject filter design.
ADVANTAGES
It provides high selectivity. It has small collector voltage. Power loss is also less. Signal to noise ratio of O/P is good. They are well suited for radio transmitters and
receivers .
DISADVANTAGES
They are not suitable to amplify audio frequencies.
If the band of frequency is increase then design becomes complex.
Since they use inductors and capacitors as tuning elements, the circuit is bulky and costly.
Numerical
Numericals
Q-1) A single tuned amplifier consist of tuned circuits having R=5ohm,L=10mh,c=0.1mf. Determine
a)resonant frequency.b)quality factor of tank circuitc)band width of amplifier Ans - Given data-: R=50ohm;L=10mh;C = 0.1mf
We know: Resonant frequency = 1/ 2 π √[ (1/LC) – (R2 / L2 ) ]
= 1 / 2 π √[(1/10*10-3 - 25/100*10-6) ] = 5.034 KHz Q = 2 π * 5.034 * 10-3 * 10 * 10-3 / 5 = 63.227
BW = FR /Q = 5.034 / 63.227
= 79.62KHzRESULT:- Fr = 5.034 KHz
Q = 63.227BW = 79.62KHz
Q2.In a class c amplifier ckt C=300pf,L=50mH,R=40ohm, RL =4M ohm.
Determine:- a)Resonant frequency b)D.C load c)A.C load d)Quality factor
Ans :- Given: C=300pf L=50mH R=40ohm RL =4M ohm
Fr = 1/2 π √ LC
= 1/2 π √(50 * 10-6 * 300 * 10 -12 ) = 1.3 MHz
Rdc = 40 ohmXL = 2 π fr L = 2* 3.14 * 1.3 * 106 * 50 * 10-6
= 408.2
Qdc = 408.2/ 40 = 10.205
Rac = Rp ll RL Rp = Qdc * XL
= 10.205 * 408.2 = 4165.681
RL = 4* 106 Rac = 4161.34
Qac = [Rac / XL ] = 4161.34/408.2 = 10.194Result: -
Fr = 1.3 MHzQdc = 10.205Qac = 10.194
Q. A circuit is resonant resonant at 455 khz and has a 10khz bandwidth. The inductive reactance is 1255ohm. What is the parallel impedance of the circuit at resonance?
Solution:Given that: fr =455 khzFrequency BW=10khz and XL Let zp be the value of impedance at resonance We know that the value of bandwidth
(BW)=fr /QSo, 10*103 =455*103 /QQ=45.5Q=XL /RXL =1255
So,R=1255/45.5=27.6 ohm
1255=2∏fr L=2859*103 LL=1255/(2859*103)L=.439*103 H
Value of capacitance reactance at resonance:XC =XL
=1255Ω1/2∏fr = 1255Therefore, C=278.7*10-12 F
And value of circuit impedance at resonancezp =L/CR =0.439*103 / (278.7*10-12)*27.6 =57*103
=57 kΩRESULT: the parallel impedance at resonance is 57kΩ.
Q: A FET has gm=6 mA/v, has a tuned anode load consisting of a 400 microH inductance of 5 ohm in parallal with a capacitor of 2500pf. Find:-1. The resonant frequency2. Tuned circuit dynamic resistance3. Gain at resonance4. The signal bandwidth
Solution:-1. Fr => resonant frequency
= 1/(2π√(LC)) = 0.159/ √ (400*2500) = 1.59*105 Hz = 0.159 MHz
2. Rd => tuned circuit dynamic resistance = L/CR = 400/(2500*5)
= 106 * 80/2500 = 0.032 * 106
= 32 k ohm 3. Av = -gm rd = 6*32 = -192
4. BW= fr / Q
Q = Wr L/ R = (2 π * 0.159 * 10 -6 * 400 * 106) / 5 = 79.92BW = 0.159/ 79.92 = 1.98 KHz
RESULT:
Resonant frequency=0.159MHzDynamic Resistance=32kΩResonance gain=-192Bandwidth=1.98khz
Q: A tuned voltage amplifier, using FET with rd = 100 kohm and gm = 500 micro s has tuned circuit, consisting of L= 2.5 mH , C = 200 pF , as its load. At its resonant frequency , the circuit offers an equivalent shunt resistance of 100 kohm. For the amplifier determine:-1. Resonant gain2. The effective Q3. The Bandwidth Solution: -Given that: gm= 500Shunt resistance=100 kohm4. Resonant gain:-
Av = -gm (rd ll Rd )/(1 + jf/fr)Av = 500(100 ll 100)/(1 + j1)Av = 17.68
2. Effective Q: -Qeff = L/CR R = 100 ll 100Q = 2.5* 103 /(200*10-12 * 50 * 103) = 2.5 * 102
3. Bandwidth: -BW = fr /Qeff
fr = 1/2 π √ LC = 225 kHzBW = 225/ 2.5* 102
= 900 HzRESULT: Resonant gain=17.68
Qeff =2.5*102 BW = 900 Hz
Submitted By:Rakesh mandiya