Tuned amplifiers

ECA UNIT 6

1. INTERNATIONAL SCHOOL OF TECHNOLOGY & SCIENCES (FOR WOMEN) DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING Prepared By: Mr . K V VENKATARAMANA , Assistant Professor . K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS II BTECH II SEMESTER ELECTRONIC CIRCUIT ANALYSIS UNIT VI

2.  Contents:  Introduction to Tuned Amplifier  Q-Factor  Small signal tuned amplifier  Capacitance single tuned amplifier  Double Tuned Amplifier  Effect of Cascading single Tuned Amplifier  Staggered tuned Amplifiers  Stability of Tuned Amplifiers K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

3. D E F INIT I O N :- 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. K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

4. C HARACTERISTICS OF T UNED A MPLIFIER  T uned a m pli f i e r s elects a nd a m plifi e s a s i n gle f r equen c y 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. K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

5.  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 Tu ned c ircuit K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

6. R e s onan c e circuits Parallel Series K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

7. 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 amplifier Large signal amplifier, low power, radio frequency Class B&C Shunt peaked tuned with higher band width K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

8. C LASSIFICATION OF T UNED A MPLIFIER Tuned a m p l i fi e r Small Signal Amplifier Single Tuned A m p l i fi e r Double Tuned A m p l i fi e r Stagger Tuned A m p l i fi e r Large signal Amplifier K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

9. CLASSIFICATION OF TUNED AMPLIFIERS  Small Signal Tuned Amplifiers :- They are used to amplify the RF signals of small magnitude. They are further classified as: Single Tuned Amplifiers :- In this we use one parallel tuned circuit in each stage. Double Tuned Amplifiers :- In this we use two m utually co u p led tuned ci r c u i ts f or e very stage both of tuned circuits are tuned at same freq. Stagger Tuned Amplifiers :- It is a multistage amplifier which has one parallel tuned circuit for every stage but tuned frequency for all stages K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

10. (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. K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

11.  SERIES RESONANT CIRCUIT I t is the c i rcu it in w hi c h a ll t h e res i s t i v e and reactive components are in series. K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

12.  SERIES RESONANT LC K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

13.  SERIES RESONANT CIRCUIT:  Impedance Of The Circuit: - Z = { R 2 + (X L – X c ) 2 } 1/2 Z = { R 2 + (ωL – 1/ ωC) 2 } 1/2  For resonant frequency:- (X L = X C ) XL = ωL = X C = 1/ ωC = 2 π f r L 1 / 2 π f r C K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

14. SERIES RESONANT CIRCUIT Since at resonance, X L = X c 2 π f r L = 1 / 2Пf r C f r = 1 / 2 π √ L C ω r f r = 1 / √LC K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

15.  RESONANCE CURVE OF SERIES RESONANT CIRCUIT : K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

16.  QUALITY FACTOR: I t is v olt ag e m ag n i f i c a ti o n that c i r c uit p r o d uc e s at resonance is called the Q factor. I max X L / I min R Voltage Magnification = = X L / R At Resonance = X L /R = X C /R ω r L / R = 1 / ω r R C K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

17. T H US Q r = ω r L / R = 1/ ω r C R = 2 π f r L / R = (2 π L / R) * (1 / 2 π √LC ) = √(L/C) / R = { tan Ф = power factor of coil } tanФ K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

18.  IMPORTANT POINTS Net reactance , X = 0. Impedance Z = R . Power factor is unity. 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 . K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

19.  REACTANCE CURVE SERIES RESONANT CIRCUIT X L = 2Πf L X = X L - X C X C = 1 2Πf C R f r K V VENKATARAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

20.  PARALLEL OR CURRENT RESONANCE K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

21.  PARALLEL OR CURRENT RESONANCE When an inductive reactance and a capacitance are connected in parallel condition may reach under which current resonance (also known as parallel or anti- resonance ) will take palace. The frequency at which this happened is known as resonant frequency. Current will be in resonance I reactive component of R - L branch. I R-L sinФ R-L = Reactive component of capacitive branch, neglecting leakage reactance of capacitor C. K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

22.  FREQUENCY V/S IMPEDANCE CURVE FOR LCR CIRCUIT K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

23. Inductance Quality factor (Q) 2 4 K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

24. RESONANT  R E S O NA N C E CU R V E O F P A R A L L E L CIRCUIT : With low resistance With high resistance current f R K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

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30. Conduc tan ce ( R ) 30 Q t e f f  Susp tan ce ( L / C )

31.  SINGLE TUNED AMPLIFIER USING FET K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

32.  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 A v but at the same time , it will give much reduced band with because bandwidth is inversely proportional to the Q- factor . • I t mea n s th at t un e d a m p li f ie r wit h r e d u c e b a nd wi d t h ma y not be able to amplify equally the complete band of signals & result is poor reproduction . This is called potential instability in tuned amplifier. K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

33. Double tuned Amplifier: K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

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42.  STAGGER TUNED AMPLIFIERS : K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

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45. Stability Considerations 45

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51. Video Amplifiers (IC MC1550) K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

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53.  APPLICATIONS OF TUNED AMPLIFIER Tuned amplifiers serve the best for two purposes: Selection of desired frequency. Amplifying the signal to a desired level.  USED IN : Communication transmitters and receivers. In f il ter de s i gn :- - B an d P a ss, l ow pa ss, Hi g h pa ss a n d band reject filter design. K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

54.  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 . K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

55.  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. K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS

56. Thank you K V VENKATA RAMANA ‖ ECA ‖ Dept of ECE ‖ ISTS