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Transcript
Tuned Amplifiers Dr. Monir Hossen ECE, KUET Department of Electronics and Communication Engineering, KUET Introduction of Tuned Amplifiers (1/2) Amplifiers which amplify a specific frequency or narrow band of frequencies are called tuned amplifiers. It used for the amplification of high or radio frequencies: It is not used for the amplification of audio frequencies as they are mixture of frequencies from 20 Hz to 20 KHz and not single. It is widely used for radio and television ckts. Department of Electronics and Communication Engineering, KUET 2 Introduction of Tuned Amplifiers (2/2) A tuned amplifier ckt. consist of: Transistor LC tuner ckt. Coupling capacitors Resistors Department of Electronics and Communication Engineering, KUET 3 Parallel Resonance (1/2) A parallel tuned circuit consists of A capacitor and An inductor in parallel Resonance is occurred when circuit power factor is unity, (i.e., applied voltage and the current are in phase) If we draw the phasor diagram of the above parallel circuit then we find: Department of Electronics and Communication Engineering, KUET 4 Parallel Resonance (2/2) The coil has its own resistance R and the coil current IL has two rectangular components: Active component ILcosφL Reactive component ILsinφL The power factor will be unity only when the net reactive component of the circuit current is zero, i.e., I C I L sin L 0 or I C I L sin L Resonance in parallel circuit can be obtained by changing the supply frequency. Department of Electronics and Communication Engineering, KUET 5 Resonant Frequency (1/2) The frequency at which parallel resonance occurs is called the resonant frequency fr At parallel resonance we have I C I L sin L xL V Now, I L ; sin L and ZL ZL So V V xL xc Z L Z L xC xL Z L2 L Z L2 R 2 xL2 C L 2 R 2 2f r L C IC V xc xL ZL R ZL= R+jxL ZL2= R2+xL2 Department of Electronics and Communication Engineering, KUET 6 Resonant Frequency (2/2) L 2f r L R 2 C L 2f r L R2 C 2 1 L fr R2 2L C 1 fr 2 1 R2 2 LC L If coil resistance R is small then fr 1 2 LC Department of Electronics and Communication Engineering, KUET 7 Characteristics of Parallel Resonant Ckt. (i) Impedance of tuned circuit: The parallel tuned circuit is used to select the resonant frequency and reject all others. At resonant condition I C I L sin L So the line current at resonant condition is: I I L cos L V V R Zr ZL ZL 1 R 2 Zr ZL 1 R CR Zr L L C L Zr CR I C I L sin L xL ZL R V xc V xL ZL ZL Z L2 xC xL Z L2 Department of Electronics and Communication Engineering, KUET L L C C 8 Characteristics of Parallel Resonant Ckt. (ii) Circuit current: V At parallel resonance line current is: I Zr (iii) Quality factor Q: The ratio of inductive reactance of the coil at resonance to its resistance is known as quality factor Q. xL 2f r L R R The Q of a parallel tuned circuit is very important because: The sharpness of the resonance curve and selectivity of the circuit depends on it. Higher the value of Q the tuned circuit is more selective Q Department of Electronics and Communication Engineering, KUET 9 Advantages of Tuned Amplifier 1) Small power loss: Due to small resistance in the tuned circuit power loss is quite low. 2) High Selectivity: It can select the desire frequency for amplification out of a large no. of frequencies simultaneously impressed upon it. 3) Smaller collector supply voltage: Because of little resistance in the parallel tuned circuit, it requires small collector supply voltage Vcc. Department of Electronics and Communication Engineering, KUET 10 Relation Between Q and Bandwidth The quality factor Q of a tuned amplifier is equal to the ratio of resonant frequency ‘fr’ to the bandwidth (BW). fr Q BW Again the Q of an amplifier is determined by the circuit component values. Generally the Q of a tuned amplifier is greater than 10. when this condition met the resonance frequency 1 is f r 2 LC Department of Electronics and Communication Engineering, KUET 11 Why Tuned Amplifiers are not Used for Low Frequency Amplification? 1) Low frequencies are never single: Tuned amplifiers are used to amplify a narrow or a single frequency but available practical low frequencies are not single, e.g., 20 Hz to 20 KHz. 2) High Values of L and C: The resonance frequency of a parallel tuned amplifiers is fr 1 2 LC If fr is low then L and C should be large. This will make the tuned circuit bulky and expensive. Department of Electronics and Communication Engineering, KUET 12 Example The Q of a tuned amplifier is 60. If the resonant frequency for the amplifier is 1200 KHz, find (i) BW, and (ii) cut-off frequencies. Soln: f r 1200 20 KHz (i) BW Q 60 (ii) Lower cut-off frequency, f1=1200-BW/2=1200-10=1190 Upper cut-off frequency, f1=1200+BW/2=1200+10=1210 Self study: Example: 15.1, 15.2, 15.3 (V.K. Mehta) Department of Electronics and Communication Engineering, KUET 13 Single Tuned Amplifier A single tuned amplifier consists of a transistor amplifier containing a parallel tuned circuit as the collector load. The values of capacitance and inductance of the tuned circuit are so selected that its resonant frequency is equal to the frequency to be amplified. Operation? Department of Electronics and Communication Engineering, KUET 14 AC Equivalent Circuit of Tuned Amplifier(1/3) Ac equivalent circuit of the tuned amplifier. Note the tank circuit components are not shorted Department of Electronics and Communication Engineering, KUET 15 AC Equivalent Circuit of Tuned Amplifier(2/3) In order to completely understand the operation of this circuit. We should consider the three frequency conditions (a) fin = fr , (b) fin < fr , and (c) fin > fr (a) When fin = fr: • At fr, the parallel circuit offers a very high impedance, i.e., act as open circuit. • All the ac collector current flows through RL. • So, voltage drop across RL is maximum, i.e., voltage gain is maximum. (b) When fin < fr : At this condition, the circuit is effectively inductive. As the frequency decreases from fr a point is reached when xc-xL = RL the gain falls by 3db. So, lower cut-off frequency f1 for the circuit occurs when xc-xL = RL Department of Electronics and Communication Engineering, KUET 16 AC Equivalent Circuit of Tuned Amplifier(3/3) (c) When fin > fr: In this condition, the circuit is effectively capacitive. As fin is increased beyond fr a point is reached when xL-xC = RL the voltage gain of the amplifier will again falls by 3db. So, upper cut-off frequency for the circuit will occur when xL-xC = RL Self-study, Example 15.7 VK Mehta Department of Electronics and Communication Engineering, KUET 17 Double Tuned Amplifier (DTA) It consists of a transistor amplifier containing two tuned circuits one (L1C1) in the collector and the other (L2C2) in the output. The resonant frequency of tuned circuit L1C1 is made equal to the signal frequency. The output from this tuned circuit is transferred to the second tuned circuit L2C2 through mutual induction. Double tuned circuits are used for coupling the various circuits of radio and television receivers. Department of Electronics and Communication Engineering, KUET 18 Frequency Response of DTA The frequency response of double tuned circuits depends upon the degree of coupling, i.e., upon the amount of mutual inductance between the two circuits. When L2 is coupled to coil L1 a portion of load resistance is coupled into primary tank circuit L1C1 and affects the primary circuit in exactly the same manner as a resistor had been added in series with primary coil L1 . Department of Electronics and Communication Engineering, KUET 19 Types of Coupling in DTA (1/2) There are two types of coupling: (i) Loose coupling (ii) Tight coupling (i) Loose coupling: When the coils are spaced apart all the fluxes of primary coils ‘L1’ will not link the secondary coil ‘L2’. In this condition, resistance reflected from the secondary circuit is small. So the resonance curve will be sharp and Q will be high Department of Electronics and Communication Engineering, KUET 20 Types of Coupling in DTA (1/2) (ii) Tight coupling: When the coils are very close together all the fluxes of primary coils ‘L1’ will link the secondary coil ‘L2’. This coupling is called tight coupling In this condition, resistance reflected from the secondary circuit is large. So the resonance curve will not sharp and Q will be lower Department of Electronics and Communication Engineering, KUET 21 Bandwidth of DTA In DTA, bandwidth (BW) increases with the degree of coupling If coupling is tighter the bandwidth is greater BWDTA = frK here, fr is the resonance frequency and K is the coefficient of coupling Example 15.8: It is desired to obtain a bandwidth of 200 kHz at an operating frequency of 10 MHz using a double tuned circuit. What value of coefficient of coupling should be used ? Soln: Here, BWDTA =200 kHz , fr =10 MHz We know: BWDTA = frK BWDTA 200 103 k 0.02 6 fr 10 10 Department of Electronics and Communication Engineering, KUET 22 Advantages of DTA 1. Bandwidth is increased. 2. Sensitivity (i.e. ability to receive weak signals) is increased. 3. Selectivity (i.e. ability to discriminate against signals in adjacent bands) is increased. Department of Electronics and Communication Engineering, KUET 23 Thanks for Your Kind Attention Department of Electronics and Communication Engineering, KUET