What is Transistor Tuned Amplifiers – Electronics Tutorial

What is Transistor Tuned Amplifiers – Electronics Tutorial
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What is Transistor Tuned Amplifiers – Electronics Tutorial

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    Transistor Tuned Amplifiers 389INTRODUCTIONMost of the audio amplifiers we have discussedin the earlier chapters will also work at radiofrequencies i.e. above 50 kHz. However, theysuffer from two major drawbacks. First, they becomeless efficient at radio frequency. Secondly, such ampli-fie...

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    390 Principles of Electronicstion. For instance, radio and television transmission are carried on a specific radio frequency as-signed to the broadcasting station. The radio receiver is required to pick up and amplify the radiofrequency desired while discriminating all others. To achieve...

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    Transistor Tuned Amplifiers 391tuned amplifier from other amplifiers? The difference is that tuned amplifiers are designed to havespecific, usually narrow bandwidth. This point is illustrated in in Fig. 15.2. Note that BWS is thebandwidth of standard frequency response while BWT is the ban...

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    392 Principles of ElectronicsResonant frequency. The frequency at which parallel resonance occurs (i.e. reactive compo-nent of circuit current becomes zero) is called the resonant frequency fr.Fig. 15.4At parallel resonance, we have, IC = IL sin φL NowIL= V/ZL ;...

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    Transistor Tuned Amplifiers 393quency curve of Fig 15.5. It is clear from impedance-frequency curve that impedance rises to a steeppeak at resonant frequency fr. However, the impedance of the circuit decreases rapidly when thefrequency is changed above or below the resonant frequency. This...

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    394 Principles of Electronicsthe resonance curve. It is clear that when the resis-tance is small, the resonance curve is very sharp. How-ever, if the coil has large resistance, the resonance curveis less sharp. It may be emphasised that where highselectivity is desired, the value of Q sh...

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    Transistor Tuned Amplifiers 395=106 × 6111010R=× = 105 Ω = 0.1 MΩΩΩΩΩNote that the circuit impedance Zr is very high at resonance. It is because the ratio L/C is verylarge at resonance.Line current at resonance isI =510 V10rVZ=Ω = 100 μμμμμAExample 15.3. The *dynamic im...

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    396 Principles of Electronics15.6Why not Tuned Circuits for Low Frequency Amplifica-tion ?The tuned amplifiers are used to select and amplify a specific high frequency or narrow band offrequencies. The reader may be inclined to think as to why tuned circuits are not used to amplify lowfreq...

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    Transistor Tuned Amplifiers 397the carrier wave frequency is 710 kHz, then the resultant radio wave has a frequency range *between(710 –5) kHz and (710 +5) kHz. Consequently, the tuned amplifier must have a bandwidth of 705kHz to 715 kHz (i.e. 10 kHz). The Q of the tuned circuit should be...

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    398 Principles of ElectronicsSolution. Fig. 15.8 shows the frequency response of an ideal tuned amplifier. The ideal tunedamplifier has zero gain for all frequencies from 0 Hz up to the lower cut-off frequency f1. At thispoint, the gain instantly jumps to the maximum value [Av (max)]. The ...

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    Transistor Tuned Amplifiers 39915.10 Analysis of Tuned AmplifierFig. 15.10 (i) shows a single tuned amplifier. Note the presence of the parallel LC circuit in thecollector circuit of the transistor. When the circuit has a high Q, the parallel resonance occurs at afrequency fr given by:Fig....

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    400 Principles of Electronics(ii) When input frequency is less than fr (i.e., fin < fr ). When the input signal frequency is lessthan fr, the circuit is effectively* inductive. As the frequency decreases from fr, a point is reachedwhen XC − XL = RL. When this happens, the volta...

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    Transistor Tuned Amplifiers 401tuned circuit offers very high impedance tothe signal frequency. Consequently, largeoutput appears across the tuned circuit L1C1.The output from this tuned circuit istransferred to the second tuned circuit L2C2through mutual induction. Double tunedcircuits are...

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    402 Principles of Electronics∴ Co-efficient of coupling, k =200 kHz1010 kHzdtrBWf3=× = 0.0215.14 Practical Application of Double Tuned AmplifierDouble tuned amplifiers are used for amplifying radio-frequency (RF) signals. One such appli-cation is in the radio receiver as shown in Fig...

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    Transistor Tuned Amplifiers 403Class C operation means that collector current flowsfor less than 180º. In a practical tuned class C amplifier,the collector current flows for much less than 180º ; thecurrent looks like narrow pulses as shown in Fig. 15.17.As we shall see later, when narrow ...

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    404 Principles of ElectronicsWhen the capacitor is fully charged, it will dischargethrough the coil and the load resistor, setting up oscil-lations just as an oscillatory circuit does. Consequently,sine-wave output is obtained.(iv) If only a single current pulse drives the LCcircuit, we wi...

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    Transistor Tuned Amplifiers 405*Because the drop in L due to d.c. component is negligible.○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○Example 15.9. In the circuit shown in Fig.15.20, C =...

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    406 Principles of ElectronicsFig. 15.23As discussed earlier, PD in class C operation is very small because the transistor remains biasedoff during most of the input signal cycle. Consequently, PD may be neglected as compared to Po(max).∴Maximum η j()()omaxomaxPP j 100 %It is worthwhile...

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    Transistor Tuned Amplifiers 407=(Np/Ns)2 × 50 = (5/1)2 × 50 = 1250 ΩΩΩΩΩ(ii) Max. load power, Po(max)=22(50)22 1250CCACVR=× = 1 WExample 15.11. In the above example, if power dissipation of the transistor is 4 mW, find themaximum collector efficiency.Solution.Po (max)=1W; PD = 4...

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    408 Principles of Electronics13. At parallel resonance, the net reactive com-ponent of circuit current is .............(i) capacitive(ii) zero(iii) inductive(iv) none of the above14. At parallel resonance, the circuit impedanceis ..............(i)CLR(ii)RLC(iii)CRL(iv)LCR15. In a parallel L...

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    Transistor Tuned Amplifiers 40932. In parallel resonance, there is .................(i) both voltage and current amplification(ii) voltage amplification(iii) current amplification(iv) none of the above33. For frequencies below resonant frequency,a series LC circuit behaves as a .................

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    410 Principles of ElectronicsProblems1. A parallel circuit has a capacitor of 100 pF in one branch and an inductance of 100 µH plus a resis-tance of 10 Ω in the second branch. The line voltage is 100V. Find (i) resonant frequency (ii) circuit impedanceat resonance and (iii) line current...