JNTU
JNTU B.Tech II ANALOG ELECTRONICS Previous Question Paper (March - 2021 - R16)
Answer any five questions. All questions carry equal marks.
Main
Draw the circuit diagram of RC coupled amplifier. Explain the operation and its frequency response.
Answer
An RC coupled amplifier uses a coupling capacitor to connect the output of one transistor stage to the input of the next, blocking DC while allowing AC signals to pass. The circuit consists of a transistor in common-emitter configuration, biasing resistors (R1, R2), an emitter resistor (RE) bypassed by a capacitor (CE), a collector resistor (RC), and a coupling capacitor (CC).
Operation: The input signal is applied to the base of the transistor through the coupling capacitor. The transistor amplifies the signal, which is then passed to the next stage via the coupling capacitor. The bypass capacitor ensures that the emitter is at AC ground, maximizing gain.
Frequency Response: The gain is constant over a mid-frequency range. At low frequencies, the gain drops due to the high reactance of coupling and bypass capacitors. At high frequencies, the gain drops due to the internal transistor capacitances and stray wiring capacitances.
In a single stage CB – amplifier circuit, RE = 20K, Rc = 10K, VEE = -20V, Vcc = 20V, RL = 10K. Find out Ri, Ro, Ai, Av and power gain in dB.
Answer
Given: RE = 20 kΩ, RC = 10 kΩ, RL = 10 kΩ. For a Common Base (CB) amplifier, we assume standard transistor parameters (hib ≈ 25Ω/IE). Assuming a typical IE = 1mA, hib ≈ 25Ω.
1. Input Resistance (Ri): Ri ≈ hib = 25 Ω.
2. Output Resistance (Ro): Ro = RC = 10 kΩ.
3. Current Gain (Ai): Ai ≈ α ≈ 1.
4. Voltage Gain (Av): Av = (α × RL') / hib, where RL' = RC || RL = 5 kΩ. Av = (1 × 5000) / 25 = 200.
5. Power Gain (Ap): Ap = Av × Ai = 200 × 1 = 200. Power Gain in dB = 10 log10(200) ≈ 23.01 dB.
For any transistor amplifier, Prove that Ri = hi / (1 - hrAv).
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Draw the h-parameter equivalent circuit for a typical common emitter amplifier and derive expression for Ai, Av, Ri and Ro.
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Derive the expression for the CE short circuit current gain Ai as a function of frequency using Hybrid - π model.
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In hybrid π model of a transistor at high frequencies, show that the gm is proportional to collector current.
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Derive an expression for current gain with resistive load.
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For a single stage CE amplifier whose hybrid π parameters are given below. What value of Rs will give 3 dB frequency fH which is half the value obtained with Rs=0. Hybrid π parameters are: gm=50 mA/V, rbb=100Ω, rb=1 K, CC=3 pF, Ce=100 pF.
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With the help of neat diagram explain the construction, working and VI characteristics of n channel JFET.
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Describe the operation of Enhancement MOSFET.
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Derive the relation between μ and gm of JFET amplifier.
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Draw and explain the CS stage with diode connected load.
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Compare the performance of BJT and FET amplifiers.
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Draw the circuit diagram of class B push pull amplifier and explain its operation. Also prove that its conversion efficiency is 78.5%.
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Discuss in detail about frequency response of tuned amplifiers.
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Design a class B power amplifiers to deliver 25W to a load resistor RL=8ohms, using transformer coupling. Vm=Vcc=25V. Assume necessary data.
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A series fed Class A amplifier operates from dc source and applied sinusoidal input signal generates peak base current 9mA. Calculate: (i) Quiescent current ICQ, (ii) Quiescent voltage VCEQ, (iii) DC input power PDC, (iv) AC output power PAC and (v) Efficiency.
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