JNTU
JNTU B.Tech II ANALOG ELECTRONICS Previous Question Paper (September - 2021 - R16)
Answer any five questions. All questions carry equal marks.
Main
With diagram, derive the expression for current gain and input resistance of Darlington amplifier.
Answer
A Darlington amplifier consists of two transistors connected in cascade such that the emitter of the first transistor (Q1) is connected to the base of the second transistor (Q2). The collectors are tied together. Let the current gains of the transistors be β1 and β2.
The total current gain is defined as the ratio of the output current (Ie2) to the input current (Ib1). Since Ie1 = (1 + β1)Ib1 and Ib2 = Ie1, the output current is Ie2 = (1 + β2)Ib2 = (1 + β2)(1 + β1)Ib1. Thus, the overall current gain A_i is approximately β1β2.
For input resistance, the input is applied to the base of Q1. The resistance seen looking into the base of Q1 is R_i = β1(r_e1 + R_i2), where R_i2 is the input resistance of the second stage. Since R_i2 = β2(r_e2 + R_E), the total input resistance is:
This configuration provides a very high input impedance and high current gain compared to a single-stage emitter follower.
Explain a low frequency h-parameter model.
Answer
The low-frequency h-parameter model represents a BJT as a two-port network using hybrid parameters. The input side is modeled as a voltage source in series with a resistor, and the output side is modeled as a current source controlled by the input current.
The governing equations are:
In this model, h_ie is the input resistance with the output shorted, h_re is the reverse voltage transfer ratio, h_fe is the forward current gain, and h_oe is the output admittance with the input open. This model is valid for low frequencies where internal transistor capacitances can be neglected.
With neat diagram explain cascode amplifier and derive the overall voltage gain of cascode amplifier.
Answer
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Explain the effect of coupling and bypass capacitors on the frequency response.
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Derive the expression for Hybrid- π capacitance of CE transistor at high frequency.
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With the help of necessary circuit diagrams and approximations obtain the expression for CE short circuit current gain and derive the relation between fβ and fT.
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Derive the short circuit current gain CE amplifier at high frequencies.
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Brief out the importance of Gain-bandwidth product.
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Analyze CSJFET Amplifier in terms of gains and Impedances.
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Draw and explain the drain characteristics of JFET and mark the regions of operation.
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Describe the construction and operation of Enhancement MOSFET and draw its characteristics.
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Discuss the need of cascading amplifiers.
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Discuss the working principle of Wein bridge oscillator and derive the expression for frequency of oscillations.
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Derive the expressions of input and output resistances for Voltage Series feedback amplifier.
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With neat diagram explain Series fed, Directly coupled Class A Power Amplifier and derive its maximum efficiency.
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Design a class B power amplifiers to deliver 25w to a load resistor RL=8 ohms, using transformer coupling. Vm=Vcc=25V. Assume necessary data.
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