JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II ELECTRONIC CIRCUITS Previous Question Paper (March - 2021 - R16)
Answer any five questions. All questions carry equal marks.
Main
Classify the amplifier circuits based on frequency range, type of coupling, power delivered and signal handled.
Answer
Amplifier circuits are classified based on the following criteria:
- Based on Frequency Range: Audio frequency (AF) amplifiers operate from 20 Hz to 20 kHz. Radio frequency (RF) amplifiers operate at frequencies above 20 kHz, typically used in communication systems.
- Based on Type of Coupling: Direct coupled amplifiers connect the output of one stage directly to the input of the next. RC coupled amplifiers use resistors and capacitors for coupling, while Transformer coupled amplifiers use transformers to provide impedance matching and isolation.
- Based on Power Delivered: Small signal amplifiers (voltage amplifiers) are designed to amplify weak signals without significant power handling. Large signal amplifiers (power amplifiers) are designed to deliver substantial power to a load, such as a loudspeaker.
- Based on Signal Handled: Small signal amplifiers process signals in the millivolt range, whereas large signal amplifiers handle signals in the volt range.
State and prove Millers theorems. Explain its significance in transistor circuit analysis.
Answer
Miller's Theorem states that a floating impedance Z connected between two nodes with voltages V1 and V2 can be replaced by two grounded impedances Z1 and Z2 at the respective nodes, where the gain A = V2/V1.
Proof: Consider a current I flowing through Z from node 1 to node 2: I = (V1 - V2) / Z. Substituting V2 = AV1, we get I = V1(1 - A) / Z. The impedance seen at node 1 is Z1 = V1 / I = Z / (1 - A). Similarly, for node 2, the current I' flowing from node 2 to node 1 is I' = (V2 - V1) / Z = V2(1 - 1/A) / Z. The impedance seen at node 2 is Z2 = V2 / I' = Z / (1 - 1/A).
Significance: Miller's theorem is essential in transistor circuit analysis, particularly for calculating the high-frequency response of common-emitter amplifiers. It allows the transformation of the feedback capacitance (C_bc or C_gd) into equivalent input and output shunt capacitances, which simplifies the determination of the upper cutoff frequency and bandwidth of the amplifier.
Discuss the circuits of voltage shunt feedback amplifier and derive the expressions for input impedance Rif and output impedance Rof.
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Show that for voltage shunt feedback amplifier trans resistance gain, Ri and Ro are decreased by a factor (1+Aβ) with feedback.
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With the relevant information explain how the negative feedback improves stability, reduce noise and increase input impedance.
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With a neat sketch explain the working of a Hartley oscillator.
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Evaluate the expression for maximum conversion efficiency for a simple series fed Class A power amplifier. What are the drawbacks of transformer coupled power amplifiers?
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With a neat diagram explain the principle of operation of class B push pull amplifier.
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Explain and analyze a transformer coupled class A power amplifier and also define the total harmonic distortion with three point method.
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Explain in detail about transistor power dissipation, thermal runway and Heat sink.
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Sketch the response of an RC high-pass for square wave input for different time constants.
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Explain the working of a simple diode comparator. Draw the output wave form for a ramp input.
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Explain the working of a transistor clipper with the help of a neat circuit diagram and wave forms.
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Explain the response of an RC high-pass filter to work as a good differentiator.
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Explain the behavior of a BJT as a switch.
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Explain with a neat circuit diagram the working of an astable multivibrator and draw the wave forms at its input and output ends.
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