Question 1

Q.1. (a) Explain micro strip line matching networks. [10]

Accepted Answer

Microstrip matching networks (quarter-wave transformers, stub matching, lumped-element L-networks) transform a load impedance to match the characteristic impedance of the feed line, minimizing reflections; the Smith chart is a graphical polar plot of normalized impedance (or reflection coefficient) used to determine an unknown impedance from measured VSWR and the position of the voltage minimum, and to design matching networks by graphically tracing impedance transformations along constant-resistance/reactance circles.

Question 2

Q.1. (a) With neat diagrams, explain the various steps involved in fabricatio…

Accepted Answer

Monolithic Microwave Integrated Circuit (MMIC) fabrication involves a sequence of steps - substrate preparation, epitaxial layer growth, photolithographic patterning, ion implantation/diffusion for active devices, metallization for interconnects/electrodes, passive component formation, via-hole etching, and final dicing/testing - carried out on a semi-insulating GaAs (or similar compound semiconductor) substrate; basic MMIC materials include the semi-insulating substrate itself, epitaxial semiconductor layers, dielectric/passivation layers, and various deposited metal layers for ohmic contacts, Schottky gates, and interconnect wiring.

Question 3

Q.2. (a) Explain the principle of working of IMPATT diode with suitable struc…

Accepted Answer

The IMPATT (IMPact ionization Avalanche Transit-Time) diode generates microwave power through a combination of avalanche multiplication and carrier transit-time delay within a reverse-biased pn or p-i-n junction, producing a current that is delayed by approximately 180 degrees relative to the applied RF voltage, giving a negative resistance characteristic usable for oscillation/amplification; a detector diode is a specially-constructed low-barrier Schottky diode used to rectify/detect microwave signals by converting the RF power into a proportional DC or baseband output, exploiting the diode's non-linear current-voltage characteristic at low signal power.

Question 4

Q.2. (a) Draw equivalent circuit of varactor diode. Explain in detail its con…

Accepted Answer

A varactor diode's equivalent circuit consists of a voltage-dependent junction capacitance in series with a small series resistance, exploiting the voltage-variable depletion-region width of a reverse-biased pn junction to provide a voltage-controlled capacitor, widely used in electronic tuning and frequency-multiplier circuits; the Gunn diode operates via the transferred-electron (Ridley-Watkins-Hilsum) effect in n-type GaAs or InP, exhibiting negative differential mobility that supports self-sustained current oscillations, most commonly in the transit-time (Gunn) mode, but also in delayed, quenched, and LSA (Limited Space-charge Accumulation) modes depending on bias and circuit conditions.

Question 5

Q.3. (a) List down the steps of MOSFET fabrication with suitable diagrams. [8]

Accepted Answer

MOSFET fabrication proceeds through wafer preparation, oxidation, photolithographic patterning, source/drain doping (implantation/diffusion), gate oxide and gate electrode formation, and metallization/interconnect steps; the n-channel JFET operates by using a reverse-biased gate-channel pn junction to control the width of a conducting n-type channel between source and drain, with increasing reverse gate bias progressively narrowing (pinching) the channel and reducing drain current, providing voltage-controlled current conduction without requiring any forward gate current.

Question 6

Q.3. (a) A certain GaAs MESFET has the following parameters - Channel height …

Accepted Answer

For the given GaAs MESFET (channel height 0.1 micrometre, electron concentration 8x10^17 cm^-3, relative dielectric constant 13.10), the calculated pinch-off voltage is approximately 5.52V; the microwave BJT uses a specialized interdigitated (comb-like) emitter-base geometry with a thin base and heavily-doped, narrow emitter fingers to minimize base resistance and transit time, enabling useful current gain and power output up to several GHz.

Question 7

Q.4. (a) Explain the operation of Magnetron and write its applications. [8]

Accepted Answer

The magnetron generates high-power microwave oscillations using crossed electric and magnetic fields acting on electrons emitted from a central cathode, causing them to follow curved (cycloidal) paths past a ring of resonant cavities, interacting with and reinforcing the cavity fields to sustain oscillation, with applications in radar transmitters and microwave heating (ovens); reflex klystron bunching is the process by which an electron beam, velocity-modulated on its first pass through a resonant cavity gap, is reflected by a negative repeller electrode and returns through the same gap grouped into bunches, delivering net energy to the cavity field, with efficiency derived from the ratio of AC power delivered to DC beam power.

Question 8

Q.4. (a) Derive an expression for cut off magnetic field for a cylindrical ma…

Accepted Answer

The cutoff magnetic field of a cylindrical magnetron is the specific magnetic flux density at which an electron just fails to reach the anode (grazing it with zero radial velocity), derived from the combined equations of motion under crossed electric and magnetic fields, giving Bc = sqrt(8*V0*m/e) / [b*(1-(a/b)^2)]; conventional vacuum tubes suffer from severe performance limitations at microwave frequencies due to excessive interelectrode capacitance, lead inductance, and electron transit-time effects, motivating specialized microwave tube designs such as the two-cavity klystron, which uses spatially-separated velocity-modulation (buncher) and energy-extraction (catcher) cavities connected by a field-free drift space to achieve amplification at microwave frequencies.

Question 9

Q.5. (a) Write a short note on backward wave oscillator. [8]

Accepted Answer

A backward wave oscillator (BWO) is a microwave tube in which the electron beam interacts with a slow-wave structure such that the RF energy propagates in the direction opposite to the electron beam's motion, providing inherent electronic tunability of oscillation frequency via beam voltage; oscillation suppression in a travelling wave tube (TWT) is achieved primarily using a resistive (lossy) attenuator section placed partway along the slow-wave structure, which absorbs any backward-travelling reflected wave energy that would otherwise regeneratively couple back into forward-wave amplification and cause unwanted self-oscillation.

Question 10

Q.5. (a) Draw the structure of TWT and explain its amplification process. [8]

Accepted Answer

The travelling wave tube (TWT) uses an electron gun, a helical slow-wave structure, a focusing magnetic field, and a collector, with amplification occurring through continuous, distributed energy exchange between a velocity-modulated, bunching electron beam and a co-propagating slow electromagnetic wave travelling along the helix at a velocity close to the beam's own velocity; the axial electric field of the helix-guided wave can be derived from the helix's characteristic sheath-helix wave equation, showing an exponentially decaying/growing field profile radially away from the helix surface, consistent with the slow-wave (surface-wave-like) nature of helix propagation.

Microwave Engineering 2