Electronics and Communication questions reward anyone who is comfortable moving between the time domain and the frequency domain. This set covers network theory, analog and digital circuits, signals and systems, control systems, communication systems, and electromagnetics. Numerical solutions keep the units visible at every step, since a dropped factor is the most common reason a correct method still produces a wrong option.
Which of the following is true about intrinsic semiconductors at room temperature?
Answer: B
In intrinsic semiconductors, thermally generated electrons and holes are in equal numbers, making ni = pi at thermal equilibrium.
Q.42Easy
A BJT in saturation mode has:
Answer: A
In saturation, both BE and BC junctions are forward biased, resulting in VCE dropping to approximately 0.2-0.3V for silicon transistors.
Q.43Easy
The thermal voltage (VT) at 300K is approximately:
Answer: A
VT = kT/q where k=1.38×10⁻²³ J/K, T=300K, q=1.6×10⁻¹⁹C gives approximately 26 mV or 25 mV.
Q.44Easy
In an enhancement-mode NMOS transistor, the threshold voltage (VTh) is:
Answer: B
Enhancement-mode NMOS has positive VTh (typically 0.5-2V) because an inversion layer must be created at positive gate voltage.
Q.45Medium
A MOSFET operating in triode (linear) region exhibits:
Answer: B
In triode region, the condition VDS < VGS - VTh allows the channel to conduct with resistance proportional to VDS.
Q.46Medium
Which statement correctly describes the Early effect in BJTs?
Answer: B
Early voltage (VA) is a parameter that depends on the base width modulation effect with collector-base reverse bias voltage.
Q.47Medium
In a Zener diode regulation circuit, increasing load current causes:
Answer: B
As load current increases, less current flows through the Zener diode to maintain constant output voltage, so Zener current decreases.
Q.48Medium
A Schottky diode compared to a pn-junction diode has:
Answer: B
Schottky diodes have forward voltage ~0.3-0.4V (vs 0.7V for Si) and fast switching due to no stored charge in depletion region.
Q.49Medium
The breakdown mechanism in a Zener diode at low doping concentrations is primarily:
Answer: A
Low doping leads to wide depletion region favoring avalanche breakdown (impact ionization) rather than direct Zener tunneling.
Q.50Medium
In a BJT common-emitter amplifier, if input impedance needs to be increased, the preferred biasing technique is:
Answer: C
Voltage divider bias with emitter resistor provides stability and increases input impedance through negative feedback from emitter resistor.
Q.51Hard
When a BJT is used as a switch in saturated condition, the overdrive factor is defined as:
Answer: A
Overdrive factor β = IB(actual) / IB(min) ensures transistor stays saturated even with parameter variations.
Q.52Hard
A JFET in pinch-off condition means:
Answer: A
At pinch-off, VGS reaches VP (pinch-off voltage), channel closes, and further VDS increase has minimal effect on ID (saturation region).
Q.53Hard
In practical MOSFET applications for RF circuits, which parasitic effect limits frequency response most?
Answer: B
The gate-drain capacitance (Cgd) creates Miller effect, which effectively multiplies the capacitive reactance in feedback, severely limiting high-frequency gain.
Q.54Hard
The noise figure of a BJT amplifier at low frequencies is primarily determined by:
Answer: B
At low frequencies, shot noise associated with base current (IB) dominates, following Poisson statistics: i²n = 2q×IB×Δf.
Q.55Medium
For a given application, selecting a BJT with higher β (current gain) results in:
Answer: B
Higher β means lower IB needed for same IC, reducing loading on the driving source and improving effective input impedance (Zin ≈ β×re).
Q.56Hard
In a MOSFET differential amplifier, the common-mode gain is reduced by using:
Answer: C
Current mirror loads increase output impedance, while source degeneration resistors provide common-mode feedback, both reducing common-mode gain.
Q.57Hard
The avalanche multiplication factor M in reverse-biased junction is expressed as:
Answer: A
The avalanche factor M = 1/(1-(VR/BV)ⁿ) shows multiplication increases exponentially as reverse voltage approaches breakdown voltage BV, where n≈3-6.
Q.58Easy
In a p-n junction diode, the depletion width increases with:
Answer: A
Reverse bias voltage increases the electric field across the junction, expanding the depletion region width. The depletion width W is proportional to √V_r where V_r is reverse bias.
Q.59Easy
The diffusion potential (built-in potential) of a silicon p-n junction at room temperature is approximately:
Answer: B
For silicon at 300K, the built-in potential is typically 0.7 V, determined by V_bi = (kT/q)ln(N_a*N_d/n_i²). This value is fundamental for silicon device calculations.
Q.60Medium
Which of the following statements about tunnel diodes is INCORRECT?
Answer: C
Tunnel diodes typically operate at frequencies below 100 GHz due to parasitic capacitances and lead inductances, though they are still very fast. All other statements are correct characteristics of tunnel diodes.
