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.
In a BJT, if the base-emitter junction is forward biased and the base-collector junction is also forward biased, the transistor is operating in which region?
Answer: B
When both junctions are forward biased, the transistor operates in saturation region where collector current is maximum and relatively independent of base current.
Q.82Medium
A BJT has β = 100 and early voltage VA = 80 V. If IC = 2 mA and VCE = 10 V, what is the small signal output resistance ro?
Answer: A
ro = VA/IC = 80/(2×10^-3) = 40,000 Ω = 40 kΩ. Early voltage determines the output resistance.
Q.83Medium
In a MOSFET, the transconductance gm depends on which parameters in saturation region?
Answer: A
In saturation, gm = ∂ID/∂VGS = (W/L)·μn·Cox·(VGS - VT), showing linear dependence on overdrive voltage.
Q.84Hard
A tunnel diode exhibits negative differential resistance. At what operating point is the dynamic resistance most negative?
Answer: C
Negative differential resistance occurs in the region between peak and valley current (Ip to Iv), where current decreases with increasing voltage.
Q.85Easy
In a photodiode, the photocurrent is primarily determined by which parameter?
Answer: A
Photocurrent Iph = η·(q/hf)·Φ, where η is quantum efficiency and Φ is incident optical power.
Q.86Medium
A light-emitting diode (LED) has a bandgap of 2.5 eV. What is the approximate wavelength of emitted light?
In a JFET, as the reverse bias voltage on the gate-source junction increases, what happens to the channel width?
Answer: B
The depletion region widens with more negative VGS, pinching off the channel and reducing ID until pinch-off voltage is reached.
Q.88Hard
A CMOS logic gate has static power dissipation. Which phenomenon is primarily responsible for this in 2024 technology nodes?
Answer: A
In modern sub-28nm nodes, static power dominates due to increased leakage from subthreshold conduction (IOFF) and gate-induced drain leakage (GIDL).
Q.89Medium
A thyristor (SCR) is triggered into conduction. To turn it off, which method is most commonly used in AC circuits?
Answer: B
SCRs turn off naturally when anode-cathode voltage reverses in AC supply. Gate signal only triggers ON, not OFF.
Q.90Medium
A DIAC is used in a light dimmer circuit. What is its primary characteristic?
Answer: A
DIAC is a bidirectional diode that switches in both directions when breakover voltage (typically 30V) is exceeded, used to trigger TRIACs.
Q.91Medium
In a BJT common-emitter amplifier with emitter bypass capacitor, what is the primary effect on voltage gain?
Answer: A
Bypass capacitor shorts RE to ground at signal frequencies, eliminating degeneration feedback and increasing Av = gm·RC approximately.
Q.92Hard
A MESFET (Metal-Semiconductor FET) differs from a JFET primarily in which aspect?
Answer: A
MESFET uses Schottky (metal-semiconductor) junction for gate control instead of p-n junction, enabling high-frequency GaAs implementation.
Q.93Easy
In a p-channel MOSFET, if VGS = -8 V and VT = -2 V, what is the gate-source overdrive voltage?
Answer: A
For p-channel: |VGS - VT| = |-8 - (-2)| = |-6| = 6 V overdrive, or algebraically VGS - VT = -8 - (-2) = -6 V
Q.94Easy
A power MOSFET has RDS(on) = 0.5 Ω and carries 20 A. What is the approximate conduction power loss?
Answer: B
Pconduction = ID²·RDS(on) = (20)²×0.5 = 400×0.5 = 200 W
Q.95Hard
In a heterostructure bipolar transistor (HBT), the wide bandgap material in the emitter provides which advantage?
Answer: A
Wide bandgap emitter (e.g., AlGaAs) blocks minority carrier injection from base, improving γE and allowing lower base doping for higher β.
Q.96Easy
Which semiconductor material has the highest electron mobility among common choices?
Answer: A
GaAs has electron mobility ~8500 cm²/Vs, significantly higher than Si (~1350 cm²/Vs), enabling higher frequency operation.
Q.97Medium
A device operates with reverse saturation current IS = 10^-14 A at 25°C. If temperature increases to 75°C, by approximately what factor does IS increase?
Answer: C
IS doubles approximately every 5°C for Si. Over 50°C increase: IS(75°C) = IS(25°C)·2^(550) = 2^10 ≈ 1000 times
Q.98Easy
In a silicon p-n junction diode at room temperature (T = 300K), the thermal voltage (VT) is approximately:
Answer: A
Thermal voltage VT = kT/q, where k = 1.38×10⁻²³ J/K, T = 300K, q = 1.6×10⁻¹⁹ C. VT ≈ 26 mV at room temperature, a fundamental parameter in semiconductor device physics.
Q.99Medium
A germanium BJT has a base-emitter voltage VBE = 0.3 V in forward bias. Compared to a silicon BJT with VBE = 0.7 V, which statement is correct?
Answer: A
Germanium has bandgap Eg ≈ 0.67 eV versus silicon's 1.12 eV. Lower bandgap requires less voltage to forward bias. However, silicon is preferred due to better temperature stability and lower leakage current.
Q.100Medium
In an n-channel depletion-mode MOSFET with Vto = -3 V, if VGS = 0 V, the device is in which region?
Answer: C
For n-channel depletion MOSFET, when VGS = 0 V (which is greater than Vto = -3 V), the channel is open and device conducts. With VGS > Vto and positive VDS, the device operates in saturation. Depletion-mode devices conduct at VGS = 0.
