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 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.22Medium
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.23Medium
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.
Q.24Medium
In a BJT, the early voltage (V_A) represents:
Answer: B
Early voltage is a measure of the output resistance of a BJT. It's the voltage extrapolated from the I_C vs V_CE characteristic where the output current would theoretically become zero, indicating the slope of the characteristic curves.
Q.25Medium
A photodiode is operated in photovoltaic mode when:
Answer: C
Photovoltaic mode occurs when the photodiode is not externally biased and acts as a light-sensitive voltage source, generating an open-circuit voltage proportional to light intensity. This is used in solar cells.
Q.26Medium
In an optocoupler (photocoupler), the coupling efficiency depends on:
Answer: B
Coupling efficiency is maximized when the LED's emission spectrum overlaps with the photodetector's spectral sensitivity. Spectral mismatch significantly reduces overall coupling efficiency.
Q.27Medium
In a MESFET (Metal-Semiconductor FET), compared to MOSFET:
Answer: D
MESFETs use Schottky metal-semiconductor junctions instead of oxide layers, resulting in lower gate capacitance, better high-frequency response (f_T/f_max higher), and simpler manufacturing process, especially in GaAs technology.
Q.28Medium
A varactor diode is used in frequency modulation circuits because:
Answer: A
Varactor (variable capacitor) diodes have a depletion capacitance C_j = C_0/(1+|V_R|/V_bi)^n that varies with reverse bias. This voltage-dependent capacitance is used to modulate oscillator frequency.
Q.29Medium
For an IMPATT diode oscillator, the 'IMPATT' stands for:
Answer: C
IMPATT (Impact Avalanche and Transit-Time) diodes operate by combining avalanche multiplication at the p-n junction with carrier transit time delay, producing negative resistance for microwave oscillation at GHz frequencies.
Q.30Medium
A CMOS inverter has asymmetric rise and fall times primarily because:
Answer: A
Since μ_n ≈ 2-3 μ_p, NMOS transistors have higher transconductance. To achieve balanced rise and fall times, the PMOS width is typically designed larger (2-3×) than NMOS. Without this, the PMOS pull-up is slower.
Q.31Medium
The output impedance of a BJT common-collector amplifier is approximately:
Answer: A
Common-collector (emitter follower) configuration provides very low output impedance ≈ r_e = V_T/I_E ≈ 26Ω/I_E(mA). This makes it an excellent impedance matching buffer. The low impedance comes from the emitter follower configuration.
Q.32Medium
Which of the following statements about Schottky diodes is incorrect?
Answer: D
Schottky diodes have LOWER junction capacitance due to the metal-semiconductor junction, making them suitable for high-frequency applications.
Q.33Medium
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.34Medium
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.35Medium
A light-emitting diode (LED) has a bandgap of 2.5 eV. What is the approximate wavelength of emitted light?
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.37Medium
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.38Medium
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.39Medium
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.40Medium
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.
