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.62Hard
The transconductance (g_m) of a MOSFET in saturation is given by:
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.64Medium
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.65Hard
The thermal runaway condition in a BJT occurs because:
Answer: A
Thermal runaway is a positive feedback mechanism: temperature ↑ → V_BE ↓ (≈ -2mV/°C) → I_B ↑ → I_C ↑ → P_dissipated ↑ → T ↑, creating instability. β also increases with temperature.
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Q.66Easy
A JFET is depletion-type because:
Answer: B
JFETs are called depletion-type devices because the channel is formed by a depletion region. The p-n junction (gate-channel) creates a depletion layer that widens with reverse bias, pinching off the channel.
Q.67Medium
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.68Hard
The charge storage time in a BJT switch is primarily due to:
Answer: C
Storage time (t_s) occurs because excess minority carriers accumulate in the base during saturation. When the input is removed, these carriers must recombine or diffuse out before reverse current can flow, causing delay in switching off.
Q.69Medium
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.70Hard
In a PIN diode used as an RF switch, the operating principle is based on:
Answer: C
PIN diodes have wide intrinsic regions. When forward biased, high carrier concentration (conductivity modulation) is established in the I-region, giving low impedance. When reverse biased, carriers are swept out, giving high impedance—ideal for RF switching.
Q.71Hard
The noise figure of an amplifier is defined as:
Answer: C
Noise Figure F = (SNR_in)/(SNR_out) = (input noise power × gain)/(output noise power). It represents the degradation in SNR due to the amplifier's internal noise contributions.
Q.72Medium
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.73Hard
The substrate effect in a MOSFET causes:
Answer: D
Body bias effect: reverse substrate bias increases depletion width, increasing V_T (back-gate bias effect). This reduces g_m and changes I_D characteristics. The effect follows V_T = V_T0 + γ(√(2φ_F + V_SB) - √(2φ_F)).
Q.74Medium
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.75Medium
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.76Easy
In a silicon BJT at room temperature, for every 10°C increase in temperature, the leakage current I_CO approximately:
Answer: A
The reverse saturation current I_0 doubles for every 5-7°C rise in silicon at room temperature. Since I_CO ≈ I_0, it also doubles per 5-7°C, or roughly per 10°C it increases significantly (by factor of ≈1.5-2).
Q.77Easy
A clamping diode in logic circuits is used to:
Answer: A
Clamping diodes protect circuits by limiting the maximum voltage that can be applied to a node. They conduct when voltage exceeds V_DD + V_D_forward or goes below V_SS - V_D_forward, preventing overstress to gate oxide and junctions.
Q.78Hard
The frequency response of a BJT amplifier is limited at high frequencies primarily by:
Answer: B
The base-collector capacitance C_bc exhibits the Miller effect, appearing as (1+|A_v|)C_bc at the base, creating a dominant pole that limits bandwidth. This is the primary high-frequency limitation in BJT amplifiers, with f_T ≈ g_m/(2π(C_bc(1+A_v) + C_be)).
Q.79Easy
A Zener diode is operating in reverse breakdown with a current of 50 mA. If the Zener resistance is 5 Ω and Zener voltage is 12 V, what is the terminal voltage?
Answer: A
Terminal voltage = Vz + Iz·Rz = 12 + (50×10^-3 × 5) = 12 + 0.25 = 12.25 V
Q.80Medium
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.
