Which of the following topologies provides the highest input impedance for an amplifier?

The correct answer is B: Common Drain FET amplifier (Source Follower). Source follower (Common Drain) provides extremely high input impedance (in gigaohms range) because the gate of the FET is the input, which has very high impedance characteristic of FET gates.

A 4-bit binary subtractor using 2's complement method requires:

The correct answer is A: 4 full adders and complementing circuit. 2's complement subtraction (A-B) converts to addition A+2's complement of B. This requires 4 full adders (for addition) and XOR gates (for bit complementing), avoiding dedicated subtractors

Which window function has the BEST frequency resolution but WORST spectral leakage?

The correct answer is C: Rectangular window. Rectangular window has narrowest main lobe (best resolution) but highest sidelobe (worst leakage)

The Z-transform of x[n] = (1/2)^n u[n] is:

The correct answer is A: z/(z - 0.5), |z| > 0.5. X(z) = Σ(1/2)^n z^(-n) = z/(z - 1/2) = z/(z - 0.5) with ROC |z| > 0.5.

In a p-n junction diode, the depletion width increases when:

The correct answer is A: Reverse bias voltage increases. Reverse bias widens the depletion region by pulling majority carriers away from the junction, increasing the width of the space charge region.

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Electronics (ECE)
Electronic Devices

Analog/digital electronics, communication

41 Q 4 Topics Take Mock Test

Difficulty: All Easy Medium Hard 11–20 of 41

Topics in Electronics (ECE)

All Electronic Devices 100 Digital Electronics 100 Analog Circuits 100 Signals & Systems 100

Q.11 Medium Electronic Devices

The output impedance of a BJT common-collector amplifier is approximately:

A r_e = V_T/I_E (very low)

B r_o = V_A/I_C (very high)

C R_L (equal to load resistance)

D R_E || r_e (parallel combination with slight reduction)

Correct Answer: A. r_e = V_T/I_E (very low)

EXPLANATION

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.

Test

Q.12 Medium Electronic Devices

A CMOS inverter has asymmetric rise and fall times primarily because:

A Electron mobility is higher than hole mobility

B The pull-up and pull-down networks have different W/L ratios

C Supply voltage affects NMOS and PMOS equally

D The threshold voltages are identical

Correct Answer: A. Electron mobility is higher than hole mobility

EXPLANATION

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.

Test

Q.13 Medium Electronic Devices

For an IMPATT diode oscillator, the 'IMPATT' stands for:

A Impact Ionization Transit-Time diode

B Impurity Modulated Transit Time diode

C Impact Avalanche and Transit-Time device

D Impedance Modulated Avalanche Transit diode

Correct Answer: C. Impact Avalanche and Transit-Time device

EXPLANATION

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.

Test

Q.14 Medium Electronic Devices

A varactor diode is used in frequency modulation circuits because:

A Its junction capacitance varies with reverse bias voltage

B It has high forward conductance

C It exhibits negative resistance characteristics

D Its reverse saturation current is voltage dependent

Correct Answer: A. Its junction capacitance varies with reverse bias voltage

EXPLANATION

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.

Test

Q.15 Medium Electronic Devices

In a MESFET (Metal-Semiconductor FET), compared to MOSFET:

A It has better high-frequency performance

B It is easier to manufacture due to Schottky junction formation

C It has lower parasitic capacitances

D All of the above are correct

Correct Answer: D. All of the above are correct

EXPLANATION

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.

Test

Q.16 Medium Electronic Devices

In an optocoupler (photocoupler), the coupling efficiency depends on:

A Only the LED forward current

B The spectral response match between LED emission and photodetector sensitivity

C Only the photodetector bandwidth

D The operating temperature of the LED

Correct Answer: B. The spectral response match between LED emission and photodetector sensitivity

EXPLANATION

Coupling efficiency is maximized when the LED's emission spectrum overlaps with the photodetector's spectral sensitivity. Spectral mismatch significantly reduces overall coupling efficiency.

Test

Q.17 Medium Electronic Devices

A photodiode is operated in photovoltaic mode when:

A It is forward biased with light incident on junction

B It is reverse biased and generates voltage without external bias

C It is not biased externally, with light generating open-circuit voltage

D It operates in breakdown region under illumination

Correct Answer: C. It is not biased externally, with light generating open-circuit voltage

EXPLANATION

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.

Test

Q.18 Medium Electronic Devices

In a BJT, the early voltage (V_A) represents:

A The voltage at which base-collector junction breaks down

B The extrapolated voltage where I_C becomes zero with slope equal to output resistance

C The voltage drop across the base terminal

D The thermal voltage at room temperature

Correct Answer: B. The extrapolated voltage where I_C becomes zero with slope equal to output resistance

EXPLANATION

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.

Test

Q.19 Medium Electronic Devices

Which of the following statements about tunnel diodes is INCORRECT?

A They exhibit negative resistance in forward bias region

B They have heavily doped p and n regions

C They operate at frequencies above 1 GHz

D They have lower switching time than conventional diodes

Correct Answer: C. They operate at frequencies above 1 GHz

EXPLANATION

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.

Test

Q.20 Medium Electronic Devices

For a given application, selecting a BJT with higher β (current gain) results in:

A Lower base current requirement and reduced input impedance

B Lower base current requirement and improved input impedance

C Higher base current and higher transconductance

D No change in circuit performance

Correct Answer: B. Lower base current requirement and improved input impedance

EXPLANATION

Higher β means lower IB needed for same IC, reducing loading on the driving source and improving effective input impedance (Zin ≈ β×re).

Test

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Electronic Devices