EETC Circuit Design & Troubleshooting 4 — Questions and Answers
Question 1: What is the primary advantage of using a differential pair (long-tailed pair) in amplifier design?
- Higher output impedance than a single-transistor stage
- Rejection of common-mode signals while amplifying differential signals (Correct answer)
- Lower power consumption than single-ended designs
- Ability to operate without a power supply bypass capacitor
Correct answer: Rejection of common-mode signals while amplifying differential signals
A differential pair amplifies the voltage difference between two inputs while rejecting signals common to both, providing high common-mode rejection ratio (CMRR).
Question 2: A technician replaces a capacitor in an oscillator circuit with one of higher capacitance. What effect will this have on oscillation frequency?
- Frequency increases
- Frequency decreases (Correct answer)
- Frequency remains unchanged
- The oscillator stops working
Correct answer: Frequency decreases
In LC and RC oscillators, frequency is inversely proportional to capacitance (f = 1/(2π√LC) or f = 1/(2πRC)); larger C lowers frequency.
Question 3: In a three-phase power system, what is the phase angle between each of the three voltage waveforms?
- 90°
- 120° (Correct answer)
- 180°
- 360°
Correct answer: 120°
In a balanced three-phase system, the three voltage waveforms are displaced from each other by 120° (360°/3 phases).
Question 4: When using the half-splitting troubleshooting method on a 16-stage circuit, what is the maximum number of tests needed to isolate a fault?
- 4 (Correct answer)
- 8
- 16
- 2
Correct answer: 4
Half-splitting (binary search) isolates a fault in log₂(N) steps; log₂(16) = 4 tests maximum for a 16-stage circuit.
Question 5: What does impedance matching between a source and load achieve in a circuit?
- Minimum current transfer
- Maximum power transfer from source to load (Correct answer)
- Maximum voltage across the source
- Minimum heat dissipation in the source
Correct answer: Maximum power transfer from source to load
Maximum power transfer occurs when the load impedance equals the complex conjugate of the source impedance, as stated by the maximum power transfer theorem.
Question 6: A clamp meter is measuring unusually low current in a motor circuit. Which condition would NOT cause this reading?
- The motor is running at reduced load
- Multiple conductors passing through the clamp in opposite directions
- The clamp jaw is not fully closed
- The power factor of the motor is very high (Correct answer)
Correct answer: The power factor of the motor is very high
A high power factor does not cause low current readings; it means current and voltage are in phase, which is desirable and normal.
Question 7: In digital circuit design, what is the purpose of a pull-up resistor on an open-collector output?
- To limit current when the output transistor conducts
- To pull the output line to a defined high logic level when the transistor is off (Correct answer)
- To filter noise from the output signal
- To increase the switching speed of the transistor
Correct answer: To pull the output line to a defined high logic level when the transistor is off
An open-collector output can only pull the line low; a pull-up resistor connects the line to Vcc, ensuring a defined HIGH state when the transistor is off.
EETC Circuit Design & Troubleshooting 4
What is the primary advantage of using a differential pair (long-tailed pair) in amplifier design?