BEE Analog and Digital Electronics 5 — Questions and Answers
Question 1: In an R-2R ladder DAC, the primary advantage over a binary-weighted DAC is:
- Higher output voltage range
- Only two resistor values are needed regardless of bit count (Correct answer)
- Faster conversion speed
- Lower output impedance
Correct answer: Only two resistor values are needed regardless of bit count
The R-2R ladder uses only two resistor values (R and 2R) for any number of bits, making precision matching much easier to manufacture.
Question 2: Which configuration of an op-amp provides the highest input impedance?
- Inverting amplifier
- Voltage follower (unity gain buffer) (Correct answer)
- Summing amplifier
- Difference amplifier
Correct answer: Voltage follower (unity gain buffer)
A voltage follower connects the output directly to the inverting input with the signal at the non-inverting input, yielding input impedance approaching infinity due to near-zero differential input voltage.
Question 3: In a JK flip-flop, setting J=1 and K=1 causes the output to:
- Set to Q=1
- Reset to Q=0
- Hold the current state
- Toggle to the complement of Q (Correct answer)
Correct answer: Toggle to the complement of Q
The J=K=1 condition is the toggle mode of the JK flip-flop, where Q changes to its complement on each active clock edge.
Question 4: A Wien bridge oscillator uses which type of feedback network to set oscillation frequency?
- LC tank circuit
- RC lead-lag network (Correct answer)
- Crystal resonator
- Transmission line stub
Correct answer: RC lead-lag network
The Wien bridge uses an RC lead-lag network that provides 0° phase shift at the resonant frequency f = 1/(2πRC), setting the oscillation frequency.
Question 5: In digital logic, the fan-out of a gate refers to:
- The number of inputs a gate can accept
- The maximum number of identical gate inputs a gate output can reliably drive (Correct answer)
- The propagation delay through the gate
- The power dissipation per switching event
Correct answer: The maximum number of identical gate inputs a gate output can reliably drive
Fan-out specifies how many standard gate inputs can be connected to a single gate output while maintaining valid logic levels.
Question 6: The Early effect in a BJT manifests as:
- Increased base-emitter junction capacitance at high frequencies
- Non-zero output conductance causing IC to increase slightly with VCE (Correct answer)
- Thermal runaway at high collector currents
- Reverse breakdown of the collector junction
Correct answer: Non-zero output conductance causing IC to increase slightly with VCE
The Early effect (base-width modulation) causes the effective base width to shrink with increasing VCE, slightly increasing collector current and creating finite output resistance.
Question 7: A 4-to-16 decoder can be constructed from:
- Two 2-to-4 decoders only
- Four 2-to-4 decoders with an enable input
- Two 3-to-8 decoders with an enable input (Correct answer)
- One 2-to-4 decoder and one 3-to-8 decoder
Correct answer: Two 3-to-8 decoders with an enable input
Two 3-to-8 decoders with enable lines can be combined — one upper address bit selects which 3-to-8 decoder is active, giving 16 total outputs.
BEE Analog and Digital Electronics 5
In an R-2R ladder DAC, the primary advantage over a binary-weighted DAC is: