Amateur Extra E7A: Digital circuits
These are the 11 official questions in group E7A of the 2024–2028 Amateur Extra question pool, with the correct answer marked and a plain-English explanation of why it's right. The real exam asks one question from this group.
- E7A01Question 1
Which circuit is bistable?
- AAn AND gate
- BAn OR gate
- CA flip-flop Correct
- DA bipolar amplifier
Why: A flip-flop is bistable: it has two stable states.
- E7A02Question 2
What is the function of a decade counter?
- AIt produces one output pulse for every 10 input pulses Correct
- BIt decodes a decimal number for display on a seven-segment LED display
- CIt produces 10 output pulses for every input pulse
- DIt decodes a binary number for display on a seven-segment LED display
Why: A decade counter outputs one pulse for every 10 input pulses.
- E7A03Question 3
Which of the following can divide the frequency of a pulse train by 2?
- AAn XOR gate
- BA flip-flop Correct
- CAn OR gate
- DA multiplexer
Why: A flip-flop divides a pulse train's frequency by 2.
- E7A04Question 4
How many flip-flops are required to divide a signal frequency by 16?
- A4 Correct
- B6
- C8
- D16
Why: 2⁴ = 16, so 4 flip-flops.
- E7A05Question 5
Which of the following circuits continuously alternates between two states without an external clock signal?
- AMonostable multivibrator
- BJ-K flip-flop
- CT flip-flop
- DAstable multivibrator Correct
Why: An astable multivibrator alternates between states with no clock.
- E7A06Question 6
What is a characteristic of a monostable multivibrator?
- AIt switches temporarily to an alternate state for a set time Correct
- BIt produces a continuous square wave
- CIt stores one bit of data
- DIt maintains a constant output voltage, regardless of variations in the input voltage
Why: A monostable switches to its other state for a set time, then returns.
- E7A07Question 7
What logical operation does a NAND gate perform?
- AIt produces a 0 at its output only if all inputs are 0
- BIt produces a 1 at its output only if all inputs are 1
- CIt produces a 0 at its output if some but not all inputs are 1
- DIt produces a 0 at its output only if all inputs are 1 Correct
Why: NAND outputs 0 only when all inputs are 1.
- E7A08Question 8
What logical operation does an OR gate perform?
- AIt produces a 1 at its output if any input is 1 Correct
- BIt produces a 0 at its output if all inputs are 1
- CIt produces a 0 at its output if some but not all inputs are 1
- DIt produces a 1 at its output if all inputs are 0
Why: OR outputs 1 if any input is 1.
- E7A09Question 9
What logical operation is performed by a two-input exclusive NOR gate?
- AIt produces a 0 at its output only if all inputs are 0
- BIt produces a 1 at its output only if all inputs are 1
- CIt produces a 0 at its output if one and only one of its inputs is 1 Correct
- DIt produces a 1 at its output if one and only one input is 1
Why: XNOR outputs 0 when exactly one input is 1.
- E7A10Question 10
What is a truth table?
- AA list of inputs and corresponding outputs for an op-amp
- BA list of inputs and corresponding outputs for a digital device Correct
- CA diagram showing logic states when the digital gate output is true
- DA table of logic symbols that indicate the logic states of an op-amp
Why: A truth table lists inputs and their corresponding outputs.
- E7A11Question 11
What does “positive logic” mean in reference to logic devices?
- AThe logic devices have high noise immunity
- BHigh voltage represents a 1, low voltage a 0 Correct
- CThe logic circuit is in the “true” condition
- D1s and 0s are defined as different positive voltage levels
Why: Positive logic: high voltage is 1, low is 0.