Question
What best describes a pull-up or pull-down resistor?
Answer Options
- A) A resistor in a keying circuit used to reduce key clicks
- B) A resistor connected to the positive or negative supply used to establish a voltage when an input or output is an open circuit
- C) A resistor that ensures that an oscillator frequency does not drift
- D) A resistor connected to an op-amp output that prevents signals from exceeding the power supply voltage
Correct Answer: B
Explanation
In digital electronics, especially when dealing with TTL (Transistor-Transistor Logic) or open-collector/open-drain outputs, a state known as ‘floating’ or ‘high-impedance’ can occur when an input is left unconnected or an output is not actively driven. This floating state can lead to erratic behavior or spurious noise pickup.
A pull-up or pull-down resistor is a resistor connected to the positive or negative supply used to establish a voltage when an input or output is an open circuit. A pull-up resistor connects the line to the positive supply, guaranteeing a logic ‘1’ state when undriven. A pull-down resistor connects the line to ground, guaranteeing a logic ‘0’ state when undriven.
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