*Question E6C07:
What best describes a pull-up or pull-down resistor?*  

*Correct Answer is  B: A resistor connected to the positive or negative supply used to establish a voltage when an input or output is an open circuit*  

### AI Response: (may contain errors)  

To understand pull-up and pull-down resistors, let's break down the correct answer and the other choices.

### Correct Answer:
**B: A resistor connected to the positive or negative supply used to establish a voltage when an input or output is an open circuit.**

- **Explanation**: 
  - A pull-up resistor is connected between a voltage supply (usually the positive supply) and the input pin of a digital circuit. It ensures that the input reads a high voltage (logic high) when the switch is open.
  - A pull-down resistor is connected between the input pin and ground, ensuring that the input reads a low voltage (logic low) when the switch is open.
  - This prevents floating inputs, which can lead to unpredictable behavior in digital circuits.

### Why the Other Choices are Incorrect:

- **A: A resistor in a keying circuit used to reduce key clicks.**
  - **Explanation**: This describes a different function related to reducing noise in keying circuits, not the role of pull-up or pull-down resistors. Pull-up and pull-down resistors are primarily used for establishing defined logic levels.

- **C: A resistor that ensures that an oscillator frequency does not drift.**
  - **Explanation**: This refers to components that stabilize oscillators, such as capacitors or inductors, not pull-up or pull-down resistors. Pull-up and pull-down resistors do not directly affect oscillator frequency stability.

- **D: A resistor connected to an op-amp output that prevents signals from exceeding the power supply voltage.**
  - **Explanation**: This describes a different application, often involving current limiting or protection, but does not pertain to the function of pull-up or pull-down resistors. Pull-up and pull-down resistors are not used for limiting output voltage in op-amps.

### Summary:
- Pull-up and pull-down resistors are essential for ensuring stable logic levels in digital circuits.
- They prevent floating inputs by connecting to a known voltage level when the circuit is open.

### Useful Rule of Thumb:
- Use a pull-up resistor when you want the default state to be high (connected to Vcc).
- Use a pull-down resistor when you want the default state to be low (connected to ground). 

This understanding will help you answer questions related to digital logic and circuit design effectively on the Extra Class exam.