*Question E7A01:
Which circuit is bistable?*  

*Correct Answer is  C: A flip-flop*  

### AI Response: (may contain errors)  

A bistable circuit is one that has two stable states and can remain in either state indefinitely until an external input causes it to switch to the other state. 

### Correct Answer: C - A flip-flop

- **Why it's correct**: 
  - A flip-flop is a type of bistable multivibrator. It can store one bit of data, having two stable states (0 and 1). It changes states based on input signals, making it ideal for memory storage and digital logic applications.

### Why the other choices are incorrect:

- **A: An AND gate**  
  - An AND gate is a combinational logic circuit that outputs a high signal (1) only when all its inputs are high. It does not have stable states; it continuously changes output based on input conditions.

- **B: An OR gate**  
  - Similar to the AND gate, an OR gate is also a combinational logic circuit. It outputs a high signal when at least one input is high. It does not maintain stable states like a bistable circuit.

- **D: A bipolar amplifier**  
  - A bipolar amplifier is used to amplify signals but does not have stable states. It operates in a linear region and is not designed to hold a state like a bistable circuit.

### Summary:
- **Bistable Circuit**: Has two stable states (e.g., flip-flop).
- **Combinational Logic (AND, OR)**: Outputs depend on current inputs, not stable states.
- **Amplifiers**: Used for signal amplification, not for state retention.

Understanding these distinctions is crucial for recognizing the characteristics of different types of circuits in the Extra Class exam.