*Question E7A05:
Which of the following circuits continuously alternates between two states without an external clock signal?*  

*Correct Answer is  D: Astable multivibrator*  

### AI Response: (may contain errors)  

An astable multivibrator is a type of electronic circuit that continuously alternates between two states without requiring an external clock signal. Here's why the correct answer is D: Astable multivibrator, and why the other choices are incorrect:

### Correct Answer: D: Astable multivibrator
- **Astable Multivibrator**: This circuit is inherently unstable and continuously oscillates between two states, producing a square wave output. It does not require any external trigger or clock signal to change states, making it "astable" (without a stable state).

### Incorrect Choices:
- **A: Monostable multivibrator**: This circuit has one stable state and one unstable state. It requires an external trigger to switch to the unstable state, after which it returns to the stable state automatically. It does not oscillate continuously on its own.

- **B: J-K flip-flop**: This is a type of flip-flop that requires a clock signal to toggle between states. It does not oscillate on its own and needs an external clock to change states.

- **C: T flip-flop**: Similar to the J-K flip-flop, the T flip-flop also requires a clock signal to toggle its state. It does not continuously alternate between states without an external clock.

### Summary
- **Astable Multivibrator**: No external clock needed, continuously oscillates.
- **Monostable Multivibrator**: Needs a trigger, returns to stable state.
- **J-K Flip-Flop**: Requires a clock signal to toggle.
- **T Flip-Flop**: Also requires a clock signal to toggle.

Understanding these differences helps in identifying the correct circuit type based on its behavior and requirements.