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

Choices:  
A: Monostable multivibrator  
B: J-K flip-flop  
C: T flip-flop  
D: Astable multivibrator    

Correct Answer is  D: Astable multivibrator  

Correct answer: D — Astable multivibrator

1) Why it’s correct
- An astable multivibrator has no stable states. It continuously switches back and forth on its own, producing a free-running square wave without any external clock or trigger.

2) Why the others are incorrect
- Monostable multivibrator (A): One-shot. It has one stable state and, when triggered, produces a single pulse then returns to the stable state. It does not continuously alternate.
- J-K flip-flop (B): Bistable. It needs a clock (or trigger) to change states. With J = K = 1, it toggles only on clock edges.
- T flip-flop (C): Bistable. It toggles on each clock pulse when T = 1. It does not free-run without a clock.

3) Useful formulas and rules of thumb
- Quick memory:
  - Astable = oscillator (no stable states, free-running)
  - Monostable = one-shot (one stable state)
  - Bistable = flip-flop (two stable states: J-K, T, D, SR)
- 555 timer in astable mode:
  $$f \approx \frac{1.44}{(R_A + 2R_B)C}, \quad t_{\text{high}} \approx 0.693(R_A + R_B)C, \quad t_{\text{low}} \approx 0.693R_BC$$
- 555 timer in monostable mode (one-shot pulse width):
  $$t_{\text{pulse}} \approx 1.1\,RC$$

These highlight that only the astable configuration runs continuously without an external clock.