Question E7A01:
Which circuit is bistable?  

Choices:  
A: An AND gate  
B: An OR gate  
C: A flip-flop  
D: A bipolar amplifier    

Correct Answer is  C: A flip-flop  

To understand “bistable,” remember: a bistable circuit has two stable output states and will stay in one until an input causes it to switch to the other. It stores one bit of information.

Correct answer: C — A flip-flop
- A flip-flop is a bistable multivibrator.
- It has two stable states, often labeled $Q=0$ and $Q=1$, and it retains its state until triggered to change.
- Examples include SR, D, JK, and T flip-flops. They are fundamental memory elements.

Why the others are incorrect:
- A: AND gate — Combinational logic only; output depends instantly on inputs with no memory. Not bistable by itself.
- B: OR gate — Same as the AND gate: combinational, no inherent memory, not bistable.
- D: Bipolar amplifier — An analog linear amplifier biased to a single operating point (one stable state). It is not designed to latch between two discrete states.

Useful rules of thumb:
- Bistable = memory element = flip-flop/latch.
- Monostable = one-shot (one stable state, one quasi-stable state).
- Astable = free-running oscillator (no stable states).
- A bistable stores 1 bit: number of states $= 2^1 = 2$.