Question E6C02:
What happens when the level of a comparator’s input signal crosses the threshold voltage?  

Choices:  
A: The IC input can be damaged  
B: The comparator changes its output state  
C: The reference level appears at the output  
D: The feedback loop becomes unstable    

Correct Answer is  B: The comparator changes its output state  

When a comparator’s input crosses its threshold (reference) voltage, it flips its output to the opposite saturated state. That’s the defining behavior of a comparator.

Why B is correct:
- A comparator is a high-gain differential stage that decides based on the sign of the difference between its inputs.
- If the non-inverting input is used for the signal and the inverting input holds the threshold $V_{TH}$, then:
  $$V_{IN} > V_{TH} \Rightarrow V_{OUT} \approx V_{OH}$$
  $$V_{IN} < V_{TH} \Rightarrow V_{OUT} \approx V_{OL}$$
- Crossing the threshold switches the output state between its two saturation levels (near the supply rails).

Why the other choices are incorrect:
- A: The IC input can be damaged  
  Crossing a valid threshold is normal operation. Damage occurs only if inputs exceed absolute maximum ratings (e.g., beyond supply rails without protection), not merely by crossing the threshold.
- C: The reference level appears at the output  
  The output does not reproduce the reference; it saturates to logic-like levels $V_{OH}$ or $V_{OL}$ near the rails. The reference stays at the comparator input.
- D: The feedback loop becomes unstable  
  A basic comparator is open-loop. If positive feedback is added (Schmitt trigger), it adds hysteresis to improve noise immunity, not to create instability. Crossing the threshold simply toggles the state.

Useful formulas and rules of thumb:
- Decision rule (non-inverting input is the signal, inverting is reference):
  $$V_{OUT} =
    \begin{cases}
      V_{OH}, & V_{IN} > V_{TH} \\
      V_{OL}, & V_{IN} < V_{TH}
    \end{cases}$$
- Effective threshold includes input offset voltage $V_{OS}$:
  $$V_{TH,eff} \approx V_{TH} \pm V_{OS}$$
- If hysteresis is used (Schmitt trigger), there are two thresholds $V_{TH+}$ and $V_{TH-}$ to prevent chatter around the threshold.
- Expect a finite propagation delay $t_p$ from crossing to output change; outputs swing to rails, not to the reference level.