Some Problems When Using Operational Amplifiers as Comparators

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Some Problems When Using Operational Amplifiers as Comparators [Copy link]

Background: The op amp uses positive feedback to set the hysteresis threshold. The signal to be compared is input into the negative terminal of the op amp, and the output of the op amp drives an NMOS tube as a logic conversion.

1. If the external signal is input first and the op amp is powered later, will the op amp be killed? Under what circumstances will this happen?

2. Is the MOS tube of logic conversion always turned on? Under what circumstances does it happen?

maychang

1. If the external signal is input first and the op amp is powered later, will the op amp be killed? Under what circumstances will this happen?

If the internal resistance of the external signal is large, the external signal is applied first and the power is applied later, which generally will not damage the op amp. If the external signal is drawn from other circuit boards, it should generally be limited to filter out high-frequency interference, and a sufficiently large resistor should be connected in series (to the extent that the op amp will not be damaged) before connecting to the op amp input.

maychang

2. Is the MOS tube of logic conversion always turned on? Under what circumstances does it happen?

Your MOS tube is driven by a Schmidt comparator, and the comparator output may certainly turn on the MOS tube. The longer the comparator is in the state of turning on the MOS tube, the longer the MOS tube will be turned on.

Fred_1977

The situation you mentioned should be a problem of circuit design error. You are referring to the two design issues of stability and reliability.