Comparator Decisions

Comparator Output Rule

greater input selects high

An ideal open-loop comparator chooses a rail from exact input voltages and a stated rule. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Greater plus input selects the high rail

The plus input is 8 volts and the minus input is 6 volts, so the ideal comparator output is 12 volts.

8 V>6 V⇒Vout=12 V8\ \text{V} > 6\ \text{V}\Rightarrow V_{\text{out}} = 12\ \text{V}
Comparator highThe output follows the strict greater-than rule.V8 VV6 VV12 VHIGHtie LOWplusminusoutput

Lower plus input selects the low rail

When the plus input is 4 volts and the minus input is 6 volts, the output is 0 volts. Linear op-amp gain and feedback are deferred.

4 V<6 V4\ \text{V} < 6\ \text{V}
Comparator lowThe output follows the exact comparison.V4 VV6 VV0 VLOWtie LOWplusminusoutput