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Operational amplifiers have two signal input connections and one signal output connection. One of the signal inputs is marked '-' and the other is marked '+'. These polarity markings have nothing to do with the supply connections - they indicate the overall phase shift between each input and the output. The '+' sign indicates zero phase shift and the '-' sign indicates 180° phase shift. Since 180° phase shift produces an inverted (i.e. turned upside down) waveform, the '-' input is often referred to as the 'inverting input'.
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