The supplies’ input and output voltages could swing to within a volt or so of the rails before running out of headroom. Offset Voltage and Crossover Distortion in Rail-to-Rail Op AmpsĮarly op-amp designs required dual power supplies (☑5 V was typical). Offset voltage drift (dV OS/dT) is expressed in terms of the number of microvolts increase per degree of temperature change (µV/☌). The offset voltage of an op amp drifts over temperature. A trimming procedure during manufacturing is used to reduce the offset voltage, and op amps that exceed the maximum allowable offset voltage after trimming are rejected. Over the years, better matching of components and improved package materials have slowly reduced offset voltages. In a typical application, the differential voltage is multiplied by the closed-loop gain of the op amp, which is determined by the external components. These factors cause a mismatch of the bias currents flowing through the input circuit of the op amp, resulting in a voltage differential at the input terminals. Stresses placed on the die during packaging also make a minor contribution. Input offset voltage is primarily due to the inherent mismatch of the input transistors and components during die fabrication.
In real devices, the offset voltage can be negative or positive in polarity and vary from die to die in the same wafer lot.
In the model, a voltage source in series with the positive or negative input terminal represents the offset voltage. Our ideal op amp has zero volts across its input pins when the output is zero, but in reality, the input terminals are at slightly different dc potentials. Offset voltage (V OS) is the differential dc voltage required between the input pins of an op amp to make its output zero. The model of a real-world op amp (b) is considerably more complex than its ideal equivalent (a).
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