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Transcript
4/30/2017 769818055 1/3 Example: The Input Offset Voltage Consider an inverting amp constructed with an op-amp exhibiting an input offset voltage of Vos: R2 i2 R1 vi v1 - i1 v2 + Vos ideal + vo - + Applying the concept of a virtual short to the ideal op-amp, we find that: v1 0 Vos 0 Vos Thus, v1 v2 ! For an op-amp with an input offset voltage, the virtual “short” equation turns out to be: 4/30/2017 769818055 2/3 v1 Vos v2 Recall, however, that the input offset voltage is typically very small (i.e., Vos 5 mV ), so that v1 v2 . The current into each terminal of the op-amp is still zero, so that: i1 i2 where: i1 vi v1 R1 i2 v1 vo R2 and: Combining, we find: vi Vos Vos vo R1 R2 Performing a little algebra, we can solve this equation for output voltage vo : vo and rearranging: Vos R1 Vos R2 vi R2 R1 4/30/2017 769818055 vo v i 3/3 Vos Hey! We could have easily found this result by applying superposition! Note that if the input offset voltage is zero (its ideal value), this expression simply reduces to the normal inverting amplifier expression: R vo 2 vi R1 Thus, the term: R2 1 Vos R 1 represents an output offset voltage. Note this offset voltage is a constant with respect to vi --its value does not change, even if the input voltage is zero!.
