The Op Amp-Based Differential Amplifier: Not As Easy As It Looks

Focus:

When an inexpensive differential amplifier is needed, designers often use the familiar circuit and just choose resistor values to achieve equality of gains in both inverting and non-inverting branches. They forget that the signal sources may have different output impedances, which may completely destroy the differential amplifier’s operation, causing an output offset and compromising the common-mode rejection ratio. But given a value of the resistor on the amplifier’s inverting input and a required gain, it’s possible to select the values of the remaining resistors so that the input impedances will be made equal. This article derives the equations for calculating those resistor values, while also accounting for their variability due to initial tolerance, thermal coefficient of resistance and aging. A design example is presented.

What you’ll learn:

• How to improve the output offset and CMRR performance of a differential amplifier through proper selection of external resistors
• How to equalize the input impedances of a differential amplifier

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Author & Publication:

Gregory Mirsky, Continental Automotive Systems, Deer Park, Ill., How2Power Today, Jul 15 2019

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