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Introduction

Most portable systems have one battery, so the popularity of portable equipment results in increased single-supply applications. Split- or dual-supply op amp circuit design is straightforward because op amp inputs and outputs are referenced to the normally-grounded center tap of the supplies. In the majority of split-supply applications signal sources driving the op amp inputs are referenced to ground; thus with one input of the op amp referenced to ground common-mode voltage and voltage bias problems are negligible.

When signal sources are referenced to ground, single-supply op amp circuits exhibit a large input common-mode voltage. The input voltage is not referenced to the midpoint of the supplies like it would be in a split-supply application; rather, it is referenced to the lower-power supply rail. This circuit malfunctions when the input voltage is positive because the output voltage should go negative; this is hard to do with a positive supply. It operates marginally with small negative input voltages because most op amps cannot function when the inputs are connected to the supply rails.

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