What defines single supply operation when talking about differential amplifiers? What are the key parameters that makes them run at single supply?
First of all I would like to point out that differential amplifiers or amplifiers in general do not carry a ground pin. All they require are two supply references with a potential difference that is greater then the minimum recomended operating voltage. That being said, when we run a differential amplifier on a single supply as we show in Figures 1 and 2, there are few things to be aware of:
Figure 1 Figure 2
1. The input common mode range of the differential amplifier: Some amplifiers have an input stage that is level shifted down so it can run DC coupled single supply with input voltages (+Vi, -Vi) centered around ground and output voltage centered around Vocm (see figure 1). These amplifiers do not like to be AC coupled (see figure 2) because this will cause the input common mode to rise to Vocm and in some cases, outside the input range of the amplifier. However, there are amplifiers that have a centered input stage and for these amplifiers AC coupling the input in single supply operation is a good idea.
2. The output swing of the differential amplifier: Most amplifiers have either a rail-to-rail output stage or require some headroom from either supply rail. That being said, individual outputs (+Vo, -Vo) can not swing below ground when operating on a single supply, they are centered around Vocm. However the differential output (+Vo - (-Vo)) will be independent of the output common mode voltage because it gets subtracted out.
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