We often receive applications questions relating to the power supply, input and output voltage range capabilities of our op amps. It can be confusing so here is an attempt to sort it out:
First, common op amps don’t have ground terminals. A standard op amp does not “know” where ground is so it cannot know whether it is operating from a dual supply (±) or from a single power supply. As long as the power supply, input and output voltages are within their operating ranges, all is good.
Here are the three critical voltage ranges to consider:
These examples (figures 1, 2, 3) are shown in a G=1 buffer configuration. A key point here… the output capability of this example in figure 1 will be limited to 2V from the negative rail and 2.5V from the positive rail and due to the limited input C-M range. This op amp would need to be configured in a higher gain to deliver its full output voltage range.
The example in Figure 1 is typical of an op amp generally used on dual ± supplies. It would not be called a “single supply” type but it certainly could be operated on a single supply just by staying within these ranges.
Figure 2 shows a so-called single supply op amp. It has a C-M range that extends to, and often slightly below the negative rail. This allows its use in a wider range of circuits that operate close to ground. So an op amp that is not called “single supply” could be used in some single supply circuits but a true single-supply type is more versatile in these applications.
In this G=1 buffer circuit, this op amp would could produce an output swing of 0.5V from the V- rail (limited by output capability) and 2.2V from the V+ rails (limited by the input C-M range).
Figure 3 shows a “rail-to-rail” op amp. It can operate with input voltage equal to or even slightly beyond both supply voltage rails as shown in figure 3. Rail-to-rail output means that output voltage can swing very close to the rails, often within a 10mV to 100mV from the supply rails. Some op amps claim only a rail-to-rail output, lacking the input characteristics shown in figure 3. Rail-to-rail op amps are very commonly used on single 5V supplies and lower because they maximize signal voltage capability on their limited supply range.
Rail-to-rail op amps are appealing because they ease signal voltage constraints but they are not always the best choice. Like other of life’s choices there are often tradeoffs with other performance attributes. But that’s why you are an analog designer. Your life is full of complex issues and tradeoffs and you love it!
Thanks for reading,
I am curious to know why many manufacturers are marketing their op-amps as "beyond the rails" at the inputs. The outputs cannot reach the rails, so as a buffer there is no issue with violating the input range. Unless it is with the internal design to ensure excellent CMRR, Vos, and Iib from each rail and the byproduct is a little bit of overhead. Amiright?
Ken-- There are some circuit configurations that require common-mode range that extends to, or beyond the supply rail. These circuits do not require the output of the op amp to swing beyond the rail. This would perhaps make a good future blog topic. I can't attach a figure in this comment but I can describe a simple circuit (not necessarily a common one). Visualize an inverting amplifier with the non-inverting input referenced to a fixed voltage above the rail. With sufficient voltage on the input resistor of the of the circuit, the output of the op amp will be in the linear output range of the op amp.
Thank you Bruce. I had always worried about the difference between dual-supply and single-supply op amps but never found a clear answer, until now.
I love your blog.
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