Single supply was quite an industry buzz word back when batteries were coming into their own. If a marketer put the label of “single supply” on their amplifier, the interest and sales would reflect the attractiveness of that type of product. This is still true, but you really need to know what this phrase means.
An amplifier typically has five terminals: non-inverting input, inverting input, positive supply, negative supply, and output. That is correct: two inputs, one output, and two power supply pins (Figure 1). Notice that there is no ground pin. In fact, a designer can connect the negative supply to 50 V and the positive supply to 55 V, as long as the product data sheet allows a power supply range of 5 V. In this configuration, if the input pins and output pin comply with input range and load requirements, a single- or dual-supply amplifier will work just fine.
Figure 1. Amplifier block diagram showing the location of the different pins.
Let’s back up and define the meaning of a “single-supply” application. With this type of circuit, the designer makes sure that the supply voltage difference complies with the battery voltage limits, or as low as feasibly possible. The ultimate goal is to reduce the circuit’s power dissipation. In doing so, the power supply range (V+ – V–) is as small as the active components will allow.
What does this mean when you are talking about the operational amplifier (op amp)? The input and output performance characteristics must be able to span as close to the supplies as possible. Actually, it is preferable that the non-inverting input (VIN+) and inverting input (VIN–) are able to go slightly above and below the power supply voltages by a few hundred millivolts.
The amplifiers output terminal is another issue. It would be nice if the output pin (VO) could travel above and below the power supply pins. However, there are internal transistors that drive loads and require a minimum amount of voltage drop to operate (Figure 2).
Figure 2. Simplified schematic of the OPA314
In Figure 2, the input common-mode voltage range extends to 200 mV beyond the supply rails (V+ and V–). With 0 to 5V supplies, the input range would be from –200 mV to 5.2V. You will notice in this complementary input stage, there is an N-channel input differential pair in parallel with a P-channel differential pair. In this circuit, the N-channel pair is active when the input voltages are close to the positive rail, while the P-channel pair is active when the input voltages are close and extend beyond the negative rail.
This device has a class AB output stage to deliver a robust output drive capability. For resistive loads up to 10 k-Ohms, the output swing typically is within 5 mV of either supply rail. For this specification, the load resistor is biased to ground.
Imagine that this amplifier has voltages on the power supply pins of +2.5 V (V+) and –2.5 V (V–). In this condition, the input swing capability of VIN+ and VIN– is equal to –2.7 V to +2.7 V (inclusive). Additionally, the output load resistor is biased to –2.5 V.
The question comes up, “Can I use my signal supply amplifier with asymmetric power supplies?” The answer is yes. Bear in mind, the amplifier does not have a ground reference. However, if you plan to do this with any amplifier, whether it is single or dual-supply, watch out for the input common-mode, input swing, and output swing specifications. You will have to spend some time in translating the data sheet specifications to your application’s power supply conditions.
For more information check out this datasheet: OPA314