Practical Tips | Operational Amplifier Input and Output Common-Mode and Differential Voltage Ranges

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Any practical op amp has a limited operating voltage range at its input and output. In modern system designs, power supply voltages continue to drop, and total supply voltages of 3 V to 5 V are now common for analog circuits such as op amps. This is a far cry from the power supply system voltages of the past, when ±15 V (30 V total) was common.

Because of the reduced voltage, the limitations of the input and output voltage ranges must be understood—especially during the op amp selection process.

Figure 1 below shows roughly the limits of the op amp input and output dynamic range, relative to the two supply rails. Any op amp is powered by two supply potentials, represented by the positive rail +VS and the negative rail –VS. The op amp’s input and output common-mode range is defined by how close it is to the voltage limits of the two supply rails.

Figure 1: Op amp input and output common-mode range

On the output side, VOUT has two rail-related limits, a high level (close to +VS) and a low level (close to –VS). When high, the range is to the upper saturation limit, VS – VSAT(HI) (maximum positive value). For example, if +VS is 5 V and VSAT(HI) is 100 mV, the upper VOUT limit (maximum positive value) is 4.9 V. Similarly, when low, the range is to the lower saturation limit, –VS + VSAT(LO). Therefore, if –VS is ground (0 V) and VSAT(LO) is 50 mV, the lower VOUT limit is 50 mV.

Obviously, the internal design of a given op amp will affect this output common-mode dynamic range, and if necessary, the device itself should be designed to minimize VSAT(HI) and VSAT(LO) to achieve the maximum output dynamic range. This is done with certain types of op amps, which are usually designed specifically for single-supply systems.

On the input side, the common-mode range available for VIN also has two rail-related limits, a high level (close to +VS) and a low level (close to –VS). At the high level, the range is up to the upper common-mode limit of +VS – VCM(HI) (maximum positive value). Continuing with the example of +VS = 5 V, if VCM(HI) is 1 V, the upper VIN limit (maximum common-mode positive value) is +VS – VCM(HI) or 4 V.

Figure 2 below shows how to determine VCM(HI) using a hypothetical op amp, as shown by the upper curve. This op amp will operate with a VCM input lower than the curve shown in the figure.

Figure 2: Illustration of op amp output common-mode range

In practice, the input common-mode range of an actual op amp is usually specified as a voltage range, not necessarily referenced to +VS or –VS. For example, a typical ±15 V operating dual-supply op amp has a specified common-mode operating range of ±13 V. At low levels, there is also a common-mode lower limit. This is usually expressed as –VS + VCM(LO), as shown in Figure 2 as the lower VCM(LO) curve. If the device also uses a ±15 V supply voltage, this represents typical performance. Taking a single supply as an example, with –VS = 0 V, if VCM(LO) is 100 mV, the common-mode lower limit is 0 V + 0.1 V (i.e., 0.1 V). This example shows the common-mode lower limit within the –VS range of 100 mV, which is actually more suitable for representing a single-supply device with a common-mode lower or upper limit (including the supply rails). In other words, VCM(LO) or VCM(HI) is 0 V. There are also single-supply devices with a common-mode range that includes both supply rails. However, single-supply devices often do not provide graphical data (such as the common-mode limits shown in Figure 2) but instead describe performance over a specified voltage range in tabular form.

In normal operating mode, the op amp is connected to the feedback loop, so the differential input voltage is held at 0 V (ignoring offset voltage). However, in some cases (such as power-up), the op amp may be subject to a differential input voltage that is not equal to 0. Some input structures require limiting the differential input voltage to prevent it from being damaged. These op amps also typically have internal back-to-back diodes on the inputs, which may not be shown in the simplified schematic of the amplifier, but a ±700 mV (maximum) differential input voltage specification is shown. In addition, the maximum input differential current specification is shown. Some amplifiers have internal current limiting resistors, but these resistors increase noise and are not used in low-noise op amps.

Most general-purpose op amps have output stages that provide short-circuit protection to ground or to either supply. This is often called infinite short-circuit protection because the amplifier can infinitely drive that current into the short circuit. The output current that should be delivered by the op amp is the output current at this point. Usually the limit is set so that the op amp can deliver 10 mA of output current for general-purpose op amps. If the op amp must have both high precision and high output current, it is recommended to use a separate output stage (within the feedback loop) to minimize the self-heating of the precision op amp. This additional amplifier is often called a buffer because its voltage gain is usually 1. Some op amps are capable of delivering large output currents. For example, the AD8534 is a quad device with 250 mA output current from each of the four sections. Note that if 250 mA is delivered from all four sections at the same time, the package power dissipation specification will be exceeded, the amplifier will overheat, and may be damaged. This problem is more severe for smaller packages with lower power consumption.

The output current of a high-speed op amp is usually not limited to a lower value because it affects its slew rate and ability to drive low impedances. Most high-speed op amps source and sink currents between 50 and 100 mA, but some are limited to less than 30 mA. Even high-speed op amps with short-circuit protection may exceed the junction temperature (due to the high short-circuit current), resulting in damage to the device due to long-term short-circuit.

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