1 Introduction

The practical output voltage range of an op-amp circuit is limited by the power supply voltage, internal op-amp design, and circuit configuration. The output range is always less than the power supply range. Thus, for a ±15-V supply, the output range can be at most ±15 V. From a practical perspective some amplifiers can get very close to the supply limits, but no amplifier actually achieves output swing all the way to the supply voltage. A CMOS rail-to-rail amplifier, for example, can have an output swing a few millivolts from the power supply rail.

There are two amplifier specifications that determine the output voltage range: common-mode range, and output swing from the supply rail. The common-mode range is the range of linear operation of the amplifier versus the input common-mode signal. The input common-mode signal is defined as the average voltage applied to the input of the op amp. However, since the op amp has a virtual short between the two inputs, the voltage is approximately the same on either input. Thus, under normal linear operating conditions, any voltage measured on either input is the common-mode voltage. The common-mode range provides the range of linear operation of the amplifier input stage relative to the power supplies. The output swing range is the range of linear operation for an amplifier output stage relative to the power supply voltage and the load current.

The common mode and output swing specification in Table 1-1 can be used to determine the input and output range of the amplifier. Use the minimum and maximum limits from the specification table with the power supply voltages to determine the input and output range for the amplifier. The OPAx206 Input-Overvoltage-Protected, 4-μV, 0.08-μV/°C, Low-Power Super Beta, e-trim™ Op Amps data sheet provides two different specification tables: one at ±5 V and one at ±15 V. Since the actual circuit is between these limits, the worst case is selected. Figure 1-1 provides an example illustrating how the data sheet specification can be applied to a specific power supply configuration to determine the input and output limitations. In this example the minimum common-mode limit can be determined by using the negative supply and minimum limit from the specification table (that is,
(V–) + 1 = (–5 V) + 1 V = –4 V). The maximum can similarly be determined using the 12-V supply and the table maximum limit (that is, (V+) – 1.4 V = 12 V – 1.4 V = 10.6 V). The same approach can be used to calculate the output swing limitation. However, in this case the load must be accounted for. In this example, the load is 10 kΩ, so the output swing is limited by 0.2 V from either supply rail ( –4.8 V < V_OUT < 11.8 V). It is also important to realize that this output limitation is the point that the amplifier is completely nonlinear and the output is saturated. As you approach the saturation limit, the amplifier output becomes distorted. Section 9 covers details explaining the difference between the saturation and linear limit for the amplifier. The common mode and output swing examples use an asymmetrical power supply arrangement of V_CC = +12 V and V_EE = –5 V. In practical circuits the supplies are more commonly balanced but can be asymmetrical as shown in the example. The example uses different values for V_CC and V_EE for instructive purposes to help identify and differentiate between the two supplies.