Op Amp Input and Output Swing Limitations

Abstract

Exceeding the input or output swing limitations on amplifiers causes distortion on the output signal. This document initially covers the basic methods for calculating these limitations. Next, a simplified explanation of how the limitation occurs on internal transistor topology is covered. The internal operation is described to provide insight to the board-and-system level designer on how different categories of amplifiers behave. For example, the operation of two different types of rail-to-rail common-mode range are compared and contrasted. Each of these topologies has specific advantages and disadvantages related to noise and distortion. Finally, the impact of output current on swing limitations, short-circuit protection, and other topics related to the op amp input and output stage are covered.

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.

Table 1-1 OPA206 Common Mode and Output Swing Specifications

Parameter: OPA206		Conditions	Min	Max	Unit
V_CM	Common-Mode Voltage Range		(V–) + 1	(V+) – 1.4	V
V_OUT	Voltage Output	R_L = 10 kΩ, V_S = ±15 V	(V–) + 0.2	(V+) – 0.2	V
V_OUT	Voltage Output	R_L = 2 kΩ, V_S = ± 15 V	(V–) + 0.35	(V+) – 0.35	V

Figure 1-1 OPA206 Example Common Mode and Output Swing Limits

2 Circuit Configuration Impact on Common-Mode Range

The Introduction showed how the common-mode limitation of the amplifier can be calculated given the supply voltages and the data sheet specification table. The amplifier circuit configuration often determines if this limitation causes nonlinear operation. The voltage follower is the configuration that is most likely to be impacted by common-mode limitations. This is because the common-mode voltage is the same as the input signal and the input signal often covers the entire supply range. Figure 2-1 provides a DC input sweep to illustrate the common-mode limitation of the OPA206 in a buffer configuration. Note that the input voltage is equal to the common-mode signal in this configuration, so the linear range is –4 V to 10.6 V as calculated in Figure 1-1.

Figure 2-1 Common-Mode Limit of OPA206 in Buffer Configuration

The same op amp in a gain configuration is not impacted by the common-mode limitations because the valid input signal range for the gain configuration is small and does not approach the common-mode limits. For example, in a gain of 10 V/V, the input range is ten times smaller than the output range. Figure 2-2 shows the same supply arrangement in a gain of 10 V/V. In this case, the best output range is –5 V to +12 V, so the valid input range is –0.5 V to 1.2 V. Since the common-mode range for this device is –4.0 V to +10.6 V, the valid input signal is never near the common-mode limits. For this example, notice that the output signal does not swing all the way to the supply limits. This limitation is because of the output swing limitations, and not the common-mode limits and are covered in detail later in this white paper.

Figure 2-2 Common Mode Does not Limit OPA206 in Non-inverting Gain of 10 V/V

In the non-inverting configuration, the common-mode voltage is equal to the input signal. For the inverting configuration the common-mode voltage is equal to the voltage applied to the non-inverting input. In general, for inverting amplifiers the non-inverting input is connected to ground or a fixed DC voltage. For inverting amplifiers, the common-mode voltage remains constant regardless of the input signal, so these types of amplifiers generally do not have common-mode issues. The example shown in Figure 2-3 shows that the common-mode signal is held constant at ground, and the amplifier does not experience common-mode limitations. This amplifier has a gain of –1 V/V, so you can substitute this configuration for a buffer configuration if common-mode limitation is a problem for the buffer. However, remember that the inverting amplifier configuration has gain error and drift due to the tolerance of the feedback resistors, whereas the buffer has a very accurate gain of 1 V/V. Furthermore, the resistors add noise and additional power consumption for the amplifier.

Figure 2-3 Common Mode Does not Limit OPA206 in Inverting Gain Configuration

3 Practical Input Limitations

Section 2 demonstrated the common-mode limitations for different configurations by running a DC sweep. The simulation indicated that the common-mode range was violated by clamping the output signal. Furthermore, the limit always occurred at the minimum and maximum limit of the common-mode range. First, understand that the common-mode range given is the worst-case range, so when measuring actual devices, the performance is often better than the specification. Nevertheless, when designing a system, assume that the worst-case specification is a possibility due to process variation. Never base a system design on lab measurement of a few units. Always design considering the published minimum and maximum data sheet specifications.

The common-mode limitation also does not necessarily clamp the output as is generally shown in simulation models. A real-world device can introduce substantial distortion when the input range is violated rather than clamp the output. Furthermore, this effect can be dependent on other factors such as input frequency, or temperature. Figure 3-1 illustrates a case where the amplifier common-mode range limitation is dependent on frequency. The data sheet specification for this device provides the worst-case common-mode range across the entire bandwidth of the device. Lab measurements at low frequency indicate that the device has common-mode range much wider than the specification, whereas the measurements at higher frequency show substantial distortion where the signal exceeds the common-mode limit. Notice that even the higher frequency case does not clamp at the common-mode limitation, but rather introduces unacceptable distortion.

Figure 3-1 Measured Common-Mode Limitation on OPA140 versus Frequency

4 Input Phase Reversal (Inversion)

Normally when an op-amp input is driven beyond the common-mode range, the output becomes distorted or clipped. During the early years of op-amp semiconductor development some amplifiers exhibited a different phenomenon when the common-mode range was exceeded, called phase reversal. When the common-mode range is exceeded for a device with phase reversal, the output actually moves in the opposite of the expected direction. Figure 4-1 illustrates this issue on a buffer amplifier. Notice that when the input moves positively beyond the common-mode range the output actually moves negatively. This problem is related to a design oversight, and all modern amplifiers are designed and tested so that this issue is no longer a problem. Even legacy devices that once had phase reversal, have generally been revised to correct this issue. This section is included in the document to alleviate concerns related to legacy literature. Most modern data sheets include a line item indicating no phase reversal.

Figure 4-1 Phase Inversion (Reversal)