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If an op-amp output suddenly jumps toward the opposite supply rail, check the input common-mode voltage first. Some op amps phase-reverse when their input common-mode voltage exceeds the range specified in the datasheet; the fault can happen even while both inputs remain between the supply rails. It is most likely in a unity-gain voltage follower, but the behavior is not limited to one input-transistor type.
What op-amp phase reversal means
Phase reversal—also called phase inversion—is an abnormal output response to an out-of-range input common-mode voltage. Instead of continuing to follow the input as expected, the output moves toward the opposite supply rail. Bill Klein described the condition in a 2005 EE Times article as occurring when the applied common-mode voltage approaches or goes beyond the device’s specified range: “The phase reversal story”.
Analog Devices explains that an input stage can lose sufficient bias when the common-mode limit is exceeded, sending the output toward the opposite rail until the input returns within range. Klein describes the out-of-range input stage interacting with the second stage in a way that appears as if the inputs were reversed; input bias current can also rise substantially. The details depend on the op amp’s design, so the output’s direction or recovery behavior should not be assumed to be identical across devices. See Analog Devices’ MT-036, Op Amp Output Phase-Reversal and Input Over-Voltage Protection.
Why it happens near a supply rail
The key limit is the input common-mode range in the specific op amp’s datasheet, not simply whether the input voltage is between the positive and negative supply rails. A voltage may be inside the supply rails and still be outside the range in which the input stage operates correctly. Conversely, a “rail-to-rail input” description does not permit voltages beyond the rails or override the device’s absolute-maximum ratings.
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This is why the problem can appear when a signal approaches a supply rail: the amplifier may no longer have the input-stage headroom needed at that common-mode voltage. Check the specified range under the actual supply voltage and operating conditions, as well as the absolute-maximum input limits. Analog Devices MT-036 gives +VS + 0.3 V and −VS − 0.3 V as common rule-of-thumb absolute input limits, not universal ratings; use the particular part’s datasheet instead.
Which circuits and op amps are more susceptible?
Unity-gain voltage followers
Phase reversal is especially likely when an op amp is configured as a unity-gain follower. In this circuit, the output is connected directly to the inverting input, so an unexpected output excursion can produce a conspicuous failure: the output may stop following the signal and move toward the opposite rail.
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Input-stage types
Analog Devices associates phase reversal most often with JFET and BiFET amplifiers, while noting that some bipolar single-supply amplifiers can also exhibit it. Klein’s 2005 article characterizes bipolar-input devices as high risk, JFET devices as close behind, and CMOS devices as the most immune. These are broad tendencies, not a substitute for checking a specific part: not every op amp phase-reverses, and input technology alone does not establish whether a particular device is safe for an application.
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- Check the input ratings. In the op amp’s datasheet, find the input common-mode range and absolute-maximum input voltage. Keep the signal within the operating range and do not treat the absolute maximum as a normal operating target.
- Account for fault excursions. If startup, shutdown, external signals, or faults can drive an input outside its normal range, determine the maximum voltage and duration the input could see.
- Add current limiting and clamps if needed. Analog Devices MT-036 illustrates Schottky diodes from the input toward the positive and negative supplies, with a series resistor labelled RLIMIT. It identifies a 1 kΩ resistor as a starting point for the example—not a universal value.
- Calculate the resistor for the actual fault. Choose a value that limits worst-case current through the input protection path. MT-036 advises limiting op-amp input fault current to ≤5 mA unless the datasheet specifies otherwise. The required resistance depends on the fault voltage, supply rails, clamp voltage, and the device’s permitted current.
- Check what protection changes. Clamp leakage and capacitance, along with resistor noise and voltage drop, can affect offset, bandwidth, and accuracy. Verify that the protection components and their current ratings suit the circuit.
- Recheck all limits. Confirm the protected input stays within the replacement or original op amp’s absolute-maximum ratings under worst-case conditions.
Choosing a replacement op amp
When replacing a part that phase-reverses in the application, compare candidate datasheets on the specifications that control both the failure and the circuit’s normal performance.
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| What to compare | Why it matters |
|---|---|
| Input common-mode range | It establishes the input voltages at which normal operation is specified for the stated conditions. |
| Rail-to-rail input coverage | It describes how close the common-mode range reaches to the supply rails; it does not authorize operation beyond them. |
| Explicit phase-reversal behavior | A datasheet statement that the device does not phase-reverse is more useful than inferring behavior from the input-transistor type. |
| Input protection and allowable fault current | These determine whether the part can tolerate the application’s fault conditions and what external protection may be needed. |
| Bias current, leakage, and input capacitance | These affect source loading, accuracy, and behavior with protection components. |
| Noise, offset, and bandwidth | A device that avoids phase reversal must still meet the circuit’s signal-performance requirements. |
A replacement described as rail-to-rail input or as having no phase reversal may be a better fit, but verify the exact common-mode and absolute-maximum limits in its current datasheet. Do not select a part on a feature label alone.
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- NOTE:Exposure to absolute maximum rating conditions for extended periods may affect device reliability. We do not provide technical support, please familiarize yourself with the parameters and performance of the purchased products in advance. Sincerely apologize for you.
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