Continuing on from part 1. We are still testing whether cheap op-amps from eBay match the specifications expected for an LM358. The goal is twofold: to determine whether the parts are likely genuine and to gain a deeper understanding of how an op-amp datasheet relates to real-world behavior.
In Part 1, we examined the op-amp’s output range (output swing) and slew rate.
In Part 2, we will explore what happens to the output as we vary the input across the full supply voltage. This will allow us to investigate the op-amp’s common-mode input range.
We will also test whether the op-amp remains stable in a unity-gain configuration and observe any behavior when the inputs exceed the specified limits for an LM358.
Methodology
We configure the LM358 as a unity-gain buffer and use a potentiometer to sweep the input voltage across the entire supply range.
The oscilloscope is connected to both the input and the output to observe the op-amp’s behavior:
Expected Results
From the LM358 datasheet, the common-mode input range extends from 0 V to VCC−1.5 V.
For our 15 V supply, we therefore expect the input and output voltages to track each other linearly from 0 V up to 13.5 V (15 V − 1.5 V).
The datasheet also specifies that the LM358 is unity-gain stable, so we do not expect any oscillations when it is configured as a non-inverting amplifier with a gain of 1.
Additionally, the LM358 is free from phase inversion errors. This means that even if the input exceeds the common-mode range, the output should not suddenly flip to the opposite voltage.
Fortunately, this simple buffer circuit allows us to test both of these characteristics directly.
Results
In all of these traces the green trace is the input, and the yellow is the output.
In all of the traces, the green line represents the input, and the yellow line represents the output.
At 6.5 V, the green input trace is completely covered by the yellow output trace, which is exactly what we expect for a unity-gain configuration.
We can also immediately see that the signal is stable, confirming that this op-amp is indeed unity-gain stable—there are no unwanted oscillations.
Next, we wind the potentiometer down to 0 V. If this is a genuine LM358, the op-amp should handle this without issue. The results match expectations perfectly: the LM358 operates normally with input and output down to 0 V.
As we increase the input voltage, the output begins to deviate once the common-mode input range is exceeded. The deviation occurs precisely at 13.5 V—1.5 V below —exactly as predicted.
We also notice that when the input exceeds the range, the output simply clamps at the maximum value rather than flipping—confirming the absence of phase inversion, as specified in the datasheet.
Here is a short video demonstrating the input voltage being swept from 0 V to 15 V:
Here is our experimental setup on hobbyist breadboard.
Just like in Part 1, these inexpensive LM358s from eBay are performing as expected.
Result: Pass ✅
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