Silicon Junction

Ultra-cheap components on eBay, AliExpress, and Amazon are a constant temptation, and—like many of you—I couldn’t resist. I ended up buying a batch of fifty LM358 operational amplifiers for just $2 USD from eBay.

The LM358 is a popular low-power dual operational amplifier designed to operate well from a single power supply, making it ideal for battery-powered and low-voltage electronic circuits. It can work with a wide supply voltage range, typically from 3V to 32V for single-supply operation, and its input common-mode voltage range includes ground.

With any eBay purchase, the first question is unavoidable: are these parts fake? (Answer: probably.)

Rather than stopping there, I figured this was a good opportunity to see whether these op-amps actually resemble the manufacturer’s specifications in any meaningful way.

Our goals are:

• Make sense of the key parameters in an op-amp datasheet
• Determine whether the eBay components reasonably match those specifications

Methodology

Most of the op-amp’s key parameters can be tested with this simple relaxation oscillator circuit:

Using this circuit, we can measure the op-amps’:

• Output swing
• Low-level output voltage
• Slew rate

Let’s examine each of these in detail and compare the measurements from this circuit with the values specified in the datasheet.

Output Swing

The output swing is the range of voltages that an op-amp can produce at its output. For example, if an op-amp can output between 2 V and 10 V, the output swing is 8 V.

The LM358 datasheet specifies the expected output swing when the supply voltage is 15 V. It also shows that the output swing gradually decreases as the signal frequency increases:

To measure the output swing, we look at the highest and lowest voltages produced by the oscillator circuit and compare them with the datasheet values.

The oscillator produced a minimum voltage of –7.07 V and a maximum of 5.59 V, giving an output range of 12.7 V. This is very close to the 12.5 V specified in the datasheet.

Low Level Output Voltage

The low-level output voltage indicates how close an amplifier’s output can get to the negative supply rail. For example, if the negative supply is 0 V and the amplifier’s lowest output is 2 V, the low-level output voltage is 2 V.

A key feature of the LM358 is that its output can get very close to the negative supply voltage. How close it gets depends on the amount of current flowing into the amplifier.

In our circuit, during the low phase of the square wave, about 4 V peak is discharged from the capacitor through a 1 kΩ resistor, meaning the LM358 is sinking roughly 4 mA.

According to the datasheet graph, when sinking 4 mA, we would expect the output to be around 0.4–0.5 V above the negative rail. In our setup, this predicts a lowest output voltage of -7.1 V.

Let’s see what we get in practice:

The actual measurement was –7.07 V, which is very close to our predicted –7.1 V based on the datasheet. Another green tick for this chip.

Slew Rate

The slew rate of an amplifier is how quickly its output voltage can change—in other words, the steepest slope the output can achieve.

The LM358 has a relatively slow slew rate of 0.3V/us making it easy to measure with our setup.

To get a clearer view of the slope, we increase the horizontal resolution and zoom in on the rising edge of the oscillator signal:

From the oscilloscope, we can see that it took just over 3 divisions of 10 µs each to traverse the peak-to-peak voltage of 12.5 V.

This gives a calculated slew rate of:

This is slightly faster than our expected 0.3V/us but I think it’s close enough that we can give this a pass.

Summary

It appears that the ultra-cheap LM358s I bought from eBay—50 for just $2—could actually be legitimate. While this testing isn’t comprehensive, it does confirm that several key parameters align with the LM358 specifications.

In this case, I got lucky—there are certainly many people who receive fake components from these unofficial distributors.