Tale of two op amps

Dipping my toe into solid state electronics after all these years of working with tubes… I ordered a few op amps and a non-inverting amp circuit board with a socket which can accomodate various 8 pin dual op amps. The circuit board is set for a voltage gain of something like 5/6 times the input voltage. I have the traditional +-15 volts powering it. One characteristic of an op amp is how fast it can raise the output voltage based on changes in the input voltage. This is called the slew rate and is specified in volts per microsecond. I placed a couple of dual op amps in this circuit and put the output on my scope.

The first is an NE5532–a very traditional ‘audiophile’ op amp in the 1990s–this may or may not be the genuine article, as in this circuit, its slew rate is far below its spec. The second op amp is a far higher performing OPA2134A. The 5532 has a bipolar transistor input while the 2134 has a FET input, so its not automatically a suitable plug in upgrade for circuits containing the 5532. I note the 2134 is at least 10 times more expensive than the 5532–as befitting its higher performance.

When working with these, I ran a 13khz square wave in. At this frequency, the slew rate limitations of the 5532 start to show. I was surprised at how dramatic the difference was, as the 2134 did not appear to exhibit any slew rate limitations at this frequency, only exhibiting a tiny ringing on the leading edge. Each square on the screen represents 10 microseconds. The output voltage here is 16 volts peak-to-peak.

A nod to Siglent for this wonderful SDS1104-XE scope that has the ability to save screenshots to a usb drive, making these kinds of notes very easy to prepare.

13khz Square wave from a NE5532
13khz square wave output from the OPA2134A

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