TL072 weird output when used as buffer

Question

Dear all, I tried to use TL072CP op amp as a buffer, however to my surprise it outputs a perfect triangular wave with reduced amplitude when given sine wave input of 150kHz and I have used a 12V and -12V rail, + & - rail supply to my op-amp. I simulated the same chip in LT Spice and it the output should be the same as input waveform with low impedance. This is very weird and I hope someone can point out to me why it does not output a sine wave same as the input waveform.

All circuits are connected properly and if you can see there is a small strip of short wire connecting pin 1 to 2

Thank you for reading and have a nice day !!

Answer 1

The first thing would be adding bypass caps next to the IC.

One problem is breadboards have many parasitics. There's parasitic inductance in the wires that slows down the current from the supply. If the load (tl027) changes and needs more current the jumper wires will restrict that current with their inductance. Also make sure the wires that go back to the supply are "big", the bigger the wire, the lower Inductance the wire will have. If you have any wires that are 28 gauge or lower that may be a problem.

Another problem with breadboards is the parasitic capacitance between rows, this can amount to a few picofarads between rows and create crosstalk.

Last problem is contact resistance, some bread boards and wires may not make great contacts so make sure the wires and the breadboard are making good contact.

Answer 2

Well, the key point is that your output has 1.19V peak-to-peak, a far cry from the 12.6Vpp on the input. This just means that your circuit is far too slow to follow the input signal. It is not a matter of too low supply voltage or the output signal would not have sharp triangular corners but a more clipped look.

Now the TL072 has a specified output slew rate of 13V/µs which is far more than we need for the signal: a 145kHz signal is roughly 1Mradian/sec and with an amplitude of 6.3V, we need to support a slew rate of 6V/µs which is half of what we should be able to do while the output is (squinting) about 0.3V/µs.

Suspect #1 would be your supply lines of ±12V. As said elsewhere, you need a bypass capacitor across the supply lines that will support surge demands of current without having to go through long connections.

Apropos long connections: you write

you can see there is a small strip of short wire connecting pin 1 to 2

but frankly, there is no "short wire" anywhere on the photograph: not a single wire is short enough to fit wholly on the photograph. With comparatively high frequencies, that is also asking for trouble.

Then the output current of the TL072 is 10mA. That means that to support a slew rate of 6V/µs, the load capacity must not exceed 1.6nF, and that leaves us with no extra current for the ohmic part of the load.

What kind of load capacity/resistance has your probe? Looks like a 50ohm cable...

The manual of your oscilloscope has some calibration instructions akin to

Check the shape of the display waveform, and if necessary, adjust the variable capacitance on the probe with a non-metallic screwdriver until the waveform displayed on the screen is “properly compensated” as shown in the figure below.

Did you properly "adjust the variable capacitance"? Because that could be the elephant in the room...

Answer 3

Here is another possibility: the TL072 contains two opamps and you leave all pins of the second opamp unconnected. That may cause it to oscillate or otherwise draw current that the first opamp needs.

Try taking the second opamp out of the equation by grounding its + input and connecting its output to its - input.

And then a blocking capacitor (something like 1uF low ESR) across the supply pins of the TL072 and without long wires might help making the current available it needs for changing its output voltage fast enough against parasitic/load capacitances.