Today I was preparing a new tutorial book that studies the fundamentals of analog electronics using an experimental approach. After I finished the write up of a few  amplifier circuits, I intended to briefly cover the frequency response, with some circuits that the learners can actually experiment. 

My choices of Opamps in the learning kit include LM358 and TL072, two classic and popular Opamps that still gain popularity in education and hobbyists. Writing the theory part is easy, plus there are plenty of resources we can refer to. The main value our tutorial, of course, is the hands-on part, and as the writer of the book I definitely want to try out the experiments with our devices and instruments.

I started from the Gain Bandwidth Product (GBP), which equals the unity gain frequency of the amplifier. For this experiment, I simply built a voltage buffer and setup the circuit as shown. 

For better demonstration purposes, I used two Zoolarks, one serves as a function generator and the other one displays the waveform. And both LM358 and TL072 based voltage buffers are built on the same circuit, using Dual Supply mode. Each supply rail is powered by an independent MEGO power supply set to 12V. 

However, while the TL072 circuit is doing OK, I realized that LM358 presented a significantly lower 3dB bandwidth than anticipated in my experimental setup. As shown in the measurement results, at 100kHz, the output is no longer a sine wave, and the output amplitude quickly drops at 200kHz.

Well, I knew LM358 was featured as a single rail supply Opamp ideal for audio applications, which I did not expect excellent performance at higher frequencies. However, the result I got was still lower than expected. Based on their frequency response curve specified in datasheet, the GBP of both Opamps goes beyond 1MHz.

I was not able to figure out the reason right away, also I was preparing to drive home before the traffic got worse in peak rush hour, so I posted my question on Linkedin just to take a chance. Fortunately, I saw a smart reply from Jonny after I got home.

The answer is well written so I will quote directly:

“the TL072 large signal response graph. The TL072 is a relatively fast BIFET OpAmp, with dual supplies and a fast output stage. It can drive a 2kΩ load with a slew rate of 13V/μs.

That's a respectable slew rate.

This means you can drive a 40Vpp at 100kHz, or a 10Vpp at 200kHz, or 5Vpp at 1MHz. If you try to drive a 40Vpp at 1MHz, it will produce a 5Vpp triangular wave, limited by the 10V/μs slew rate.

Now the LM358 is a single-supply, low frequency, OpAmp, with an output stage that is incapable of driving the load with a high dV/dt. Its slew rate is < 500mV/μs. This is not in the data sheet, but there is a typical Large signal frequency response, and the voltage follower pulse response that shows it.”

I was like “ah-ha” once I saw the comment. While I appreciated Jonny’s awesome reply, I realized the value of hands-on experiential based learning, which brought me to challenges and unknowns yet interesting enough to motivate myself with an active thinking process. Yet math and theories are important, you can only gain the practical part of the learning by actually doing it. I also appreciate the value of Lab-On-The-Go, which allows me to conveniently setup the experiment before any sense of laziness takes hold.