Op Amp Gain Bandwidth NanoVNA

I have been thinking for a while about using high speed op amps as IF amps in a receiver. As mental as that might sound to some, it actually makes practical sense in someways. Gain is easy to set, impedance is easy to set, being that the IF is at a fixed frequency you are not worried about being broad banded and can tailor the circuit to suit by using a suitable op amp with sufficient bandwidth to do the job.

And that is where the problem lies, op amp gain bandwidth is given at unity gain, IE a gain of 1 and as soon as you start adding gain, you start losing bandwidth. This means that you need a unity bandwidth of Gain x MHz to be somewhere close and then you also need a op amp with a fast enough slew rate to deliver the waveform amplitude you desire.

Now there are what are called current controlled op amps that give much better gain bandwidths above unity, but they are kind of expensive and so that leaves using voltage controlled op amps and working around all its limitations, but as you start to get up there in unity gains above 300 MHz even they start becoming non cheap items also.

So a few weeks ago I was on one of the Chinese parts sellers just looking at all the different crap they have and for some reason I ended up in the op amp section and found an op amp with a few hundred MHz unity gain bandwidth for pretty cheap. And by cheap, i am talking in the 40c each kind of space. So i bought a few to try out.

So i built up the non inverting circuit as shown above. Which is quite simple to set the gain and the impedance’s just by changing the value of a few resistors.

This is the circuit built on the test board. While the op amp is an SMD part, its SOIC 8 so its big enough that even a dummy with coke bottle glasses could hand solder, but I am kind of slack in that regard so I used paste and hot air, i mean why not. LOL

You can see from the S21 gain plot that there is usable gain from 40m to 10m. I am not sure what that notch is, but i suspect that its an artifact from the nanovna, because a manual sweep of that section of spectrum using a function generator and oscilloscope did not show that dip.

Oh I should say that I have the gain set to 6x for this test. And slew rate was not an issue for 7MHz to 30MHz, with the op amp able to deliver 1.3v peak to peak quite happily. Below that, particularly around 80m, the op amp could not deliver much more than 500mV peak to peak. So for a 9MHz or 12MHz IF amplifier, the op amp might be a credible option.