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What will happen if I use an op amp as a buffer amplifier for signals with frequency greater than Gain Bandwidth Product. More specifically:
1) Say I have a signal with 2 frequencies: a) 2 MHz and b) 20 MHz. Say the Gain Bandwidth product is 5 MHz. Will the 20 MHz frequency be severely attenuated and 2 MHz signal pass as it is?
2) If that is the case, what can prevent me from using the buffer as a Low pass filter, to pass 2 MHz signal only?
No, this is not a good idea.
One reason is that only the minimum gain-bandwidth product is specified. You can count on it being at least that, but it could be, and probably is in any one part, somewhat higher. The net result is that frequencies above the gain-bandwidth product will have unpredictable attenuation.
Another issue is that while gain-bandwidth is a useful spec, it is only a very simplified model of what the opamp does. There are other issues, like slew rate and the difference between large signal and small signal responses.
Yet another issue is that the opamp won't magically pass all frequencies below the gain-bandwidth product. Generally you want to stay 10x below it to be able to ignore it. The 2 MHz signal is only 2.5x below the gain-bandwidth product, meaning you can only count on a gain of 2.5 at that frequency. That means the simplifying assumptions of a typical feedback system are being violated. Those are based on the gain being infinite, or at least "large" (again, 10x is a common margin) compared to the closed loop gain.
If you want to pass 2 MHz and attenuate 20 MHz, get a amp that can pass the 2 MHz properly, then add a deliberate filter to attenuate the 20 MHz.
But wait, there's more. As the input frequency goes up, the opamp no longer works like a opamp at all. Feeding it signals much above the unity gain frequency can cause side effects, like intermodulation distortion, rectification, or other nasty non-linear phenomena you can't predict. So get a opamp that can properly buffer up to 2 MHz, then add a low pass filter before the opamp to attenuate the unwanted higher frequencies.
The answer depends on the actual circuit - that means: Do you operate the buffer really as a buffer only or is the buffer part of the low pass feedback newtwork (as for Sallen-key filters)? In any case, the 2 MHz signal will NOT pass "as it is". Of course, it will be damped by the buffer´s frequency-dependent gain. Show us the real circuit - and it will be possible to answer in more detail.
Very interesting idea. I see no reason why it wouldn't work, but I have a few concerns, and it may not work well.
The first is that your "corner" frequency would be fully determined by your gain. Thus, in your "pass band", the gain may be too big or too small.
The nature of the filter produced would not be "optimal" in any sense, and might not drop very fast. Your attenuation at the high frequency target might not be big enough to suit your purposes.
I'd also be concerned about phase issues and harmonic distortion.
So, if you want to try it, I recommend looking over the relevant figures in the datasheet, see if your paper pass looks acceptable, then try it and see if you like the results.
Personally, I would lean toward not doing it, just because of the time needed to verify its doing what I want it to do, and the issues surrounding the relationship between gain and cutoff frequency. Your results would be more predictable by shooting for a GBW product sufficient to pass your signals and adding the appropriate capacitor for a one-pole LPF.
Of course, if the issue of a cheaper op amp or lower part count were paramount, that would push me to give it a try.
