Op Amp: Gain Bandwidth Product and LP Filter

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?

• GBP is not directly related to frequency i.e. it is incorrect when you say "with frequency greater than Gain Bandwidth Product" - it's like asking if you can have 3A when the power is 3 watts - it all depends on voltage of course. Jul 9, 2014 at 12:36

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.

• That's how I was originally thinking, but what if the parameters were a bit blurrier -- say a tiny 200 Hz signal of interest that could accommodate a gain of about 100 and 25 MHz noise source? Jul 9, 2014 at 13:01
• This opamp can certainly pass 200 Hz with significant gain. That's 25k below the gain-bandwidth product, so it could do up to 2500 or so gain before you have to consider the opamp bandwidth limitation more carefully. 2500 gain could be too much for other reasons, but that's a different discussion. In you're example, a simple R-C filter at 1 kHz would leave the 200 Hz alone but attenuate 25 MHz by 2500. Do it before the opamp to keep frequencies away from it that it might not react nicely to. You could get various non-linear effects from frequencies significantly above the gain-bandwidth. Jul 9, 2014 at 13:08
• Well, I did simulate the above scheme. Barring the fluctuations in frequency ( + - 5 KHz) compared to LC ladder filter, its ok. And it does not have weird component values as in LC filter. But I am not sure whether it would work if I actually went ahead and physically built that. And besides, such fluctuations are really unwanted. Jul 9, 2014 at 13:13
• @Pluto: I doubt the simulator models the nasty non-linear things the opamp could do with frequencies a few times higher than its unity gain frequency. Jul 9, 2014 at 13:15
• @Pluto: OpAmp macromodels have the typical values from the datasheet, and for that reason simulations must be used with care: they are an indication of circuit performance, not a guarantee. In addition, the model may have some default for frequencies well beyond the GBW which may well be completely different to the real response you might see. Dec 17, 2015 at 13:25

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.

• Actually, the output is from a frequency mixer; the two frequencies are 500 KHz and ~29 MHz. I plan to use a TL072 with GBP 3 MHz. My point is, if I only need 500 KHz frequency, can't I simply use a buffer to filter out the 29 MHz ? Why construct an LC filter then ? Jul 9, 2014 at 11:21
• Depending on the modulation you might need a better filter than a lousy LP made from a buffer. Again, a little more informations might come in handy. Jul 9, 2014 at 11:31
• More information like ? I am building a superhet receiver for SSB. I have used a ring diode mixer and frequencies it produces are 500 KHz( IF) and 29 MHz( unwanted freq). What else ? Jul 9, 2014 at 11:54
• No filter can "filter out" a signal at 29 MHz. All you can do is to suppress such an unwanted frequency. Hence, it is up to you to require a certain damping. This damping amount determines the filter order you need (first, second, ...). Question: Is 500kHz the maximum frequency that should pass the filter? (Because earlier you mentioned 2 MHz) ?
– LvW
Jul 9, 2014 at 12:55
• Yes. 2 MHz was supposed to be an example. 500 KHz and 29 MHz are actual frequencies involved. The op amp is with GBP 3 MHz. Jul 9, 2014 at 14:55

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.