## Re: [softrock40] OpAmp question on GBW

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• ... You can t exceed the GBW, it s a physical limitation of the op amp being considered. In fact, as you approach the unity gain frequency, the op amp is no
Message 1 of 10 , Jul 1, 2007
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At 12:49 PM 7/2/2007, you wrote:

>Thanks for the response Bill. I was just wondering what happens if
>you exceed the GBW ....say you have a frequency range into the OpAmp
>and a gain which when multiplied by the frequency range is the GBW.
>If you further increase the gain by changing Rfb what happens ....is
>the frequency bandwidth automatically reduced or will the OpAmp not
>respond properly to Rfb/Rg. Or is it just a distorted waveform over
>the frequency range ?

You can't exceed the GBW, it's a physical limitation of the op amp
being considered. In fact, as you approach the unity gain frequency,
the op amp is no longer acting as an op amp by definition, since one
part of the definition includes having very high open loop gain, and
the familiar formulas no longer apply (you have to take the lesser
gain of the device into consideration).

73 de VK3JED
• ... You ll get less gain and/or bandwidth than you hoped for, for starters. If there are any non linearities in the op amp at high frequencies, these will
Message 2 of 10 , Jul 1, 2007
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At 01:03 PM 7/2/2007, you wrote:

>I should say "try to exceed" the GBW.........

You'll get less gain and/or bandwidth than you hoped for, for
starters. If there are any non linearities in the op amp at high
frequencies, these will also become apparent, because there is less
negative feedback available (due to the low open loop gain).

If you build an amplifier with 10x gain (at low frequencies) and
attempt to run it at the unity gain frequency, then the actual gain
will be around unity (may be more or less, because there is also like
to be a phase shift in the output, which affects the feedback
loop). It certainly won't be 10.

73 de VK3JED
• ... The frequency response of the amplifier is affected by the gain. As you increase the gain (by changing the feedback resistors), you lower the frequency of
Message 3 of 10 , Jul 1, 2007
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At 01:02 PM 7/2/2007, you wrote:

>Thanks for the response, Milt. I think that's basically what I was
>saying...... The GBW is the max frequency for a gain of 1, you can
>also max out at that frequency/100 if your gain is 100, or at that
>frequency/200 if your gain is 200. If you are at the max, further
>trying to increase the gain by changing Rfb/Rg does nothing at that
>frequency ? or does it start automatically trading off frequency vs
>amplification (loose the Rfb/Rg ratio formula for gain) to keep the
>GBW which is constant. Or does it just start distorting the
>amplified waveform.

The frequency response of the amplifier is affected by the gain. As
you increase the gain (by changing the feedback resistors), you lower
the frequency of the corner point in the frequency response, so yes
you could say (in simplistic terms) you are trading gain for
bandwidth and vice-versa. If you need a higher gain bandwidth
product, you have to either choose a device with a higher gain
bandwidth product, or use multiple lower gain stages in cascade (e.g.
2 x 10dB vs 1 x 20dB should give you a little over 3x the bandwidth
at 20dB gain, all other things being equal - 10dB is a voltage ratio
of 3.16 roughly).

73 de VK3JED
• Thanks Tom, and the other guys. I understand, or at least have a much better idea of what happens. 73 Kees K5BCQ -- Tony Langdon wrote: ...
Message 4 of 10 , Jul 2, 2007
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Thanks Tom, and the other guys.

I understand, or at least have a much better idea of what happens.

73 Kees K5BCQ

-- Tony Langdon <vk3jed@...> wrote:

At 01:02 PM 7/2/2007, you wrote:

>Thanks for the response, Milt. I think that's basically what I was
>saying..... . The GBW is the max frequency for a gain of 1, you can
>also max out at that frequency/100 if your gain is 100, or at that
>frequency/200 if your gain is 200. If you are at the max, further
>trying to increase the gain by changing Rfb/Rg does nothing at that
>frequency ? or does it start automatically trading off frequency vs
>amplification (loose the Rfb/Rg ratio formula for gain) to keep the
>GBW which is constant. Or does it just start distorting the
>amplified waveform.

The frequency response of the amplifier is affected by the gain. As
you increase the gain (by changing the feedback resistors), you lower
the frequency of the corner point in the frequency response, so yes
you could say (in simplistic terms) you are trading gain for
bandwidth and vice-versa. If you need a higher gain bandwidth
product, you have to either choose a device with a higher gain
bandwidth product, or use multiple lower gain stages in cascade (e.g.
2 x 10dB vs 1 x 20dB should give you a little over 3x the bandwidth
at 20dB gain, all other things being equal - 10dB is a voltage ratio
of 3.16 roughly).

73 de VK3JED