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If the gain bandwidth product of an operational amplifier is a constant and |Vout/Vin| is also a constant does that mean that the operational amplifier will not function for all but 1 frequency? Please explain.

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    \$\begingroup\$ Who says the gain is constant. Look at just about any op-amp datasheet and you'll find a graph of typical gain (varying) vs frequency. \$\endgroup\$ – The Photon Sep 16 '15 at 14:31
  • \$\begingroup\$ I did, which is why I am confused. If |Vout/Vin| is calculated to be 1, isn't gain a constant for an ideal op amp? \$\endgroup\$ – user3897744 Sep 16 '15 at 14:33
  • \$\begingroup\$ GBW is about open loop gain, not closed loop. \$\endgroup\$ – The Photon Sep 16 '15 at 14:37
  • \$\begingroup\$ ok that makes sense, thanks. But is GBW useful for closed loop gain? Does it have any significance when you are working with an amp in a closed loop? \$\endgroup\$ – user3897744 Sep 16 '15 at 14:42
  • \$\begingroup\$ Yes, an opamp with a GBW product of 1 GHz will give you a 1 GHz BW if configured as a 1x amplifier. But 0.1 GHz BW when configured as a 10x amplifier. And 10 MHz at 100x gain. You see: Gain x BW stays the same. \$\endgroup\$ – Bimpelrekkie Sep 16 '15 at 14:58
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Open loop Vout / Vin isn't constant - it rolls off with frequency and obeys the gain-bandwidth product approximation: -

enter image description here

What you may be being confused with is the DC open loop gain and, from the diagram above that would be quoted in a data sheet as 1 million.

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  • \$\begingroup\$ Lets say ina closed loop circuit with op amp that Vout/Vin = R3/R1 both are resistors of some value. Does gain bandwidth product hold here or is that just for open loop? \$\endgroup\$ – user3897744 Sep 16 '15 at 14:39
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    \$\begingroup\$ If you choose a resistor ratio that gives you say a gain of 10 then you will find that the gain is ten up to a certain frequency then it slopes off back to unity. In the graph above, at about 1MHz the op-amp circuit would be down 3dB at 1MHz and this marks the real end-stop of performance for that design. Lower than 1MHz the gain will be much closer to ten i.e. at 100kHz it will be a tiny fraction of a dB down and at 10kHz you'd not be able to measure the error. \$\endgroup\$ – Andy aka Sep 16 '15 at 14:45

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