Difference between gain bandwidth product and unity gain frequency

[Junus2012]

Dear friends,

if the opamp AC performance perfectly compensated it will behave like single pole low pass filter, no doupt.
I have read that in this case, the Gain bandwidth product and the unity gain frequency (UGB) will be the same.

My question is: how they can be the same when the GBW is extracted from gain when it drops by 3 dB while the UGB is extracted at the gain is equal to unity?

Thank you

 

[arthorios]

My question is: how they can be the same when the GBW is extracted from gain when it drops by 3 dB while the UGB is extracted at the gain is equal to unity?

As you said, GBW=UGB. If you have an OTA in unity gain, its UGB will be equal to the GBW. For single pole response and a closed-loop gain below the OTA DC gain, the GBW = (CL-gain x f-3dB) will be constant. These terms are usually differentiated because, in practical cases, you might not have a single-pole response. If you want to know where does the GBW reasoning comes from, get the transfer function of a single-pole amplifier and solve for a gain of 1.

 

[nitishn5]

For a single pole OpAmp, with, lets say,
A_dc = 1000 = 60dB
f_-3dB = 1Hz
.
GBW = 1000 x 1Hz = 1000 Hz
.
With the 20dB/decade Gain Roll-off
UGB = f_0dB = 60/20 decades = 1000 Hz.
.
Hence GBW = UGB.
.
I think you are trying to calculate it by the gain at the f_-3dB which in this case would be 57dB.
The above calculation as well as all standard calculations are done with "DC" gain considered and the plots are assumed to be asymptotic.