For GBP datasheet parameters of Op-amps, I have seen datasheets with test conditions like gain and Rl, but there are also datasheets with ftest as their conditions. How do we calculate the GBP of the product by using the ftest? Is 'ftest' the cutoff frequency where the gain drops -3db?
Like this op amp, conditions for GBW contains ftest: http://www.farnell.com/datasheets/1830512.pdf 'GBW Gain-Bandwidth Product fTEST = 20kHz, RL = 10k'
GBP (Gain Bandwidth Product) is calculated from the slope of gain vs frequency. If the actual curve is not a straight line at -20dB/decade then a test frequency may be specified to provide best fit. In the case of the LTC6091...
...above ~80kHz the curve steepens, so choosing a higher frequency would produce inaccurate results. Also the maximum output swing decreases at higher frequencies. The LTC6091 can produce 140V at 20kHz, but almost nothing at 2MHz - so 2MHz would not be a good test frequency! Finally, (not shown on this graph) all op amps have a low frequency corner below which the gain flattens off, so measuring gain at a very low frequency would not be accurate either.
BTW having a GBP of eg. 12MHz does not necessarily mean that you can use it as a unity gain buffer at that frequency. In this case the deviation from ideal slope causes unity gain to occur at ~2MHz.
