Hi,

Two questions here:

A) I was thinking **where the poles of an Op amp IC are likely to be**. Example I check the datasheet of an Op amp AD713, and a straight line is shown for the frequency range 30Hz to 10MHz (page 7 of datasheet, Rev F, 2011). Is it proof enough to assume that first pole < 30Hz and second pole > 10MHz?

B) I found AD713 **used as an unity gain amplifier in a design, and was checking why it would have been used**. Do these reasons hold good, and what else could have determined the choice:

i) That it has a good gain margin (gain approx ~15dB below zero dB when phase is -180 Deg)

ii) Appears to have a very huge phase margin (phase is 50 degrees when gain is 0 db, which make phase margin as 50 degrees difference -180 degrees, ie 230 Degrees)

Are my points i) and ii) above correct? **Any more reason that AD713 would have been used.** Frequency of operation is less than 1kHz.

Regards

-Sathyadev

Hi Sathyadev,

The frequency plot will start to roll-off at a rate of 20dB/decade once it hits a pole. So for the same graph that you indicate, it does look like there's a pole that occurs at a frequency before 50Hz. Outside of this graph, there is not much more we can conclude regarding pole locations.

I would say that your assumptions are correct. In general, a circuit is considered stable when the phase margin is greater than 35 degrees, so for this case where the phase margin is about 50 degrees, as you say, this is stable.

We also have a rule of thumb to use an amplifier with 10 times the bandwidth of your circuit's frequency requirement to ensure stability. Given the frequency of operation of 1kHz, the 4Mhz of the AD713 is more than enough.

The choice of amplifier is really determined by the application. It can be determined by the input source and what is expected at the output, basically what the application requirements are. The AD713 has relatively good DC characteristics, fast settling time and relatively high slew rate, though, and these may have been factors in the choice of this amplifier.

Thanks!

Kris