Simulating Gain-Bandwidth—the generic op amp model

RE: Simulating Gain-Bandwidth—the generic op amp model

Bruce Trump — Mon, 11 Mar 2013 00:28:28 GMT

Thanks, LLC. I fixed the link and it now works. -- Bruce

RE: Simulating Gain-Bandwidth—the generic op amp model

HaveEE_WillTravel — Sun, 10 Mar 2013 21:08:23 GMT

The link to TINA is wrong. It should be <www.ti.com/.../tina-ti>

RE: Simulating Gain-Bandwidth—the generic op amp model

Tim Sobering — Sun, 10 Mar 2013 17:27:48 GMT

Thanks Bruce. I got it backwards. Not enough coffee yet this morning apparently. I'm recalling Bob Pease's discussion of DC gain now. As you note, active filters can really exacerbate these problems. While you are changing the f_t frequency in your simulation, the effect on the active filter is due to the reduction in the "headroom" between the filter gain and the open loop gain curve at the filter cutoff frequency, i.e. the remaining loop gain in the system. Clearly headroom and GBW and f_t are all interrelated, but if your filter cutoff was at 50kHz instead of 1MHz, you wouldn't see much change in the filter performance over the 20x change in GBW, just as variations in DC gain have little impact on low-frequency performance. I guess maybe I'm nit-picking, but I'm kind of obsessed with looking at loop gain (and phase!).

RE: Simulating Gain-Bandwidth—the generic op amp model

Bruce Trump — Sun, 10 Mar 2013 15:23:57 GMT

Tim-- The GBW of a given op amp type is typically well-controlled with variation in the range of +/-15% or so. The DC open-loop gain is not so well-controlled and can vary over quite a wide range. This means that the first pole frequency actually varies a lot. Don't panic--this rarely affects circuits. What matters is the GBW. Simulating with a fixed DC open-loop gain and variable first pole is just an easy way to fiddle with the GBW. Sorry I can't draw a picture in this comment. Does this need a blog to explain? -- Bruce

RE: Simulating Gain-Bandwidth—the generic op amp model

Tim Sobering — Sun, 10 Mar 2013 14:56:53 GMT

Bruce,

I've been told that IC manufacturers control the DC open loop gain quite well, which means the frequency of the dominant pole and the unity gain crossover frequency can (and must) vary (as you show in figure 2). Is that correct, or is more attention paid to f_t? What about the second pole...does it move in relation?

Tim