On Apr 29, 2010, at 6:50 AM, Russell Shaw wrote: > Gene Heskett wrote: >> On Wednesday 28 April 2010, Dave McGuire wrote: >>> On Apr 28, 2010, at 3:40 PM, John Doty wrote: >>>> Well, you started out complaining about a 741 model. I'd call that >>>> a very rare, obsolete part: I haven't actually seen one in a >>>> circuit in over 30 years. I guess it's still in textbooks (read >>>> Stephen J. Gould's rants about textbook authors' tendency to copy >>>> from previous textbooks sometime), but why would anyone use it in a >>>> new design? >>> Very rare?! I see 741s everywhere. WTF? >>> >>> -Dave >>> >> Sorry to bust the bubble, but he's right. The 741 is well over 40 years >> old, and its open loop first response pole, where the 6db per octave rolloff >> begins, is a measly 10 hertz. > > The opamp is 1MHz unity BW. The higher the gain, the lower the first pole. > An even better opamp would roll off at 1Hz. > >> Today there are $1.00 opamps with a working gain of 20 when feedback is >> applied, with output slew rates of several thousand volts per second. Thats >> working bandwidth to several hundred megahertz at the sort of levels found >> in either a modern broadcast audio mixer, or a production video switcher, >> and either of those are driving 60 ohms for audio, or 75 for video. > > Those are video buffers. They have much less closed-loop gain and inferior > offset voltages. They're also noisy and are very prone to oscillation with > any stray capacitance or with certain feedback resistors. > >> Slew rate limits alone in the 741 means you can't honestly ask it for more >> than a volt of output at full audio bandwidth. > > dV/dt = 2.pi.Vm > > at 20kHz and 1V/us, Vm=8Vpk > > quite ok for most apps below 5Vpk. > >> At 3 volts the slew rate distortion is so bad even these 75 year old ears >> can hear it. Even a TLO-72 or 74 can mop the floor with a 741, and output a >> +- 15 volt rail to rail signal doing it, but into the old 600 ohm std load. > > LM741 has 1mV OS typical. TL072 is 3mV > > LM741 would be better than TL072 for control apps, and cheaper.
Yes, but there are much better devices for control apps than a 741, with its high power consumption, high bias current, and poor voltage ranges for common mode, output, and power. Indeed, there are so many that it's a pain to choose. What should I replace the obsolete OP220 with? Stepping back, this discussion reinforces the point I was trying to make. We frequently have newbies to gEDA complaining "why doesn't gEDA support my common/standard needs straight out of installation?". But the universe here is large, and nobody sees more than a bit of it. What you see as essential depends on where you sit. When it comes to parts selection, Gene thinks audio/video because that's what he works with. You seem to be cost sensitive. I'm a scientific instrument designer: parts cost is usually a negligible part of the budget, but noise and power are a big deal. We look at this stuff different ways. Moving from parts selection to the broader issues of EDA, we again see a great deal of diversity. There really are no common/standard needs beyond the basics that gEDA does pretty well. If you believe that there are, I think you need to broaden your horizons. gEDA's unique strength is that it supports that diversity well. John Doty Noqsi Aerospace, Ltd. http://www.noqsi.com/ j...@noqsi.com _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
