Re: gEDA-user: A little puzzled about the purpose of gschem

[John Doty <jpd@xxxxxxxxx>]

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/
jpd@xxxxxxxxx




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