Dave Dilatush wrote:
> michael brown wrote...
>
>> ...Fairchild and National for example, state it at only one or
>> two supply voltages.  Since Fairchild only states it for one supply
>> voltage, it's kind of hard to extrapolate the rest.  :-?  I did find
>> one datasheet with a chart, and it told me the answer.  I did RTFM
>> and I looked the schematic over, from that I couldn't immediately
>> see any reason it wouldn't go closer to the rails.  Heck, I can't
>> find one datasheet that shows a minimum supply voltage.  Everyone
>> but me and the manufacturers seem to know it's 6V.  ;-)  But if you
>> happen to need the external characteristics of the package, about
>> one half of the datasheet is dedicated to it.  I'm trying, geez
>> doesn't that count for something.
>
> Yes, it does.
>
> There's a solid reason for what you're seeing on the LM741
> datasheet, and also for what you're NOT seeing.
>
> The LM741 is a very old, even ancient, opamp; and back in the
> days (some thirty years ago, IIRC) when it was new, nobody but
> NOBODY would even dream of trying to operate it, or any other
> opamp, on anything other than +/- 15 volts.  It was taken as a
> given that ALL such devices required several volts of headroom,
> from both supplies, on both the input and output.
>
> When the 741 was new, the notion of "single supply" opamps (like
> the LM324, with its input common-mode range including the minus
> supply terminal) had not yet appeared, and the "rail-to-rail i/o"
> opamps we take for granted today were not yet even a faint
> glimmer in an IC designer's eye- much less RRIO opamps that can
> operate on just a couple of volts.
>
> Again: back then ALL analog systems operated on +/- 15 volts;
> that's "just the way things are done".  Anyone who wanted to do
> otherwise was on his own, as the part's guaranteed specs applied
> only for +/- 15 volt operation.  The manufacturer might show
> "typical" curves for operation at lower voltages, but there was
> no operability guarantee and that's why you don't see any minimum
> supply voltage listed.
>
> I haven't looked at this circuit you're trying to build, but if
> it uses a 741 and you want to operate it at a low voltage, try
> substituting one section of an LM324 quad; you'll probably get
> satisfactory results.
>
> Hope this helps a bit...

Definitely, it makes complete sense from that perspective.  ;-)  I built
the circuit, and it worked pretty much as expected except for the
headroom surprise.  Well, that and no output without a negative feedback
resistor, I just don't get that.  I would like more gain, but the 741 is
performing as the datasheets would predict at 25kHz coming in somewhere
near 100.  I was just intrigued by the extra margin required by the chip
over what the schematic would seem to indicate.

I lowered the suggested feedback resistor by a factor of 10 to only call
for a gain of 100 instead of the unobtainable 1000 as designed.  As I
had hoped, it made no measurable difference in the performance at 25kHz.
I'm hoping it serves to lower unnecessary gain on the undesired lower
frequencies.  I don't think it will really make any difference in the
end, as the transducer has a high Q and is virtually deaf to anything
but it's resonant frequency.

Thanks Dave  I'll pick up some more op-amps some time this week, the 324
looks like it would give me better gain.

michael

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body