[sdiy] IN your mind, what is ....

[Scott Gravenhorst]

Harry Bissell Jr <harrybissell at prodigy.net> wrote:
>Perhaps Bob's opinion is based on manufacturing
>variations that might make any solution for one batch
>of parts invalid with another.  THAT would make for a 
>poor opinion as well.
>
>Most of the apps I've played with used the LM3900
>because of its dirt cheap price and used it as a poor
>opamp... not for the magic tricks it is capable of.

So where does one find a nice description of the magic things
it does?  I have tube, but have never used one.  Now I'm not
sure why I would want to, given the low gain (compared to an
opamp) and low input impedance (input signal current eater) ?
 I've got the National data sheet which has a ton of circuit
examples in it, but they like to use it as an opamp in most
of them.

I guess my question is: what magic trick does an LM3900 do
that is BEST done by an LM3900 ?

Wow, what a thread.  Starts out as "name the most versatile
IC" and evolves into an LM3900 bashing session.  Fine with
me, I'm learning a lot.  At least I know what things NOT to
use it for.

>H^) harry
>
>
>
>--- jhaible at debitel.net wrote:
>> > > Quoting the LM3900 designer once more:
>> > > "Because application design approaches are very
>> different
>> > > from those used with standard opamps, the LM3900
>> is usually
>> > > not called an opamp to avoid confusion with
>> standard
>> > > opamps."
>> > 
>> > Indeed.  Technically the LM3900 is a Norton opamp.
>> 
>> Sure. Maybe it even was the other way round, and the
>> LM3900
>> first coined that word, Norton opamp?
>> Are there any predecessors for such a current mirror
>> input
>> device?
>> 
>> 
>> > But hey, the Philbrick "brick opamp" of the 1960s
>> didn't have much gain 
>> > above 100 either!  
>> 
>> Yes, that's technical progress ...
>> Today's state of art is tomorrow's junk - or vintage
>> device, depending
>> on your application. For many audio applications,
>> you don't need
>> an "operational" amp at all - you just need an amp.
>> Discrete, when
>> I have the choice. And with a single transistor
>> input stage, because
>> the second half of a differential pair will add 3dB
>> of noise. (;->)
>> (But then again, try building a GIC or FDNR filter
>> from a LM3900 or a 
>> Philbrick opamp ...)
>> 
>> BTW, when Harry speaks of Bob Pease dissing that
>> LM3900 (Hi Harry!)
>> this is from _today's_ point of view. Back then, he
>> wrote the
>> foreword to that Fredriksen Book where the 3900 is
>> presented as an
>> important part of National's opamp history. And he
>> gave me that book
>> with the comment that it's still valuable reading
>> today. (;->)
>> 
>> JH.
>> 
>> -------------------------------------------------
>> debitel.net Webmail
>

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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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