Ned Konz wrote :
> On Friday 26 October 2001 11:42 pm, Chris Carr wrote:
> > ----- Original Message -----
> > From: "Bob Blick"
> >
> > > The LM3900 is a Norton opamp, you can easily put your two photodiodes
> > > from positive then into each input of one of the amps. Then bias the
amp
> > > with a resistor(say 10 meg) from positive to the noninverting input,
and
> > > a 5 meg resistor from the output to the inverting input.
> >
> > I believe that the LM3900 has been obsolete for some time now
> > (but I bought so many when they were current that I still have some in
> > my junk box) LM259 is still available and if I remember correctly
> > Motorola (On Semiconductor) used to (still do ?) produce a Norton
Amplifier
> > but it was never a popular as the LM3900.
>
> This is what, a 30 year old chip?

Thanks, I like been reminded that I'm considerably closer to the Grim Reaper
than
the time I popped into this world  8-)

It does make you realise the problems that they must have keeping ancient
planes like the Concorde in the air, that also is 30 years old. It must rely
on 555 timers for everything electronic.

>
> The Motorola MC3401 was close or equivalent. But I think it's discontinued
> now too.

That number looks sort of familiar, you are probably right about it's lack
of availablity.
Can we construct a replacement with a PIC ?  8-)
>
> The LM259/LM359 is a dual; I don't know if there's any quads any more.

Probably correct but in the old days when we used Maths, 2 Duals made a
Quad.
Now we have had, probably in the interests of Globalisation, Math imposed
things are probably different......Sorry I couldn't resist the temptation.
>
> > Are they not also known as Transconductance Amplifiers? Just a random
> > thought, too early in the morning, insufficient caffeine in the system.
> > Probably wrong.
>
> No, a transconductance amplifier has a current-mode output, not input.
>
Yes, I've just had a look at the datasheets for a LM13700 and you are
correct
I must keep my caffeine levels up.

This question of obsolescence may be worth exploring as an OT

Regards

Chris Carr

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