John N. Power wrote:
> The following is from National Semiconductor application note=20
> AN-72, "The LM3900 - A new current-differencing quad of=20
> +/- input amplifiers. (September 1972)",

It took me less than 1.5 min to download 300K datasheet=20
"LM2900/LM3900/LM3301 Quad Amplifiers" February 1995


> page 3:
> " The open-loop gain changes only slightly over the complete=20
> power  supply voltage range and is essentially independent of=20
> temperature changes."

I've searched the 1995 datasheet with the word "slightly" and=20
"essentially": - no occurrences were found. If they used the=20
1972 vocabulary now they would get kicked off the market=20
shortly.


> Clearly, there is no expectation that the mirror gain changes
> significantly.

What is it "expectation" and how much is "significantly" - 0.01%=20
or 10 times? Either this must  be specified in a datasheet or this=20
MUST NOT be talked about at all, I think.


> There are numerous examples of triangle, sawtooth and=20
> ramp generators in this apnote. A change in mirror gain with=20
> input current would cause a supposedly linear output to be=20
> curved.=20

An app note is just a note, not a datasheet. And what is it=20
"supposedly"?


> The applications given in this note would not work if this
> happened.=20

"Triangle, sawtooth and ramp generators" will work but with
unspecified distortions.


> I would breadboard a circuit myself and check this,
> but I can't do it right now.=20

LM3900 production has been discontinued in former=20
Millennium by National (Don't know about Motorola). Does this=20
chip deserve to get breadboard now?


> I trust National to know something about these matters.

Yeah, they are great guys. Their modern op amps are far better=20
than obsolete discontinued  LM3900 (1972).


> In any case, the amplifier does have limitations. The most pronounced
> is its lack of speed. It is suitable only up to audio frequencies. It
> is not intended for precision work, and that was not the original
> application. The chip is second sourced by Motorola, has been
> around for a respectable amount of time, as you can see from the
> date on the apnote, and is still available. This would not happen
> if it had serious problems with linearity.


Regards,
Mike.

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