John N. Power wrote:
> > By the way, "plus input" current is about 0.02 ma for LM2900. Ever
> > seen a phototransistor with the 0.02 ma output current?
>=20
>       This particular amplifier doesn't work that way. Each input
appears as
>       a diode to ground. A current into the positive input is
subtracted from
>       the current into the negative input by means of a current
mirror. The
>       current difference drives the output. Because of this current
sinking
>       property, this amplifier is good for sources which produce
current
>       directly, such as phototransistors...
>       The 3900 can sink current without inverting.
>=20
>       The maximum current into each input is 5mA max at 25C, and
>       reduces to 3.8 mA at 70C. Within this range, the inputs can
>       accomodate a perfectly reasonable amount of current.


Have a look at the LM3900 datasheet, please.=20
Mirror Gain is specified only for 0.02 ma and 0.2 ma input current. And
the difference of Mirror Gain for these two input currents could be as
big as 5%. For usual light intensity range 2^12=3D4096 one could suspect
the difference of Mirror Gain as big as 20 times. Pretty linear, yeah.

And you can't compress the signal, cause even National failed to=20
specify temperature drift of Mirror Gain.=20


> > Not to say, two transistors without negative feedback
(phototransistor
> > and "plus input" transistor) are real killers for the converter
> > linearity and even stability. Temperature signal shift will be
> > much more than working signal range.
>=20
>      Consider a current source like a phototransistor working into a
diode
>      load to ground. The current in the diode controls the output
voltage.
>      This  is quite linear.


Yeah, only 20 times change in amplification over the range.


>                                                             ... Try to
make a conventional
>       virtual ground input using a normal opamp and a single 5 volt
>       power supply. You might try to bias the positive input with a
divider,
>       but remember that this limits the voltage on the
phototransistor. Also,
>       the output is inverted which causes problems if the amp output
can't
>       go within a volt of ground.

I wouldn't. I'd rather simulate current mirror using xtal PIC's PWM,
precision resistor, capacitor.


> Bush is a Yale graduate, so I find that flattering.=20

Fattering to Bush? Does Bush deserve flattering? At his Yale=20
he even didn't learn how to comment code and use linker. :-)
PICList membership is better.


> Which "stile"[sic] is yours?

"Style", sorry for the typo, thanks for the hint.

Mike.

P.S. My current subscription options [OT], [EE]

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