Take a look at this sketch...
http://www.analog-innovations.com/SED/EL7900_IDSS.pdf
I replaced the upper P-channel "enable" device with a current source
of value IDSS. (One of my special Spice tools... can't pull to
infinity, stops at zero volts across it
Clearly, when the sum of Iphoto and M×Iphoto exceeds IDSS, the output
will collapse.
(There's a multiplication symbol between "M" and "Iphoto"... in my
fonts, maybe not in yours.)
The shape of that collapse will depend on the particular CMOS process
it was made from. (A very sharp collapse would indicate that the
"mirror" is actually made up as a cascode (maybe even combined with
the enable device)... which is likely, if it was done properly.)
Without conducting testing with a calibrated light source, I can't
tell whether it is you simply overloading the device above its design
range, or if it is defective.
...Jim Thompson
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