Some references describe diode tempco as -2.1 mV per degree C and also as 3300 parts per million. If parts per million, then the tempco would vary with the forward voltage of the diode, for example, with a very low forward current, Vf could be half a volt; at 3300 parts per million that comes to about
-1.7 mV. So is it -1.7 or is it -2.1; in other words is tempco a constant or is it a proportion of Vf?