For the simple LED or 2-diode type sources, a major error is the
voltage divider formed by the bias resistor and the dynamic impedance
of the led or diodes. A resistor from the transistor emitter to ground
can cancel this error. Or drive the led/diodes with a very simple,
ground-referenced current source. Or bypass the hell out of them.
He doesn't show using an opamp to close the loop around a transistor
or fet, which gives better dynamic and temperature performance; maybe
in part 2?
John