Very interesting tutorial.
I've been busy with some (high) gain stages for a DC power analyzer a little while back (college project). I ended up using an instrumentation amplifier of +/-15V. The purpose was ofcourse to measure currents through a 0.1ohm high-side shunt with supply voltages up to 12V or so, and to measure current (transients) of devices. Should be able to measure uA's (in addition to detect in-rush currents of 1-2A)..
The first stage was a very high gain stage of 1000x. The next stages added an additional +/-1x to 50x (depending on the range- the 16-bit A/D didn't had enough resolution for full scale).
I did saw some appnotes about transistor amplifiers that would get very very low noise results, in the order of 3 times lower than I used (I abused a microphone amplifier, INA103). I believe it was like 0.33nV/sqrt(Hz). It was indeed a set of matched transistors..
I still wonder, how is the 1/f (DC) noise with transistor amplifiers compared to op-amps? I really wished we did more analysis on transistor amplifiers in college..
In the end the DC analyzer was 'not bad', but ofcourse not calibrated. With some oversampling (16x, 1kSPS samples sent out) and additional filtering I ended up with <0.5uA noise on a +/- 2.5mA range.