Many thanks again magic: for the time, for the patience and for the hints.
Was always wrongly thinking that 1uA was equivalent to no current - BJT not working. And Ic current of 140uA was already nothing... only in the mA range was worth of interest..
You are right, again: originally the two I_source and I_sink legs are applied in a complementary way to the same load (470 ohm) via an external analog switch leaving the other leg floating ("flappin' in the breeze")..That's why I had connected the other leg to a 10Meg resistor.
I am anyhow more interested on the Sink state.
So some considerations to maybe clean some confusion in my head:
FOCUS on Q3 during SINK STATE:
It has floating collector: despite this, Q3 is anyway in active region (due to the voltage 3.6V - (Viref+VR14 drop) > VturnON. The Ie current 150uA is forced out the Ib despite the Ie=hfe*Ib. Kirchoff node current overbears. RIGHT?
Or is Ie current only due to the difference of the voltage drop on R14 due to Kirchoff's Voltage??
VCE drops from 5.4mV to 5.6mV since it has a terminal floating but Ve changes from 3.6 to 3.58 during OFF -> ON state.. so only PN junction (BE) is working.
Q2 is a PNP BJT. Ib current flowing outside. If it would be an NPN the IeQ3 would be injected inside base of Q2 contributing to the current boost of the mirror below. So 150uA*hfe of Q2. (picture HP1) RIGHT?
Same would be for Q1. But NOT THAT CASE. All Ib Q3, Q2 and Q1 blocks the way (or better subtract their contribution of fraction of uA to total current) so the 150uA is forced to drop across R2.
LTspice gives me IR2 = 300uA approx (150uA from IcQ1 + 150uA). in global way adding to the net current that flows outside Ic of Q1 it "depletes" the current available on the RSinkLoad boosting up the VbQ1.
So do you think that removing the sink leg would fix the circuit?
Thanks again