Hi,
with regard to Q4, it has a a resistor of 2 Mega Ohm connected to the base. Wouldn't the base current by too small to drive the transistor? For a 3.6 V input voltage to the base-emitter, the base current is about 1.45 uA. It is not possible on the Vce- Ic- Ib curves of the transistor (datasheet) to visualize it so that one can see if the load resistor value is enough to get the transistor into saturation.
May you please give some hint how the base resistor R12 is set at 2 Mega Ohm?
Thank you very much.
Kind Regards,
Amr