In allen Holberg I find this page and I see that mathing of two transistor is a point of problem (ie. current density should be equal and I do matching NMOS to NMOS)
I`m trying to make Bandgap ref 1V supply, now I have problem with systematic offset, which I see between Vgs(N_CLl) and VDS( N_CLr). This two voltage should be equal right?
But in my circuit the second stage is PMOS( P_BGRl) and there is a problem with mathing right? Because I can do same current density but this two device has different threshold voltage. So I try to make current density equal but Im not sure if this is good solution.
When You using symetrical OTA You don't worry about matching between any OTA current mirror transistors and driving stage.
In folded cascode case the loading current mirror on pfets (I assuming input pair made also on pfets) should be match to driving transistors.
Whenever you use an NMOS Active Load referred to ground to drive a PMOS gain stage referred to VDD you'll have a systematic offset.
Symmetrical OTA is an option or use a 2 stage amplifier (higher DC gain -> better Vp/Vn matching)
You can only get rid of systematic offsets if matched devices are operating in the same region. The VDS of the two devices are are not the same but if they are both deeply saturated in strong inversion the IDS current offset is very small. That is why you only see 0.5mV of offset at the input of your diff pair.