Understanding Low voltage folded cascode op amp

[batmanbeginz]

Hi all,

I am new to analog design and gaining more knowledge gradually
So I have designed a folded cascode Op amp which works around ~ 0.6V with mos transistors in subthreshold:
[I need low voltage design only]. Please have a look at figure below:

Though I got it working at an open loop gain of ~45db, I still didnt quite get the complete functioning of this design. From my knowledge [from standard texts like razavi etc.] the first stage M1-M3 is just a diff pair for sensing input, the 2nd stage : M4-M13 is folded cascode, [ in which I didnt get why only 2 stacks of pmos used and 3 of nmos ? specifically the role of M6/M7 and why pmos M4 and M5 share the same gate voltage as of nmos M8 /M9]. The 3rd stage [M14-M16] I didn't get completely and the fourth output [M17-M18] seems to be kind of source follower.

Please help me in resolving the functionality and clearing these doubts. Also as open loop gain is not very high, I read on gain boosting techniques, but in the end I am a bit confused on which particular transistors be picked for applying gain boosting here

Any help will be great !!

Thanks a lot !

 

[hanspi]

Hi batman!

M1-M3 is indeed a Diff Pair, and M4-M13 is a folded cascode. The structure M10,M12 is a self-biased cascode (gates connected together). That structure, and also M11,M13 and M15,M16 are the outputs of a current mirror whose input looks the same. That input generates Vbias. This makes it possible that the current through M14 tracks the currents in the two folded-cascode paths quite well.

M14 therefore has a constant current and acts as a voltage level shifter between the drains of M7,M9 and the gate of M18. Since the drains of M7,M9 are connected to the gate of M17, this means that there will be a constant voltage difference bewteen the gates of M17 and M18. The circuit would also work if you omitted M14-M16 and connected both gates of M17,M18 to the drains of M7,M9, but then a huge current would flow from Vdd to Vss through M17,M18 in the operating point. The circuit M14-M16 is there to reduce that operating-point current. Only if the drains of M7,M9 move down (or up) will M18 (or M17) start to deliver much more current than in the operating point. This is therefore a class-AB output buffer.

I hope this helps!
Slainte!
H.