I have the following circuit and I'm wondering a little bit about the functionality of the output transistors M12-M19.
simulate this circuit – Schematic created using CircuitLab
I know that M1,M2,M6 and M3,M4,M5 are two differential pairs and I also think, that M19,M12 and M16,M14 redirect/combine the current from positive and negative input to one current.
What I don't fully understand, though, is the idea behind M13,M15,M17,M18. I believe, that the node connecting all four gates and the drains of M18 and M17 should always have a voltage exactly at the mean of M18's and M17's sources. So I was thinking, that M13 and M15 could be cascodes, with M18 and M17 being a self-biasing network.
Is this pretty much it or is there more to the eight transistors on the right?
Best Answer
You're correct M16, M17, M18 and M9 are mainly for biasing. Note how they're all "MOS diodes", all in series and between Vdd and ground. They are the biasing for M12 - M14.
At the same time M12, M19 and M14, M16 are current mirrors that push the current difference from the differential pairs in the direction of the output.
The biasing current through M16 - M19 is not constant, M1 and M3 add/subtract current and that influences the voltage at M17 and M18.
M13 and M15 are indeed cascodes even though their gate voltage is not constant. Their gate voltage will not vary much though. M13 and M15 provide a low impedance at their source so that the currents from M2 and M4 can flow into the string M12 - M15.
This cascode structure is similar to a "folded cascode", see this lecture sheet 17 and onwards.