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Close BOTH switches to start.
This is an improvement to my previous regulator ( http://everycircuit.com/circuit/4659192062017536 ). This version uses a PMOS (IRF4905) as the "upper regulator" element and an NMOS (IRF540N) as the "lower regulator" element. This arrangement offers rail-to-rail output voltage swing. The control circuit is more complex, with 2 seperate op amps controlling the 2 MOSFETs, unlike the previous version, with a single op amp controlling both MOSFETs (in a Class AB amp topology). Even though the previous version was simple, it has an obvious disadvantage of a high 3-4V dropout, which is overcome in this version.
Note 1: The diode connecting the gate of the NMOS to the output of the op amp controlling the PMOS is needed to suck gate current from the NMOS and shut it down during startup. It does not conduct current after startup. Without the diode, both MOSFETs turn on during startup. Delete the diode to see what I'm talking about, an observe the MOSFET gate voltages and op amp output voltages to see why I connected the diode that way.
Note 2: The circuit seems to have stability issues, which I have overcome using a 1mV voltage source. Remove it to see the issue. All real-world op amps are different, so this should not be a problem in real life. I am yet to test this circuit in the real world.
Note 3: By supplying a negative voltage to the circuit and making some adjustments to the feedback network, the regulator becomes a 4-Quadrand one.
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