What is the internal structure difference between a dual supply opamp and single

[opa627bm]

Hello everyone, I was playing around with a ldo circuit
http://www.maximintegrated.com/app-notes/index.mvp/id/3657

and I noticed that some of the opamps doesnt work in the circuit. Later I found out is because some of them only works in dual supply mode so the feed back need to be grounded in the middle of the rails.  What is the internal structure difference between a dual supply opamp and a single supply one? and Why the opamp is acting as an inverting amplifier to drive the MOSFET? thank you!

[SebG]

There really isn't any difference since as far as the op-amp is aware the voltage that powers it is Vcc - Vee.  The reason some op-amps might not work is that they can not output a voltage close or at the power rails so if the lower voltage (Vee) is at GND the lowest output can be something like .7 or 1V or 2V depending on the device.  Rail-to-Rail op Amps can output voltages closer to the power rail.  Setting one up as a dual supply allows the op-amp to output 0V very easily.

[SebG]

Also the mosfet is P-ch so it needs an op-amp with an inverted output.  The mosfet turns on as the Vg is less then Vcc - .7 (or whatever the threshold voltage is) so the mosfet needs an decreasing voltage at the gate for an increasing V+ input (proportional to the circuit output voltage, OUT)

[T3sl4co1l]

Typically, a "dual supply" type op-amp stops working if one or the other input gets closer than 1-3V to the positive or negative rail.  The voltage range, and how it "stops working", both depend on design.  What usually happens is, the input stage goes into cutoff, or input bias is shunted out of the input terminals, and the output saturates to +V or -V.  (It's always the same direction, so if the output slammed to +V by pulling +in to -V -- phase reversal -- the amp can get stuck that way!)

It's fairly easy to make an op-amp that works up to one supply, e.g., some JFET amps work up to +V, some bipolar amps (LM358, etc.) work down to -V.  The most common method of making an amp that handles both is to simply invert and repeat half the circuit.  So the normal input circuit might have PNP transistors (that work down to -V), the inverted section would use NPN transistors (that work up to +V).  Somewhere inbetween, the two sections cross over, with the result that input offset voltage changes suddenly (because it's like using two distinct op-amps in parallel, each with its own offset voltage).  TLV2371 is a MOSFET amp that does this.

Output stages also vary, for similar reasons.  Outputs that can reach the supply rails are more difficult to design, and usually have worse power consumption, higher distortion and so on.

Tim