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This opamp design using pmos differential amp

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opamp2.jpg
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1)Most opamp use nmos differential amp, but this one uses pmos. why? nmos is faster.
2)the third stage, buffer stage gain, and on the circuit, it shows 2 pmos(m8, m9). why? isn't it inverter is used as buffer? how's 2 pmos is a buffer?
 

Hi,

From the very little I have read, PMOS have a characteristic related to PSRR, noise or similar, but don't quote me on that specific as I haven't looked through three books to find the exact sentences, sorry, which can be preferable to NMOS, I think. PMOS inputs can reach ground, NMOS can't, that's why rail-to-rail use both N and P input devices.

Maybe, maybe not, the output stage PMOS M8 is needed to level shift the NMOS gain stage, M6, but perhaps and probably I'm interpreting the schematic incorrectly. M9 appears to be an active load/current source.
 

A P-device accepts a signal which is referenced to the more positive leg. This makes it a natural choice when the power supply is negative polarity (and/or when the signal is referenced to the more positive rail for some reason). Thus we can construct an op amp (voltage regulator, etc.) which operates in the negative region, as a symmetrical counterpart to typical op amps we are familiar with.
 

Working with signals close to the negative rail is the main
reason to prefer a PMOS or PNP front end - either the diff
pair itself, or source / emitter followers preceding it (then
the diff pair could be N type). Single supply op amps of the
normal sort are always PNP or PMOS (or, overlaid NMOS and
PMOS in rail-rail input types, but the PMOS remains essential).

Final stage is a PMOS source follower buffer with PMOS
current source load. Not great for any interest in ground
(negative rail) hugging output, while ground-hugging input
is a design feature. If I was calling this a single supply op
amp I'd at least flip the final stage to NMOS follower &
sink load (4 transistors instead of 2). But neither choice
is great.
 
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