# Type one compensator...why is everybody giving it a phase of -90 degreees

1. ## Type one compensator...why is everybody giving it a phase of -90 degreees

Surely a type one compensator phase vs frequency Bode plot should always be drawn with -270 degrees phase?
I mean, -180degrees for the inverting opamp...then -90 degrees for the capacitor in the feedback path...thats -270degrees...so why do most people say the phase is -90 degrees?..its non-intuitive.

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Same thing tfor the type 2 compensator on page 5 of this...
http://www.ti.com/lit/an/slva662/slva662.pdf

..surely the phase should start off at -270 degrees, not -90 degrees?
Starting it at -90 degrees ignores the -180 degrees contributed by the inverting opamp...the inversion gives the -180 degrees....why do all articles ignore it?

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2. ## Re: Type one compensator...why is everybody giving it a phase of -90 degreees

Often the 180 contributed by the opamp is left out during analysis. It's then accounted for by calculating phase margin relative to -180 degrees, rather than -360.

So long as it's done consistently, it gives the same answers. Just a preference for some engineers.

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3. ## Re: Type one compensator...why is everybody giving it a phase of -90 degreees

Type I is just a single pole integrator. Poles are understood to introduce a 90 degree phase lag or -90 degree phase shift.

Other phase shifts can be seen as external.

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