In a 3rd order system precisely an operational amplifier of which negative feedback is applied to it. it is said that for the system to experience oscillation, then the loop gain must have a phase shift of -180 degree and a loop gain of 0dB and above. and we know that <loop gain=<OLG- <CLG since our <CLG = -180 , then our<OLG must be -360 before we can have a <LG=-180. MY QUESTION ; WHERE IS -360 DEGREE COMING FROM ,SINCE WE KNOW THAT OUR AMPLIFIER HAS 3 POLES OF WHICH EACH CONTRIBUTES -90 phase shift EACH ?
Not always true.
1/ When bootstrapped it can be positive.
2/ If its used as a comparator and you need hysteresis.
If you are getting oscillation then you are getting positive feedback somewhere. Either the op-amp stages have feedback not 180 degrees or if you use over all feedback then that is not 180 degrees.
In a stable amplifier, the dominant pole, ie the lowest frequency, reduces the OLG to 0db before (below the frequency at which) the other poles contribute a total of another 90 deg. OP-amps and audio amplifiers typically use pole-splitting compensation which moves the VA amplifier pole down while moving the output driver poles up by reducing the impedance they are driven with.
Pole splitting - Wikipedia
Pole splitting - Wikipedia
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