# Phase compensation

Discussion in 'Electronic Design' started by [email protected], Nov 1, 2008.

1. ### Guest

I have some doubts about phase compensation.....
puts a zero of feedback function at frequency with 0dB loop gain. So,
supposed that 2 first poles (open loop gain of opamp and pole of
feedback loop) are totally evolved, he obtains a phase margin of 45°.
This is ok, but there is a frequency range where phase loop is 360°:
so i think oscillations are present. Why in stability analysis only
phase margin is considered and not the entire phase evolution?

2. ### Guest

Yoda sais: open loop gain not is > 1 at 360 degrees.
Why?
i have this system:
1) two poles before |A(s)B(s)|=1 =>phase shift=180°
2)one zero at |A(s)B(s)|=1=>phase shitf at |A(s)B(s)|=1 equal to 45°
3)before |A(s)B(s)|=1 open loop gain greater than 1
So, if phase shift= 180° plus 180° of inverting configuration, total
phase shift is equal to 360°. In this range |A(s)B(s)|>1....this
system is stable or not?

3. ### MooseFETGuest

Something that oscillates has infinite gain or more. Consider only
the infinite gain for a moment.

The gain of a feedback loop is G / ( 1 + GH)

where:
G is this gain of the amplifier (Complex number)
H is the gain of the feedback (Complex number)

To obtain infinite gain, the GH part needs to be exactly -1 so the
phase must be 180 degrees when the abs(gain) is one.

To make a system stable, this is the condition that must be avoided.

At lower frequencies, where the gain is more, the phase doesn't matter
much because if you plug the numbers into the G /(1 + GH) you find
that you still get reasonable numbers.

4. ### Simon S AysdieGuest

Where can I get an amp that has greater than infinite gain?

5. ### MooseFETGuest

Just add positive feedback to an ideal op-amp.

6. ### JamieGuest

When you kick the bucket and make it to the other side! (Maybe)

http://webpages.charter.net/jamie_5"