I have some doubts about phase compensation..... I'm reading Graeme book about transimpedance amplifiers with opamp:he 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=B0. This is ok, but there is a frequency range where phase loop is 360=B0: so i think oscillations are present. Why in stability analysis only phase margin is considered and not the entire phase evolution? Thanks in advance
Phase compensation
Nov 01, 2008
8 Replies
On a sunny day (Sat, 1 Nov 2008 03:46:45 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :
Yoda sais: open loop gain not is > 1 at 360 degrees.
Why? i have this system:
1) two poles before |A(s)B(s)|=3D1 =3D>phase shift=3D180=B0 2)one zero at |A(s)B(s)|=3D1=3D>phase shitf at |A(s)B(s)|=3D1 equal to 45= =B0 3)before |A(s)B(s)|=3D1 open loop gain greater than 1 So, if phase shift=3D 180=B0 plus 180=B0 of inverting configuration, total phase shift is equal to 360=B0. In this range |A(s)B(s)|>1....this system is stable or not?On a sunny day (Sat, 1 Nov 2008 06:15:21 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :
Yoda sais: Remeber the force and be happy.
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.
Where can I get an amp that has greater than infinite gain?
I want one, too ;-)
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at
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| 1962 | It's what you learn, after you know it all, that counts.
Just add positive feedback to an ideal op-amp. :)
When you kick the bucket and make it to the other side! (Maybe)
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