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Improve Common Mode Gain PM (Read 1153 times)
kanishxmas
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Improve Common Mode Gain PM
Feb 06th, 2018, 9:34pm
 
Hello guys,

I have a dual stage fully differential amplifier (first stage telescopic, second stage common source) with common mode feedback (CMFB) and it has been biased properly so that all transistors are operating in the "saturation" region.
Below is the response of my Differential Mode gain and phase:


At some point the phase on CM goes beyond 180 as it can be seen from the picture. I am wondering whether you can suggest a couple of things I could do to improve my phase margin? Right now is negative based on the graph.

Thanks


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Working_Response_-__CM.PNG
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kanishxmas
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Re: Improve Common Mode Gain PM
Reply #1 - Feb 6th, 2018, 9:35pm
 
This is the DM response:

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Working_Response.PNG
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Tako
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Re: Improve Common Mode Gain PM
Reply #2 - Feb 7th, 2018, 12:23am
 
In your second post, where phase is blue and gain is pink you have:

- bandwidth = 849 MHz
- phase margin = 60.29 degrees

So everything is correct.

I do not understand the picture from your 1st post, where phase is red and goes up then down. Gain is yellow.
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kanishxmas
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Re: Improve Common Mode Gain PM
Reply #3 - Feb 7th, 2018, 12:55am
 
Hi Tako, thanks for the reply. The problem I am having is with the first image on my first post. That graph represents the Common Mode gain and phase. I am trying to find ways of fixing the zeros and poles you see on the graph and at the same time I do not want to sacrifice my DM results (the ones shown in the second picture).
I was told I can either change the gm of my differential cascode amplifier tail current or I could increase the "capacitance" of the sense circuit I am using as feedback. I just want to confirm whether this is the correct way of approaching the problem. I pretty much do not want a negative phase margin at 0dB for my CM graph.
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Tako
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Re: Improve Common Mode Gain PM
Reply #4 - Feb 7th, 2018, 1:10am
 
Hi kanishxmas,

Absolutely. Now, I get your problem correct:
- DM = Differential Mode
- CM = Common Mode

I am surprised by the plot of CM gain and phase. When I designed CMFB circuit a couple years ago, I had typical "opamp" response. The one like your DM gain and phase.

Typical approach is to increase compensation capacitors Cc of CMFB circuit and this is something I would propose to you to try firstly.
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kanishxmas
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Re: Improve Common Mode Gain PM
Reply #5 - Feb 12th, 2018, 9:35pm
 
The compensation capacitor (Cc) was the game changer (2pF--->6pF). It did help me improve my DM and CM phase margin. Thanks.
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Tako
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Re: Improve Common Mode Gain PM
Reply #6 - Feb 13th, 2018, 2:29am
 
Great! Happy you solved the problem and notified us here.
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