Miller Compensation

[KGF KING]

Hi Everyone,

I'm trying to stabilize my bandgap design. Initially it was advised to me to add miller compensation ( which is adding a resistor and capacitor in series) between first stage (diff amp) and second stage (CS amp). So i tried to add some random RC value to visualize the improvement. It does but after a certain value it doesn't get better (means i couldn't bring my Unity Gain at between 80 to 45 degree). As of now i couldn't found out the why reason. So i'm seeking some advise on how to know what is the right value to find the correct compensation value and how to calculate it. Is there anything else i need it to consider such as how to know the Id and Gm is enough or not? I mean in terms of calculation. As i'm still quite new to designing world I'm still couldn't master certain fundementals such as this.

 

[sidun.av]

As I can see from your previous post, your Bandgap uses 2-stage amplifier. In this case, it would be quite tricky to compensate the entire bandgap, because you basically working with 3-pole system - 2 poles are coming from 2-stage amplifier and the third one is the bandgap itself. First of all, make sure that your opamp is stable, then proceed to the bandgap compensation. I can recommend two approaches:
1. Replace your opamp with one-stage design, i.e. folded cascode amplifier - that will make your life much easier in terms of compensation;
2. Alternatively, remove the Miller compensation on the bandgap's level, perform AC analysis and understand where each pole comes from. After that, add a small value Miller compensation and see how it changes the AC response. After that you should be able to find the correct values of the compensation.

 

[KGF KING]

Hi Sir,

Thank you for your feedback. Can you explain more about the suggestion number 2 because as of now i'm just running the stb analysis because i thought for close loop system is similar and ac analysis. And if AC analysis is difference how to find where the each pole comes from?

 

[sidun.av]

You can use STB analysis, it is the same as AC analysis, but more user-friendly. Your first two poles (opamp) are coming from input diff pair and output stage. You can play a bit with the width (increasing/decreasing the capacitance) and you will see the corresponding pole moving in frequency.