Hi everyone. I am not that good in writing English so the text below is generate by AI, but the questions and context are mine.
The situation is that I’ve just started working on a personal project from scratch after finishing Razavi’s Design of CMOS. However, I’ve realized that there is a huge gap between theory and practice.
I’m currently designing a two-stage op-amp with five OTA transistors. When I watch YouTube videos, I often see people clearly define their specifications from the very beginning, such as gain, GBW, CMRR, slew rate, etc. What I don’t understand is how they are able to determine these specifications so precisely in the first place. More importantly, somehow, they manage to meet all of these specifications, at least to some extent.
As a beginner, I thought I would try limiting the number of specifications I work with, so I decided to focus on gain, power consumption, supply voltage, output swing, and phase margin.
However, this seems to make the problem even harder. For example, GBW and slew rate provide relationships between different parameters, such as gm and Cc, or I_ss and Cc . With the specifications I’ve chosen, though, I’m still struggling to figure out how to select appropriate design parameters and establish meaningful relationships between them.
Another issue is that I’m not sure what the right approach to the design process should be.
For example, suppose I start with the gain requirement. If I want a gain of at least 60 dB, then theoretically, the product of the gains of the two stages (A1 and A2 ) must be at least 1000. My initial guess would be to allocate a gain of 50 to the first stage and 20 to the second stage.
Then, for example, I could express the second-stage gain as gm_p * (ro_p // ro_n)
The problem is that, in nanometer-scale CMOS design, gm and ro can vary significantly and behave quite poorly as functions of square-law and other related parameters (at least with the pdk I am having). This makes it really difficult for me to determine the appropriate values and figure out how to proceed with the design.
So, I’d like to ask the community two questions:
How do you choose specifications that are interconnected? In my case, if I deliberately limit the number of specifications, is there a way to determine suitable design parameters and establish relationships between them?
What is your general approach to calculating and designing a circuit? In particular, how do you approach hand calculations when working with nanometer-scale devices, where the models seem to become noticeably less accurate and the device characteristics are much less ideal?
I’d really appreciate any advice or insights from people with more experience in analog IC design!
Edit: Sorry for the missing parameters in the text. I have updated.