This is one of the rather difficult topics in analog circuit design. A lot of techniques have been discussed in the literature. But one of the better techniques was taught to me at KU Leuven. It is called the asymptotic gain method. I describe the method in this article. I also give an example using one dependent source.
Watch out for an update where I add an example with two independent sources!
Dual path PLLs is a way to decrease the gain of a VCO without sacrificing on reference spurs. Because the gain of the VCO drops, the capacitors can be sized smaller, saving PLL area. It also will help in decreasing the PFD+CP noise. It does not use complicated tuning mechanism which requires extensive digital circuits. Its a pure analog technique.
Some people argue that a student needs to focus on the basics of analog design without any EDA play. I have vehemently disagreed with it for sometime now. Once a concept is understood, it really helps if someone is able to verify them. It also promotes the idea of ‘execution’. When I started out, I could understand the concepts but never got to the execution part. I had to push myself out of that, and learn to simulate and check my understanding.
Unfortunately, students don’t get access to Cadence tools outside the university environment (and sometimes even universities lack that!). It helps to have an open source simulator. NGSpice is one of them. In this presentation (which is being continuously updated), I try and give a basic idea about using NGSpice simulator and how to design cascode current mirrors!
Update: An industrial grade simulator from Spectrum Software, called MicroCap is now free!! I will play around with it and I will have an update about it real soon!
Analog Interview Questions
I am inundated with requests from recently graduated students from India on how to prepare for interview for analog design positions. This will hopefully be an answer to them.