IC Design Interview 3: Bandgap Reference

Life has intruded so not much update on the boost converter. I do now understand there is a brand new shiny zero that has to be taken care of. I also now understand the gate drive needs to forced to oscillate at some duty cycle regardless of the output value. I need to rework the feedback compensator to account for the boost AC characteristics instead of the buck that is in there now. Until I have that ready, here's some more interview notes.


The bandgap equations must be derived sometime during every integrated circuit design interview. At least it seems that way. It is a standard, useful circuit found in nearly every chip. I think the discussions about the circuit are better able to gauge a candidate's knowledge and experience, though. 

Start with a schematic like the one shown below. 

That last equation isn't the final answer, though I've never been asked in an interview to go beyond it. V_BE has a negative temperature coefficient and the V_T has a positive temperature coefficient so appropriate values of resistors and bipolar multipliers can be found.In general R₁ and R₂ are made equal. Then the bipolar multipliers are set based on layout. Pick a rectangular array that allows Q₁ and Q₂ to be arranged in a common centroid configuration. Then set the ratio of R₁ and R₂ for zero temperature coefficient.

There are at least four other sub-topics that I will put off until a later blog entry.

• Current conveyor instead of an op-amp
• Lower voltage reference
• Producing a VPTAT (Voltage Proportional To Absolute Temperature)
• Start-up issues

Bruce McLaren

IC Design Interview 1: Basic Device Equations

I've been on both sides of the hiring desk and can say that interviewing for an IC design position is an interesting process. There aren't really that many of us, so we either know each other or know someone who knows the other. Everyone involved (the hiring manager, job seeker, other designers) is busy with things other than the interview. 

The usual process is for an interviewer to meet with the job seeker for an hour or so. This repeats about six times with a break for a group lunch. The interview is usually extremely technical in nature. There will be equations derived and schematics drawn. This seems to surprise people in other professions. 

I memorize equations poorly. This is bad when I know I will be required to recite a multitude of equations. These equations are basic to our profession and are used regularly, I just can't spit them out during an interview. So I spend several days ahead of the interview cramming as if for a test. I have prepared seven pages of basic information to be memorized before an interview with a few pages that I add on depending on the specific position. The first page of notes are the basic device equations. I present these here without explanation; the explanation would encompass an undergraduate electronics course.

I am still learning how to use google docs and embed the result in this blog.

Until next week.

Bruce McLaren