Introduction

I was watching a rather interesting EEVBlog video where Dave Jones showed a tracking pre-regulator design someone named "John Barnes" sent him (hopefully I got that right). Unlike his first video where he took Linear Technologies example circuit which uses the threshold voltage of a MOSFET to set the pre-regulator voltage, the new design uses a PNP transistor and resistors to calibrate the pre-regulator voltage. This design has several advantages over the MOSFET design. The main big one is that the threshold voltage can be set indepedendant of the specific component parts used, where-as the MOSFET design was highly dependant on the exact part used. Additionally, the ability to find the specific MOSFET with the correct threshold was unlikely. Dave did say you could compensate for this by adding diodes, but really this is a poor design in my opinion and he seemed to agree. Dave did some spice simulations with some actual parts and was able to parametrically evaluate what resistor values to use for a given target pre-regulator voltage. This is great if you have the available parts in your simulation software, but I wondered if I couldn't arrive at the solution using some math so I could design with any components I wanted. I also was curious as to why the thing worked to begin with, which the simulation software doesn't give too much indication towards.