[sdiy] Patchell's Boggling 13700 Expo Converter Thing???

[Scott Bernardi]

That's the part of the circuit that I contributed, and it goes back to my early days
as a analog IC designer.
Bipolar semiconductor transistors have a Vbe with a negative tempco (our famous
-3300ppm/deg C), and we are familiar with the kT/q thermal voltage, which has a
positive tempco that is proportional to absolute temperate (PTAT).  By taking a Vbe
and adding to it a scaled version of kT/q, there is a "magic" voltage where the
tempcos will cancel out. For silicon, that magic voltage is 1.25v and is called the
"band-gap voltage" (and it's also why the commercially avalable voltage reference
IC's are some multiple of 1.25v). If you used germanium transistors, there would be
a different "magic" voltage (about .78v). The band-gap voltage characteristic is
also what determines the scale factor for our exponential generators: approx. 18mV
per octave or 60mV per decade for silicon.
Using the difference between two Vbe's running at different current densities is the
most common way to genate a PTAT kT/q term. In Jim's circuit we convert it to a
current to drive an OTA two quadrant multiplier which multiplies the input CV
voltage times a PTAT term that will cancel the negative tempco term of the
exponential transistor.


Here's a Bob Pease article on the bang gap reference:
http://www.national.com/rap/Application/0,1570,24,00.html

Here's one with some theory (for you guys that like equataions):
http://amesp02.tamu.edu/~sanchez/689-604-BandGAPREG.PDF

Ian Fritz wrote:

> At 06:27 PM 6/21/2003, Jim Patchell wrote:
>
> >         The second block is the temperature sensor. The temp sensor is
> > made up of Q6A, Q6C, U10 and U11B.  The configuration is a Band Gap Reference.
>
> This is something I'm still confused about.  I looked up everything I could
> find on bandgap references and they seem to be something different.  They
> use a PTAT generator like the Q6A and Q6C arrangement, but they use this
> with further circuitry to generate a stable reference voltage.  Can someone
> clarify the reason this is being called a band gap reference?
>
>    Ian

--
Scott Bernardi
sbernardi at attbi.com