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

[ASSI]

On Sunday 22 June 2003 05:37, 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?

In short, a bandgap reference consists of a deltaVbe circuit and a 
forward biased diode. The voltage over the diode and the deltaVbe are 
added so that the combined TC becomes zero to first order 
approximation. This happens when the sum of both voltages is equal to 
the bandgap voltage at absolute zero. Due to some second order effects 
the actual reference voltage for which the TC is zero is slightly 
higher. The forward biased diode is typically the low current side BE 
diode of the delta Vbe circuit.

In Jim's circuit, the two servo opamps make Vcb zero. If you take that 
for granted, you find that the interesting part of the circuit looks 
like (view with fixed width font and excuse the feeble ASCII art):

  ----+-----------+---------+---
      |	          |         | 
       \ |Q6A      \ |Q6C   | 
       	\|          \|      | 
         |-----------+------+ 
      	/|          /|        
       / |         / |        
      |	          |           
      /	R104      / R105      
      \	          \           
      /	          /           
      \	          \           
      /	          /           
      \	          \           
      |	          |           |\ U11B
      +-----------------------|-\      
      |	          |           |	 \     
      |	          |           |	  >----
      |	          |           |	 / 
      |	          +-----------|+/  
      |	          |           |/
      /	R103      / R106      	
      \	          \           
      /	          /           
      \	          \           
      /	          /           
      \	          \           
      |	          |           
      V           V              
      
This topology is very often used in bandgap circuits, so I guess Jim is 
right to call it a bandgap. The "reference voltage" is not the output 
of U11B, but the voltage difference between upper and lower rail.


Achim.
-- +<[ Q+ & Matrix-12 & WAVE#46 & microQkb Omega sonic heaven ]>+ --

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