[volt-nuts] Making a Reference IC

[Charles Steinmetz]

Dave wrote:

>Just thinking... would it be possible to make a reference with similar
>characteristics with discrete components (a low tempco Zener and a
>transistor)?  They would likely have to be closely coupled thermally
>and maintained at a constant temperature
>
>What criteria would apply to the selection of the parts?

(1) the absolute tempco (mV/C, not ppm/C) of the transistor B-E 
voltage and the Zener voltage need to be very accurately matched over 
the expected temperature range.  (2) the temperature of the 
transistor and Zener substrates must be identical to within very 
small parts of a degree C.  The only practical way to come even close 
with respect to (2) is if both devices are on a monolithic 
substrate.  Even separate dice thermally mounted to a very 
thermally-conductive substrate in one package is a less than optimal 
solution (although that is how the original reference was 
constructed).  Starting with packaged components will just end in tears.

I concur with other advice you have received -- ditch the 1960's 
technology and use one of the excellent low-drift, low-noise voltage 
references that are plentiful these days.  The 731s just have the 
reference IC mounted out in the open, which feeds a 1960s op-amp 
(LM301A in the case of the 731A, LM308A in the case of the 731B) 
mounted out in the open with 1% MF (731A) or precision WW (731B) 
resistors, also mounted out in the open, to produce 10v.  If you do 
nothing but use an LM399 reference and change the op-amp gain 
resistors to suit, using the same type resistors that are in your 
unit now, your 731 will be better than the day it left the factory.

If you replace the op-amp with a modern precision part (I suggest the 
OPA277 -- others may try to talk you into an auto-zero op-amp, but I 
think that would be a mistake in this application), and upgrade the 
gain-setting resistors to ones with better tempcos, it should be very 
much better than the day it left the factory.

None of these changes requires any more design or construction 
expertise than making a new reference IC on the pattern of the 
original, and unlike that effort, they will actually improve the 731 
instead of ruining its performance.

Best regards,

Charles