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Analysis of TC compensated Voltage Reference / Discrete Linear Regulator
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AG7CK:
In this thread I would like help in understanding and analyzing a certain group of precision voltage reference circuits used in - among other applications - 4.5 to 6.5 (7.5) digits DMMs and voltage reference standard boxes that can be used to calibrate DMMs up to 8.5 digits. We are talking about circuits / devices that usually output "constant" DC voltage 10.0000x volt (x usually less than +- 10 microvolt) to 10.00000x or even more precise (with good long time stability, low noise and low temperature coefficient).

In order to follow and/or contribute to the discussion you will have to understand the tree topologically very similar circuits shown below, and a few very important differences between them.


Fig. 1




Fig. 2




Fig. 3




Equally important is it to recognize that Fig. 3 utilizes the so called +- 2mv temperature compensation scheme of a series connection of a zener (i.e. avalanche) diode and a PN junction (or diode) that has the "highly desirable feature ... that the output is somewhat self compensating for temperature changes by the opposing changes in VZ and VBE for VZ ≈ 10 volts. With the zener having a positive 2 mV/°C TC and the transistor base to emitter being a negative 2 mV/°C TC, therefore, a change in one is cancelled by the change in the other." [Source https://www.onsemi.com/pub/Collateral/HBD854-D.PDF page 34, paragraph after the paragraph containing formula (27)]

I will the similarities and differences I have found in a post below.


AG7CK:
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AG7CK:
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AG7CK:
The first circuit I would like comments about and analysis of is this one from HP 3450A/B (from late 1960s or so - imo a good starting point in order to understand newer circuits that will follow):


Fig. 4



Source: https://www.aef.se/Instrumentmuseum/Hewlett_Packard/3450B/HP_3450B_manual.pdf



Whether you agree or not with my opinion that this circuit is just and application / sophistication of the regulator in post #1, please comment on why this circuit is as it is

and

How it resembles and/or differs from the "general" regulator circuits Fig. 2 and Fig. 3 in post #1.

Thank you.
David Hess:
They are all the same topology but there are some mind bending alternatives like the first example shown below.

The improvements in figure 4 involve operating the reference and error amplifier at a constant operating point for better performance.  R31 and R32 provide constant currents to the combined error amplifier transistor and reference.  R31 is biased by a higher voltage from CR12 to increase its value for a given current so that the voltage gain of the transistor is greater.  (1) The differential pair buffers the output of the error amplifier preventing a change in operating point to improve regulation and further increases open loop gain.  Also note Kelvin sensing for the reference output although oddly enough not the ground as shown in my second example below.

I remember when RCA was advertising their combined zener+transistor references which have the advantage of placing the temperature compensating PN junction and zener diode in the same thermal environment and if you are going to use a temperature compensating diode, it might as well be the transistor which either acts as the error amplifier or as in the later integrated references like the LM399 and LTZ1000, acts to lower the reference's output resistance.

(1) The transistor's collector resistance limits how effective this can be so only raising the value of R31 further or replacing it with a high impedance constant current source yields diminishing returns.  Plus you only need so much open loop gain before other error sources become dominant anyway.
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