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Analysis of TC compensated Voltage Reference / Discrete Linear Regulator
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AG7CK:
Thank you very much - Kleinstein and David Hess.


--- Quote ---It looks like a separate ground is used for the reference and feedback divider return currents to provide Kelvin sensing for the reference output like in the example I posted.

--- End quote ---

As this thread hopefully progress towards cascaded refamps, virtual grounds / potential splitting and current cancellation op amps as in Fluke 5700A and 732B respectively, I will ask more questions about implementation. For now, I concentrate on design / theoretic function.

I have too many questions - I don't know where to start. So I just start:



--- Quote ---The collector current of the ref-amp is used to trim the TC.

--- End quote ---

- The HP 3450 circuit Fig. 4 has a base resistor R1 8k25 (?) in the error amp / refamp. The fluke 341A circuit Fig. 5 has not a base bias resistor. Suppose one trims the output of the HP circuits by a small change in the sampling divider. This will give a small base current change in the relatively low impedance HP divider, and the resulting small change in Ic is sufficient for the output voltage adjustment because of high refamp and diffamp gain. Is this correct?

- Since Fluke has no base resistor (but relies on the divider for current limiting), a relatively higher / more sensitive current change and change of base bias voltage in the high impedance Fluke divider will be the result. Is this correct?

- What is it that Fluke "adjusts" with R21? It is not Ic, because Ic is Beta x Ib? (I appreciate it is a one time "selection" - not a customer adjustment option. But some other Fluke boxes have 2 collector resistors in series, which could mean iterative selection / adjustment - as in "trimming" of the selected part.)

- Is it so that Fluke sets the (already known) current for which the TC is zero by adjusting the tapping point on the sampling divider. Then the divider is adjusted for exact output voltage. Finally the collector resistor is selected for the desired operating point Vc? (HP can skip the last step because of much higher gain and a "constant" temperature cooler?)

In general, I do not yet understand the most important aspect of the refamp "zero TC" regulator: The mechanism and the steps in selection / trimming.
AG7CK:

--- Quote from: David Hess on June 03, 2018, 02:20:53 am ---...
So the collector current is 0.1 milliamps which is typical for a high beta low noise transistor and the zener diode sees 3.0 milliamps total which is typical.  Those are suspiciously nice round numbers so probably not an accident.  I think that is an RCA part but Motorola made them also they give specifications for a 5 milliamp zener current and 250 microamp collector current.
...

--- End quote ---

I have found no data/-sheet neither for SZA263 nor DH71005A nor DH80417B nor LTFLU-1, so I wouldn't know. Empirically, almost all hardware I have seen on the forums are run at 3mA or thereabout.

Kleinstein:
I would consider the base resistor in the HP circuit as a kind of precaution to prevent to much base current, especially on startup. With about 1 µA of base current there is only a few mV of drop on the resistor. The 15 V of the fluke circuit naturally results in a higher impedance of the divider.

The feedback loop adjusts the voltage over R21 to a constant value. The collector resistor R21 in the fluke circuit set's the collector current - the base current is a result of this.

I would expect the adjustment procedure to first adjust the TC with R21 and than adjust the voltage level. Changing the divider would not have a significant effect on the TC.

With temperature control the HP circuit does not need the TC adjustment - especially not at that level of resolution.
David Hess:

--- Quote from: AG7CK on June 03, 2018, 07:53:26 pm ---
--- Quote ---The collector current of the ref-amp is used to trim the TC.
--- End quote ---

- The HP 3450 circuit Fig. 4 has a base resistor R1 8k25 (?) in the error amp / refamp. The fluke 341A circuit Fig. 5 has not a base bias resistor. Suppose one trims the output of the HP circuits by a small change in the sampling divider. This will give a small base current change in the relatively low impedance HP divider, and the resulting small change in Ic is sufficient for the output voltage adjustment because of high refamp and diffamp gain. Is this correct?

- Since Fluke has no base resistor (but relies on the divider for current limiting), a relatively higher / more sensitive current change and change of base bias voltage in the high impedance Fluke divider will be the result. Is this correct?
--- End quote ---

I am not sure what is going on with R1 but the higher divider resistance of the Fluke suggests to me that the added base resistance compensates for variation of transistor Vbe or hfe with temperature.  Some current mirror implementations do something similar with added base resistance.


--- Quote ---- What is it that Fluke "adjusts" with R21? It is not Ic, because Ic is Beta x Ib? (I appreciate it is a one time "selection" - not a customer adjustment option. But some other Fluke boxes have 2 collector resistors in series, which could mean iterative selection / adjustment - as in "trimming" of the selected part.)
--- End quote ---

Since the collector voltage and reference output are constant, they are adjusting Ic and I assume trimming Ic for best temperature coefficient like Kleinstein wrote.  Instead of Ic=beta*Ib, in this case it is Ib=Ic/beta.

In a modern design, I doubt it is worth going to all of this trouble versus using a differential pair where the impedance at both bases can be matched instead of selected.  A differential pair doubles the noise but that is easy to make up for with a higher tail current.


--- Quote ---- Is it so that Fluke sets the (already known) current for which the TC is zero by adjusting the tapping point on the sampling divider. Then the divider is adjusted for exact output voltage. Finally the collector resistor is selected for the desired operating point Vc? (HP can skip the last step because of much higher gain and a "constant" temperature cooler?)
--- End quote ---

Fluke is doing something like that.  I don't know how they established the value for R21 but I suspect they tested the reference units at two different temperatures.  See below.


--- Quote ---In general, I do not yet understand the most important aspect of the refamp "zero TC" regulator: The mechanism and the steps in selection / trimming.
--- End quote ---

I doubt the methods were widely advertised or documented outside of Fluke and HP.

The production processes I have seen in the past used a test assembly to measure the temperature coefficient with two different temperatures and then calculated the values for the selected parts for each unit.  I have written the programs for doing these calculations a few times.
AG7CK:
Thanks again. I think I understand a new point:

The adjustment of / change in Ic is not because they "push" current (Vout - Vc)/Rc into the collector thereby increasing Ib. It is more like the "inverse" relation Ib=(1/Beta)xIc arises because adjusting Rc changes Vc which is the input signal to the diffamp. So a change in Rc marginally adjusts Vout and hence Vb and Ib?
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