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Just for fun: a superreg with <12 discretes??! ?! ?
Just for fun: a superreg with <12 discretes??! ?! ?
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Old 17th January 2019, 05:36 PM   #11
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Elvee View Post

Click the image to open in full size.

Unsurprisingly, all of this greatly improves the noise performance: it is now lower than 0.4µV for the 10Hz to 10kHz bandwidth.
I am presently unable to tell how much lower it is: I have reached my measurement limit.

I may be able to give more information in one or two weeks.
I have been able to refine the measurement, now that I have a proper measuring gear: the noise in the 10Hz to 10kHz range is revised to 0.25µV, but this does not represent the ultimate limit: the circuit is still in the breadboard form, and most of the visible noise are bursts from the PLC, pervading everything.
The second contributor is 50Hz/100Hz + harmonics induced from the environment.
"True" noise is thus somewhat lower, probably in the 0.15µV region
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Old 17th July 2019, 03:04 PM   #12
Mark Johnson is offline Mark Johnson  United States
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Just for fun: a superreg with <12 discretes??! ?! ?
The schematic in post #11 confuses me a little bit.
  • Is the base of transistor Q5 actually connected to the regulator output node?
  • Does any current flow in resistor R3? Its two terminals appear to be shorted.
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Old 17th July 2019, 03:24 PM   #13
basreflex is offline basreflex  Spain
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Q5 is a cascode, and the output net is a convenient level for its base.
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Old 17th July 2019, 03:59 PM   #14
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Mark Johnson View Post
The schematic in post #11 confuses me a little bit.
  • Is the base of transistor Q5 actually connected to the regulator output node?

Yes, it is, for the reason given by Basreflex.
However, it leaves little breathing space for the transistors, and it demands a low Vto from the PMOS to be able to operate in ~LDO mode, which is why I opted for a divider in the practical application I used it for:

Click the image to open in full size.
  • Quote:
  • Does any current flow in resistor R3? Its two terminals appear to be shorted.
It is one of the open-loop enhacement tweaks: when the short is removed, this small resistor (it could be a few mm of track) creates a negative resistance that compensates the residual output impedance.

It can be made larger, to pre-compensate the downstream wiring resistance for example.

R1/C1 forms the open-loop ripple compensator when connected to the emitter of Q5.

All that is explained in the first post
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Old 13th October 2019, 06:08 PM   #15
jean-paul is offline jean-paul  Netherlands
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Hi Elvee, is C2 really 47 µF? Are D1 and C4 connected right? Just asking
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Old 13th October 2019, 08:44 PM   #16
Elvee is offline Elvee  Belgium
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Yes, C2 is actually 47µF, and has a 100V WV, which might look vastly excessive, but the lowish capacitance resonates (slightly) with this particular transformer's leakage inductance and mandates a high enough voltage rating to cope with the reactive role, something borderline for standard Ecaps.

This value just balances the the two supply polarities with the drain imposed by the LNA.

These values only apply to this particular transformer, used in these particular conditions, but once the principle is understood, it can be applied elsewhere.

While we are it, the role of D6 (which apparently does nothing useful) is to prevent a polarity inversion of C2 during random, unfavorable cold-start conditions.

D1 and C4 are connected correctly: C4 (very close physically to the diode) eliminates possible switching noise due to snap-recovery (relatively rare nowadays in modern slow diodes), and more importantly prevents it from acting as a PIN diode, imposing a 50Hz modulation to the RF haze pervading our modern environments.
This supply was custom-built built for a sensitive LNA, which explains those precautions (and no, I am certainly not a paranoid crackpot chasing non-existent problems, I am 100% objectivist, and these precautions are a result of real-world, measurable experiences).

This pic illustrates the operation of rectifier part of the supply, and I also include the asc if you want to check by yourself


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