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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 23rd December 2018, 08:31 PM   #1
Elvee is offline Elvee  Belgium
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Default Just for fun: a superreg with <12 discretes??! ?! ?

During a discussion about an add-on for upgrading 317/337 regulators, a question has surfaced:

Would it possible to design a "superreg" using only 12 components or less, preferably all discretes?

Here is an answer (to be taken with of pinch of salt, and in the spirit of Christmas magic, something that participants to the previous thread seem to have missed).

This example uses 11 discretes, and outperforms a cap-fitted 317 on all key parameters (without using any cap).
Does it qualify as a superreg?

Certainly not by today's standards, and the DC stability is approximate because of a crude, first-order temperature compensation, but the central question was noise-related, and it does not perform too poorly in this regard (and there is a lot of room for improvements, since the circuit is based on the <12 components constraint, and no cap).

May I remind everyone that this site is named DIYaudio, and members like to fiddle with creative, cheerful and unorthodox solutions (which is not synonymous with worthless), which is why I post this circuit.
Grinches, etc., please don't look further and move along....

Here is the PSRR of the basic circuit: ~80dB

Click the image to open in full size.


This is the output impedance: ~1.6mΩ

Click the image to open in full size.

The measured noise, in a 10Hz to 10kHz bandwidth varies between ~2µV and 4µV, depending on the LED used and the error amplifier transistor.
The most favorable combination (I only made a limited number of tests) is an old Siemens green LED (CQ-something) and a BC337-40 resulting in ~2µV, and if the LED and transistor are from 6N139 coupler (an attractive solution because of the thermal tracking, the single component and the good temperature compensation), this becomes ~2.5µV.

It is possible to compensate some of the parasitic parameters like Early effect to the first order by adding some tweaks.
The practical range of this type of tweak cannot exceed one order of magnitude, ~20dB: beyond, the adjustment become too sensitive to be of practical use, but for DIY applications, 20dB is realistic.
Here is the result with tweaks active (becomes >12 components, of course)/

Click the image to open in full size.

Click the image to open in full size.

Note that R3 is not a physical component: it simply materializes the order of the components on the output track: R5 needs to be upstream of D1.


It is important to note that the circuit is not self-starting: this can be seen as an advantage or a drawback, depending on the application.
In a previous example, I have used this as a "feature".

Disclaimer:
The circuit in its present condition is suboptimal in many respects: its main purpose is to deliver nice figures within the 11 discretes constraint, and although it works (I had to make actual measurements on the breadboarded version), I certainly do not recommend using it in its raw state.

It could become really useful with only cheap and minor amendments/additions, but it will probably remain unable to beat the best silicon $£€ can buy, made by the top foundries.

I will discuss later the practical adjustments required if the circuit has to be used in the real world, by people that do not necessarily have access to Farnell, Mouser, etc. and for whom a 317 is already a kind of luxury (2/3rd of the world population?)
Attached Images
File Type: png SupRpsrr.png (91.4 KB, 604 views)
File Type: png SupRZo.png (91.4 KB, 595 views)
File Type: png SupRpsrT.png (93.6 KB, 573 views)
File Type: png SupRZoT.png (89.3 KB, 586 views)
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Last edited by Elvee; 23rd December 2018 at 09:51 PM.
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Old 23rd December 2018, 10:06 PM   #2
Elvee is offline Elvee  Belgium
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I posted the output impedance graph in an unpractical format.

Since editing of the attachments doesn't seem to be allowed, here it is in a more legible form (if a moderator feels like it, he/she can replace the file and delete this post).
Also included is a pic of the test setup

A clarification:
The picture shows the optocoupler transistor in the cascode role: I made this test, to check whether it would improve the noise, but it didn't, so the error amplifier location is the best possible, both for temperature compensation and noise (irrelevant).
Attached Images
File Type: png SupRZo.png (103.9 KB, 96 views)
File Type: jpg SuperrPic1.jpg (444.3 KB, 120 views)
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Last edited by Elvee; 23rd December 2018 at 10:11 PM.
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Old 25th December 2018, 07:50 PM   #3
Elvee is offline Elvee  Belgium
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I have tested the "lowest noise version", the one using two large caps at critical locations (C2 and C3).

Now, the noise originating from the LED is mostly eliminated, as is the noise caused by the resistance of the feedback divider, and the noise gain is brought to 0dB.

The only significant cause of remaining noise is now the transistor

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 tested both the BC337 and the 6N139 transistor as error amplifier.
As can be expected, the BC337 is marginally better, but so marginally that an additional discrete doesn't seem worth the trouble, based on the present data.

This is an interim result; it may change when I refine my setup (and ultimately, the noise performance will inevitably be dictated by this transistor)
Attached Images
File Type: png Supercap.png (57.5 KB, 225 views)
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Old 30th December 2018, 09:01 AM   #4
basreflex is offline basreflex  Spain
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Default bc337 replacement

the dominant noise will be voltage noise,of both emitter and base resistance.

as standard NPN's have a thin base for high gain, and the base is made of P-material, base resistance is usually higher for NPN's. RF transistors are optimized in geometry for low base resistance.
I found the BFU530A with 0.9dB NF, a good possible replacement candidate..
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Old 30th December 2018, 09:54 AM   #5
EUVL is offline EUVL  Europe
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We have been using a version of this for our headphone amps for the last year :
Voltage Regulator From Discrete Components

Of course not comparable to the Jung/Didden in pure specs.
But it is really simple (9 components without current limit), good stability, and very low noise.
Noise is basically 1x Vbe of 2SC3324/2SA1312 + 1x HLMP6000.
The latter is known for its very low noise.
(Photo is from an older version using LM329 instead of the LED.)
The current source is key to PSRR performance.

Perhaps someone can come up with an even simpler one ?


Happy New Year,
Patrick
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Attached Images
File Type: jpg 171001 SDR Frt.JPG (84.6 KB, 150 views)
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File Type: asc SDR LED PSRR Freq Public.asc (6.6 KB, 33 views)
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Old 30th December 2018, 01:14 PM   #6
Elvee is offline Elvee  Belgium
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Those simple discrete regulators come basically in two flavors: one is the follower output, as shown by Patrick, the other is the common-emitter output.

Both have their peculiarities and advantages: the common-collector is self-starting and more familiar, but the C.E. is LDO by nature, and it has the potential to provide better performances, since the reference voltage will generally be created from the output.
This however makes it non-self-starting. This can be seen as a liability but also as an advantage.
I generally manage to make an advantage of it, like in this example:
A PSU controller on a shoestring

Other starting methods are possible: completely self-starting, with a pre-bias or a capacitive kick-start for example, with two push-buttons, under the control of a µcontroller, etc

Another advantage of the CE topology when used with a LED as a reference is the first order temperature compensation of the Vbe by the LED.
With a 6N139 coupler, the compensation is very close to ideal.

It is in general preferable to actively bias the reference element (LED) rather than just relying on the "natural" current.
In the CE topology, this is inherent, but it can be added in the CC configuration: in Patrick's circuit, this result in ~5dB improvement for PSRR and internal impedance.
It costs one more resistor of course....
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File Type: png PatSuReg.png (125.1 KB, 111 views)
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Old 30th December 2018, 03:45 PM   #7
lineup is offline lineup  Sweden
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Quote:
Originally Posted by Elvee View Post
Other starting methods are possible: completely self-starting, with a pre-bias or a capacitive kick-start for example, with two push-buttons, under the control of a µcontroller, etc
Have you consider making the start-up by generating a start-up pulse?
It only takes
- a capacitor, like 1uF
- a resistor to set the time, like 100kOhm
- a diode, like 1N4148 to block further current when discharged

I have used this method frequently when using common-emitter output regulators
and when supplying reference from the output.
I have seen others using this.
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Old 30th December 2018, 04:15 PM   #8
Elvee is offline Elvee  Belgium
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Yes, that's what I named "capacitive kick-start"
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Old 31st December 2018, 03:18 PM   #9
catd is offline catd  Europe
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Hi!

A similar approach of mine from the late 1970s.
(The drawing is from then, too)
with permanent shutdown after overload
The 1M pot is to minimize hum.
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Old 16th January 2019, 04:46 PM   #10
Elvee is offline Elvee  Belgium
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Here is the first real-life application of these regulators:
Sanity-check + end-result: a x 1000 measurement preamplifier

They are used inside the low-noise wall-wart supply.


The main adaptations are the inclusion of a permanent startup circuit (R11, R12, D5) and the splitting of the cascode resistor into R5 and R10, to give Q2 more room at low dropout voltages.
I didn't include LED bypass capacitors or Early tweaks, as the performance was already more than sufficient
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File Type: png supRegLNA.png (72.4 KB, 82 views)
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