abbey road d enfer

Re: Current boosted three terminal regulators and noise
« Reply #20 on: May 18, 2020, 01:36:53 AM »
So it seems to me there is a separate feedback mechanism operating where the the BJT senses the regulator output current independent of the feedback via the diff amp in the regulator.
It's not a separate NFB mechanism. The way the error is sensed is the same, only the internal pass transistor is not the only one under control. The external transistor is controlled by the chip's input current.

Quote
To me, the more obvious topology would have been to use and NPN BJT as an emitter follower so the regulator only has to provide bas current like this:
The regulator raises the NPN base to ensure it is turned on enough to maintain 1.25 across the resistor between the OUT and ADJ terminals and the NPN definitely IS in the feedback loop.

But of course this will not work because with a three terminal regulator we cannot connect the actual supply output to the diff amp input so you have to invent some other method (such as the one in the first pic). To do the emitter follower version you need a four terminal regulator??
Some regulator IC's provide connections for this configuration. Just read recently about it, haven't kept the link though...  :(

EDIT: found the link
https://www.analog.com/en/technical-articles/increasing-output-current-of-the-ultralow-noise-ultrahigh-psrr-lt3042-200ma-linear-regulator.html#
« Last Edit: May 18, 2020, 01:44:34 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


moamps

Re: Current boosted three terminal regulators and noise
« Reply #21 on: May 18, 2020, 05:47:42 AM »
EDIT: found the link
https://www.analog.com/en/technical-articles/increasing-output-current-of-the-ultralow-noise-ultrahigh-psrr-lt3042-200ma-linear-regulator.html#

I posted this link on the previous page.
Anyway, I will just repeat that internal transistor is usually  NPN type which forms the CFP or Sziklai pair with external PNP transistor. This CFP can be analyzed as one high power, high gain NPN transistor which is, of course, inside the feedback loop. 
https://sound-au.com/articles/cmpd-vs-darl.htm



ruffrecords

Re: Current boosted three terminal regulators and noise
« Reply #22 on: May 18, 2020, 05:54:41 AM »
It's not a separate NFB mechanism. The way the error is sensed is the same, only the internal pass transistor is not the only one under control. The external transistor is controlled by the chip's input current.
 Some regulator IC's provide connections for this configuration. Just read recently about it, haven't kept the link though...  :(
Aha! Light bulb. Of course - the error is always sensed the same way - and in the series pass case the result is a change of current through the regulator which is what drives the series pass transistor.

And moamps you were right all along.

Thanks guys.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

abbey road d enfer

Re: Current boosted three terminal regulators and noise
« Reply #23 on: May 18, 2020, 05:59:22 AM »
I posted this link on the previous page.
Yes, I missed it. Sorry for disturbing S/N ratio.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

JohnRoberts

Re: Current boosted three terminal regulators and noise
« Reply #24 on: May 18, 2020, 12:20:56 PM »
Yes, I missed it. Sorry for disturbing S/N ratio.
sometimes the light bulb doesn't go off right away and answers get repeated.

JR

Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Re: Current boosted three terminal regulators and noise
« Reply #25 on: May 18, 2020, 01:18:39 PM »
MCI used NPN 2n3055 pass transistors pretty heavily in pretty much all of their PSU designs. Might be worth a look.

abbey road d enfer

Re: Current boosted three terminal regulators and noise
« Reply #26 on: May 18, 2020, 01:26:48 PM »
MCI used NPN 2n3055 pass transistors pretty heavily in pretty much all of their PSU designs. Might be worth a look.
2N3055 was the period workhorse. A tad short on voltage specs for today.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

JohnRoberts

Re: Current boosted three terminal regulators and noise
« Reply #27 on: May 18, 2020, 01:52:20 PM »
MCI used NPN 2n3055 pass transistors pretty heavily in pretty much all of their PSU designs. Might be worth a look.
The old school 2n3055 was reliable workhorse (brick sh__ house) when higher voltage power transistors could be pretty fragile. The original 2n3055 had something like a 200kHz ft (slower than dirt).  Modern 2n3055 devices only resemble the original parts in part number.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Re: Current boosted three terminal regulators and noise
« Reply #28 on: May 18, 2020, 01:56:58 PM »
Sorry for my ignorance here, but would that sluggishness affect its performance as a pass transistor? If so how?


JohnRoberts

Re: Current boosted three terminal regulators and noise
« Reply #29 on: May 18, 2020, 02:04:32 PM »
Sorry for my ignorance here, but would that sluggishness affect its performance as a pass transistor? If so how?
It doesn't help, but they were used because they were literally hard to kill....  8) (Academic now since they are long gone).

Voltage regulators already have rising output impedance with high frequency due to falling internal open loop gain at HF...

Back decades ago I discovered that around a 1,000uF cap across the output of a typical 3 terminal regulator would flatten out that rising output impedance for another octave or two above audio passband of interest. Modern regulators are faster than the ones I put on my bench back then, but capacitors still supplement power rail HF behavior.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.


abbey road d enfer

Re: Current boosted three terminal regulators and noise
« Reply #30 on: May 18, 2020, 02:07:28 PM »
Sorry for my ignorance here, but would that sluggishness affect its performance as a pass transistor? If so how?
In essence, a voltage regulator is a power amp that amplifies DC; however, it also must counteract the assaults of load variations. A slow pass transistor will have trouble dealing with fast load variations. Depending on the application, it may or may not be an issue.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

ruffrecords

Re: Current boosted three terminal regulators and noise
« Reply #31 on: May 18, 2020, 02:16:35 PM »
2N3055 was the period workhorse. A tad short on voltage specs for today.
But still in common use in linear power supplies. International Power must use truck loads of them and I can attest their power supplies are very  quiet. (they also use the LM723 as a regulator).

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

JohnRoberts

Re: Current boosted three terminal regulators and noise
« Reply #32 on: May 18, 2020, 04:19:14 PM »
But still in common use in linear power supplies. International Power must use truck loads of them and I can attest their power supplies are very  quiet. (they also use the LM723 as a regulator).

Cheers

Ian
I used an ua723 VR for the PS in my first Phoenix Systems audio kit (BBD delay line) back in 1976. I haven't used one since.

JR 
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Re: Current boosted three terminal regulators and noise
« Reply #33 on: May 22, 2020, 02:53:26 PM »
MCI used NPN 2n3055 pass transistors pretty heavily in pretty much all of their PSU designs. Might be worth a look.

And they kept blowing up like fireworks!


 

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