Current boosted three terminal regulators and noise

shop deals on amazon
Shop deals on ebay
Advertise with us
GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
ruffrecords said:
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.
Click to expand...
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.

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??
Click to expand...
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#
 
abbey road d enfer said:
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#
Click to expand...

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


 
abbey road d enfer said:
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...  :(
Click to expand...
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
 
MCI used NPN 2n3055 pass transistors pretty heavily in pretty much all of their PSU designs. Might be worth a look.
 
RSRecords said:
MCI used NPN 2n3055 pass transistors pretty heavily in pretty much all of their PSU designs. Might be worth a look.
Click to expand...
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
 
Sorry for my ignorance here, but would that sluggishness affect its performance as a pass transistor? If so how?

 
RSRecords said:
Sorry for my ignorance here, but would that sluggishness affect its performance as a pass transistor? If so how?
Click to expand...
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
 
RSRecords said:
Sorry for my ignorance here, but would that sluggishness affect its performance as a pass transistor? If so how?
Click to expand...
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.
 
abbey road d enfer said:
2N3055 was the period workhorse. A tad short on voltage specs for today.
Click to expand...
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
 
ruffrecords said:
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
Click to expand...
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 
 
RSRecords said:
MCI used NPN 2n3055 pass transistors pretty heavily in pretty much all of their PSU designs. Might be worth a look.
Click to expand...

And they kept blowing up like fireworks!
 

Similar threads

ruffrecords
OPTO specs believable??
2 3
Replies
40
Views
3K
Cqwet Dbdfte
Cqwet Dbdfte
A
Interesting resonant low-pass filter
Replies
1
Views
346
amageddian
A
V
Adapting the LM3914 Linear Bar-Graph to a Logarithmic Scale
Replies
3
Views
1K
vmsa
V
V
Design an analog K-weighting filter
Replies
3
Views
2K
mbn
mbn
S
Plate Reverb Circuit Feedback
Replies
18
Views
3K
Philbo King
Philbo King

Latest posts

Back
Top