Lowe level linear regulator: what does "stabilize" mean? - Page 1
Forum section: electronics / projects

author: miceuz
author_rank: Frequent Contributor
timestamp: April 17, 2012, 05:41:11 pm
content: Hi all,i've hooked up a basic linear regulator as Dave suggests and it simply works -http://blog.hardcore.lt/mic/archives/011064.htmlI've put it under some load of amp or two and it seems to be working just fine - i've seen no oscillations on the scope.Now, i don't believe that it could be so easy, so could someone explain, what does "stabilize" under different loads mean? How do i test it to see it's imperfection?

author: T4P
author_rank: Super Contributor
timestamp: April 17, 2012, 06:12:05 pm
content: I saw in the link that BJT's are not voltage controlled . Wrong .http://amasci.com/amateur/transis.html

author: Kremmen
author_rank: Super Contributor
timestamp: April 17, 2012, 09:13:44 pm
content: Now that was a truly confused explanation. Or actually, the word explanation is wrong because basically the text not only made little sense but it didn't actually explain anything in a quantitative sense.. I don't wish to be needlessly offensive but frankly, that was borderline crackpot and best ignored. A sure sign of this is the statement that "everything we know is wrong..." Let me see... ahh, no. Most everything we _do_ know about the workings of semiconductors is right and we know it to be so, as proved time and again when the equations actually work.You'll get much further by sticking to the standard run of the mill explanations how semiconductors in general and transistors in particular work.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 18, 2012, 02:24:50 am
content: Quote from: Kremmen on April 17, 2012, 09:13:44 pmWith all the "crackpot" phrasing of some of the comments,I expected to have to put my Aluminium foil hat on,but it was pretty much "mainstream" stuff!The author seems to be trying to create a spurious impression of controversy,where none exists.He didn't really tell me anything I hadn't already learned from the semiconductor courses I have attended over the years.They usually included all the points he has made,then went on to point out that the easiest way to work with transistors is to regard them as current controlled devices.There was never any effort to indoctrinate us with a "They are current controlled & that's that!" view.One problem with BJTs is that if you are using one in Class"A" to amplify a sine wave,& you examine the base voltage waveform,it apears distorted,but the collector voltage waveform is not.The grid voltage waveform of a valve (vacuum tube),or the gate voltage waveform of a FET,again operating in Class "A" in each case,do not appear distorted.This makes it easier to accept the concept of current control,as we can imagine the current waveform as still being a pristine sinewave,& the effect on the base voltage waveform being due to the "non-linearity of the base-emitter junction".I have always had the private opinion that as both voltage & current are involved in the base operation of a BJT,that they are "power controlled" devices

author: amspire
author_rank: Super Contributor
timestamp: April 18, 2012, 04:44:33 am
content: Quote from: miceuz on April 17, 2012, 05:41:11 pmThat particular circuit you refer to has an excellent chance of being stable, as the MOSFET does not add to the loop gain at all.To check for stability, for a first test you can find the biggest, lowest ESR electrolytic you can get your hands on and attach it to the output. See if there is any waveform on the opamp output, not the supply output. The big capacitor will hide the oscillation on the output. There will probably be a limit on how large a capacitor you can put on the output without oscillation, but hopefully it is too big to worry about.Second test. If you have a digital oscilloscope, connect a switch to the output so you can switch between no load and full load. You will see some overshoot and undershoot on the supply output as you switch OFF and ON the load. Look for ringing after the transient. You can look for ringing on the opamp output too. Ringing that decays indicates the loop is stable, but under-damped, and it can be a problem, particularly if you connect a load that includes a switching circuit at the same frequency as the ringing.Richard.

author: miceuz
author_rank: Frequent Contributor
timestamp: April 19, 2012, 06:16:36 am
content: Thanks for the answer, Richard,The cirquit already has a capacitor on the output, so you mean, cirquit has limits to the capacitance on the output before it oscilates?

author: amspire
author_rank: Super Contributor
timestamp: April 19, 2012, 06:40:07 am
content: Yes. In general, the hardest load in regards to stability for a feedback stabilized power supply is a large capacitor with an extremely low ESR (effective resistance). For many of the fully specified voltage regulators, if you look into the fine print, they specify a minimum ESR for the output capacitor. Basically, ESR means that as you increase frequency, the capacitor more and more looks like a resistor, and a resistive load is no problem stabilizing.Richard

author: miceuz
author_rank: Frequent Contributor
timestamp: May 31, 2012, 11:48:36 am
content: So... here comes time to stabilizeas i've learned on the net, i had to isolate opamp output from external capacitance and add a capacitor on negative feedback - C7. At first i've tried adding a cap from output directly to IN-, it worked, but i've noticed that when i add feedback after mosfet output, it works better.When i've added current limiting, the output would oscilate heavily when in current limiting mode and the only way to stop that was to add RC network R13, C8 and a feedback cap C6 - this one worked only when i connected it on transistor output.What i'm concerned about is that negative feedback caps are rather big, way bigger than anything i was able to find on the net as examples. And if i add a cap as in Daves design - from VSET to ground - it destabilizes the circuit.Am i doing it right?As i've understood, every gain stage adds a phase shift - every opamp and every transistor on the feedback path, as every capacitive filter. So if i add a feedback cap after transistor output - i'm cancelling it's action in phase shift. Am i right or am i delusive?

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 31, 2012, 02:38:04 pm
content: Quote from: DaveXRQ on April 17, 2012, 06:12:05 pmvery very very interesting read! my conventional "positive current" knowledge start to shattered. i am (was?) among the one who think Ic = hFe.Ib is holy as i've no proper "education" on that.Quote from: vk6gobut he did tell you that some people "believe" Ic = hFe.Ib is holy (religious) right? bet you didnt catch thatQuote from: kremmensome people are stucked with "quantitative" measure. but the "nature" i believe its both "quantitative" and "qualitative"... human made it "quantitative", and choosed to ignore the "qualitative" measure, maybe because its relative/subjective nature thats hard to explain.... "quantitatively"err, where am i? opps, wrong thread! sorry! chiaow!