Talk:Current source

message: Looking at the information shown here there is nothing showing a circuit that has a jfet in it. See http://www2.eng.cam.ac.uk/~dmh/ptialcd/csink/csink.htm for an example. I am willing to draw up such a circuit.
Chaney44145 (talk) 18:50, 28 December 2007 (UTC)
replies: text: Heh. That's from my engineering course 1st year (currently in second year). Anyway, my actual point is related... the symbol we've been taught to use for a current source is different to those used here (we use two overlapping circles to represent a current source)... unless my university is completely out of touch, there must be a lot of people who are used to seeing this - could it be listed as an alternative symbol scheme? Whose standard is the one adopted by wikipedia? Andipi (talk) 08:50, 20 May 2009 (UTC)
thread_title: Missing jfet Circuit

message: Most people seem to want to limit the discussion of current sources to those of which they have heard and are not prepared to look at the wider view of the concept.
If people want to limit this page's content to current source circuits based on semiconductors then that is fine by me. But then it should be called 'current sources made from semiconsuctors' or such like and we put the other sorts of current sources in a different article.
Exactly the same argument I apply to te voltage source page where people are trying to limit content to what they have personal experience of. Surely Wikipedia is not intended to be narrow minded like that is it?? Comments please from other interested parties Light current 16:22, 8 August 2005 (UTC)
thread_title:  Scope of Term 'Current Source' 

message: How about this for terminology, this may seem obvious but shouldn't the simple transistor constant current source be called a constant current sink?  It is not sourcing current, but rather sinking current from the load...Just a thought.
I agree with this last comment - all the circuits described are current sinks rather than sources. Mike. 80.195.172.5 (talk) 10:05, 7 March 2012 (UTC)
thread_title:  Source or Sink? 

message: Please read this article: http://www.eecs.utoledo.edu/~rking/elecdesign/EDmanualB.pdf
The bootstrapping technique is described just before the middle of the article. Although this schematic also has an emitter-follower, according to it, R2A + R2B (R1 + R2 as per my description & schematic) determines the Q-point current of the transistor - A point that I made, and was opposed by Light Current. Also the article has no mention of the emitter follower being required (or otherwise), so I guess my assumption for the sake of describing it is neither right nor wrong. The effect of the emitter follower is simply that the voltage at the output end of the bootstrap capacitor is just less than that at the collector - in my case it would simply be equal. In short, the resulting constant current is given by (K.Vx + Vb)/R2B as per the article. In the PDF article, K < 1 (due to emitter-follower Vbe drop), Vx is the collector voltage and Vb is the voltage at the emitter of the emitter-follower which is 0.65V less than Vx. In my case, looking at the schematic, K = 1 and Vb = 0 since no emitter-follower is there, so as per the equation it still works. It becomes Vx/R2 in my schematic.
I hope this clarifies my point of view a bit. Rohitbd 15:12, 12 August 2005 (UTC)
replies: text: The statement shown on p56 of of the article http://www.eecs.utoledo.edu/~rking/elecdesign/EDmanualB.pdf
is incorrect. R2a and R2b do NOT determine the Q point current of Q2. What they do do is determine the VOLTAGE at the collector of Q2 (in conjuction with the bias network (which is probably an amplified diode voltage source)). The rest of the page does appear to describe bootstrap operation correctly although I have not checked every line. This article apears to have been written by a student and therfore can not be given much credence. Any one agree??Light current 15:27, 12 August 2005 (UTC)
replies: text: ON second thoughts, this article looks to me like a draft form of a book judging by the page numbering etc. But it looks as if the author(who is probably a lecturer not a student (as I had erroneously first assumed)) has not yet picked up his own error about the Q point of Q2. Light current 09:32, 13 August 2005 (UTC)

text: Hey 'O' ,in a spirit of cooperation, if you were to do the bootstrap diagram ( I havent found how to do diags yet - too busy editing), I would be prepared to write some text to go with it. Then we could both look at it and agree a compromise edit on it. How does that sound to you? Light current 15:54, 12 August 2005 (UTC)
replies: text: Actually, the emitter follower is crucial to the operation of the circuit.  The circuit drawn above without the follower has no feedback but it does have a pole and zero in the transfer function due to CB.  Inserting the emitter follower amounts to the insertion of a current amplifier.  NOW, there is feedback.  The output of the current amp is 'fedback' to the input of the current amp via CB and R2.  It is the current amplification by the emitter follower that produces the additional base drive current that is the heart of the operation of a true bootstrap circuit. Alfred Centauri 15:00, 15 August 2005 (UTC)
thread_title: Article about bootstrapping 

message: I have reordered a bit the galvanostat voice and have written a brief comment on its talk page Talk:Galvanostat: please give it a look :). Daniele.tampieri 14:54, 24 December 2006 (UTC)
thread_title:  About the merging of galvanostat voice under the current source voice 

message: Alfred Centauri, I highly appreciate your last edits. Now, the section about the so-called "resistor current source" is where it has to be - at the beginning of the page.
*Only, I do not agree with the first sentence: "By Norton's theorem, a voltage source in series with a resistor is equivalent to a current source in parallel with the same resistor". Norton's theorem (or maybe better, Norton's equivalent circuit) might be mentioned in an eventual section about current source applications as it is made in current-to-voltage applications or in the page of voltage source (if there is any need of this at all). The reason of that is that this arrangement shows how to build a simple voltage source by using a current source. Instead, the section of resistor current source is about how to build a simple current source (and not how to use it). So, I suggest using a phrase similar to "The simplest current source consists of a voltage source in series with a resistor".
*Also, I suggest removing the insertion "(ideally infinite)". If you keep it, you have to explain the phenomenon that an infinite voltage divided by an infinite resistance gives a finite current. In order to do that, I suggest to my students to build consecutively 1mA current source first by using 10V voltage source and 10K resistor, then - 100V and 100K, after - 1000V and 1000K, ...and finally - infinite V and infinite K. In addition, I show that it is a well-known paradoxical trick from our humane routine - to depreciate a harmful quantity by another many times bigger harmful quantity. As a result, they understand the phenomenon.
*From this viewpoint, the assertion below (see Article about current sources on ESP) is just not true
:"A perfect current source can be constructed very simply, requiring only an infinite voltage and an infinite resistance. Unfortunately, this will also provide an infinite current (which will be infinitely constant), but is (infinitely?) excessive for an infinite number of applications."
*By the way, the next chain of thoughts in this source (Article about current sources on ESP) is very not true:
:"A commonly held belief is that local emitter feedback reduces the output impedance - this is simply not true - the output impedance of a common emitter amplifier is equal to the collector resistance, regardless of emitter degeneration. To reduce the output impedance, it is necessary to apply feedback around the entire circuit - from the output back to the input. The proof of this is outside the scope of this article, but it is nonetheless a fact."
:Actually, the collector-emitter part of the transistor of a common-emitter amplifier acting as a current source behaves as an imperfect dynamic resistor (because of the Early effect). The emitter follower with a constant emitter resistor acts as a quite better current source since it is a true circuit with 100% series negative feedback applied exactly around the entire circuit - from the output current back to the input voltage. What more do we want from this bare circuit?
:Circuit-fantasist 17:04, 9 April 2007 (UTC)
replies: text: Alfred, you are absolutely right! Thank you for the advices about my Wiki contributions; I will consider them. As you can see, the true is quite simple - I have a philosophy in teaching electronic circuits and want to share it with a large audience. Maybe, I will really begin creating Wikibooks and will compress the existing Wiki pages where I have contributed. I would be happy, if you help me to simplify and clarify these pages. Thank you again. Circuit-fantasist 17:22, 10 April 2007 (UTC)
thread_title:  Resistor current source 

message: Solar cells are generally regarded as a current source in parallel with a diode and a shunt resistance with an extra resistor connected in series. My background is in Physics as opposed to electronics but are there any objections to my adding this to the page? —The preceding unsigned comment was added by 155.198.186.155 (talk) 17:30, 16 May 2007 (UTC)
thread_title:  Solar Cells 

message: The simplest way to drive an LED from line-voltage AC is with a large series resistor, which functions as a current source.  Very inefficient, due to power loss in the resistor.
The article implicitly only covers DC current sources.  AC is a related but more complex subject.  But some simple cases are important.  We could also drop line voltage to a low voltage to drive an LED with an AC capacitor.  We can calculate power in the capacitor by V x A, but the result is not real watts - the answer is ideally all "imaginary" VA?  In the real world, how do we calculate power dissipation in real capacitors used thus: how hot they would get, what would be safe design, etc?  For example, if two LEDs are anti-parallel, and you want the total average current through the pair to be 20 mA, from a series capacitor, at 120 VAC, what capacitance is needed, what capacitor types would be appropriate, and would power issues in the capacitor be a concern?-69.87.204.197 13:20, 28 May 2007 (UTC)
thread_title:  Capacitor AC 

message: 91.75.70.167 (talk) 04:08, 3 February 2008 (UTC)
thread_title:  wef 

message: "The current then flowing is the IDSS of the FET."
Please list typical values for IDSS.  What is the physical reason for the limitations on this value?  It seems from CLD datasheets that this is only a solution for currents below about 15 mA.  See , for instance.
If more than one CLD can be placed in parallel on a PCB, why can't they manufacture multiple CLDs in parallel on the chip?  Wouldn't that be exactly the same as paralleling a bunch of transistors on the same chip to make a power transistor? — Omegatron 18:47, 21 March 2008 (UTC)
thread_title:  Limitations of current-limiting "diodes" 

message: A section title says "Current sources with series negative feedback" and then the first
part "Simple transistor current source" doesn't even mention the word feedback. Later there is a negative series feedback circuit presented; however it is not mentioned that the first bjt circuit  already in itself has a negative series feedback.
If temperature doesn't change, then Re emitter resistor is not necessary (in theory only!) as the Ic collector current is set by the Vbe base-emitter voltage. But in this case we can't make the assumption that Vbe=0.65V or so, because a very little change in Vbe results in big changes in Ic. (See http://diranieh.com/Electrenicas/Figures/BJT_EbersMollGraph.jpg for example, don't know why the bjt article doesn't have these curves). And of course that's one reason we use the RE resistor, as we can't guarantee a constant base potential (though the zener diode is a good try). The Re resistor kind of forces the current onto the transistor, as small changes in  Vbe will not change the emitter resistor's current much. But still it is possible to change the output current not only by Re but by changing the base voltage (if using a potmeter instead of a Zener). 
Now for the feedback: the voltage across emitter resistor Re is proportional to the output current. If the temperature rises, then Ic collector current increases - but then the Ve emitter potential will also increase. As the Vb base potential is fixed by the diode or resistor divider, and Vbe=Vb-Ve so Vbe will decrease, which acts towards decresing the collector current. This is a series negative feedback. If you simulate the circuit without the emitter resistor you get a much worse temperature dependency. Of course Re is only good so far, but can be useful in many cases.
As for earlier comments: in the output curve, the collector current does have a very little dependancy on Vce, which can be a problem for a very precise circuit and this is also compensated by the feedbacks. (Also the finite value of h22e is also due to this). 
Hoemaco (talk) 11:35, 21 April 2011 (UTC)
replies: text: Your thoughts are valuable. For the first time I see an appeal to emphasize the role of negative feedback in these circuits. BTW, I have problems when using the terms "series negative feedback" and "parallel negative feedback"; as a rule, people do not understand what I mean when saying them. Circuit dreamer (talk, contribs, email) 21:58, 21 April 2011 (UTC)
thread_title:  simple transistor design 

message: I think the load resistance in this circuit diagram should be located above the transistor. The reason for this is that if most of the voltage drops at the load (as intended), the voltage at the emitter of the transistor can easily become too big for it to conduct (since the base voltage needs to be at least 0.7V above the emitter voltage). For an PNP transistor the depicted way would be okay, but this is an NPN transistor. A detailed discussion of this kind of circuit is given at http://www.mikrocontroller.net/articles/Konstantstromquelle#Konstantstromquelle_mit_Operationsverst.C3.A4rker_und_Transistor (sorry, in German).
I already produced a corrected version of the image but can not replace the file because it's imported from wikimedia.  — Preceding unsigned comment added by 5.158.164.237 (talk) 01:09, 12 May 2013 (UTC)
thread_title:  Mistake in Figure 7: Typical op-amp current source 

message: I think we need to include the "interlinked circles" symbol for a current source.  See link.   This is very commonly used in UK engineering practice and in text books/app notes/papers, and is what was used throughout my university course.  I much prefer this symbol because it can be quickly and unambiguously interpreted as a current source. Amj4321 (talk) 22:31, 22 November 2015 (UTC)
thread_title:  Current Source Symbols 

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