Common Circuits You Should Know: The Current Mirror
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- เผยแพร่เมื่อ 26 พ.ค. 2019
- Common Circuits You Should Know: The Current Mirror
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I am not a newbie, I have been into electronics as long as the fella tutoring us, and he is teaching me tricks I didn't know. Thanks for sharing, you are helping people you will never know about. Sharing the knowledge is awesome!!!
Thank you
The reason the current dropped after the third LED was added is the source voltage wasn't high enough to bias all three yellow LEDs and the voltage dropped across Q2 (you only have 6.99VDC). I suspect those LEDs need over 2VDC each to be able to draw the targetted 6mA.
Worth mentioning: this works best if the transistors are closely matched and you also have to bias them such that they are in their active region so they have plenty of wiggle room. Don't saturate them (watch the Vbe!) if you want the current mirroring to work.
Awsome, love these base level electronics vids, really pleased you don't forget us newbies.
Thank you for sharing.
I am interested in electronics. Can you help me know further about this subject.
Thanks again Paul, Hope you are feeling better. Keep the basic coming. I learn something new everyday :)
The problem with current mirrors is they require transistors with matched characteristics. They find application in analogue ICs, where the transistors are matched by the production process that produces them at the same time in the same process.
To work well in a discreet solution really requires the transistors to be matched, same HFE, same VBE. It also helps if the cases of the transistors are pressed together so that they are operating at the same ambient temperature.
There are surface mount IC packs that contain just two transistors, ideal for this application.
Personally I have never found current mirrors particularly useful, because of its limitations, but each engineer has their pet circuit building blocks, be that Darlington circuit or cascode amplifier. I once spent a few weeks working with a French engineer on a rf design. His pet circuit was the cascode amplifier, and his receiver design was littered with them.
It is sometimes easy to identify the designer of a particular circuit by their use of pet circuits, but then it is also possible to identify the designer by their use of particular resistor ,capacitor or inductor values. Those new to electronics sometime believe that every component value is fixed by equations that relate to its place in the circuit. This is certainly true of critical components and it is certainly true that every component value should be selected for a reason that can be justified and defence in a peer design review, but sometimes that reason can be it is the designers favourite value and has therefore been used in other places in the circuit. It reduces cost to keep the bill of materials as short as possible by reusing the same value as many times as possible. This is certainly true of supply decoupling capacitors.
Where complex designs have been produced by a design team, it is not uncommon for the number of component values used to be rationalized during the first complete circuit design review. Taking out the pet values and replacing them with standard values across the whole design. It is not uncommon for designers to draw the circuit topology for building blocks and leave the choice of component values to the integration stage where the whole design comes together.
Nigel Johnson Nice observation! I know I have my pet Circuits and topologies.
@@KissAnalog we all build a library of circuit building blocks that we know work.
Hmm pet circuits? I'm trying to think of what mine are! Not sure if I have one, but I like to use filament light bulbs as quasi constant current sources. And if I can do it with a few transistors, I'll tend to avoid ICs. That may possibly be because I tend to avoid PCB boards when making up one off designs. Tag strip, point-to-point, Manhattan style don't cater easily for ICs. Usually when I use them, that's when it's time to break out the vero board! lol
The circuit demonstrations are very interesting. Nice job Paul.
Thank you for this - such a resource. Just learning basics while building a synthesiser. This is so useful from the point of view of being creative. If I can understand these ‘basics’ (as you call them lol) it will expand my creative possibilities. Much appreciated
Good one Paul I not quite sure where it be used but I love the way you explained it in great detail awesome job thank you for the education.
Great as always Paul, n love the logic shirt, good stuff.
*i love all of your videos, hope you’re doing well*
Do more of these circuits you should know videos. Thanks!
The Q1 with its base connected to its collector is connected as a diode that is forward biased. Why not just use a diode? Because the forward bias current through its base to emitter junction is matched with the forward bias current through Q2's base to emitter junction. Both Q1 and Q2 split the bias current between them. If the base to emitter junctions of both transistors are matched, then the current will split equally. Since both base currents are equal, then both collector currents are equal, or in other words, Q2's collector current *mirrors* Q1's.
Current mirrors are used in Opamp ICs. The transistors are made at the same time using the same process, so the base to emitter junctions of both are matched -- actually the whole transistors are matched. As the temperature changes, both transistors change equally and the collector current changes very little.
Nice explanation and video.
I almost bought a t-shirt, but 30 bucks shipped was just a little too much for my bank account. I'll just spend the rest of the day memorializing. Thanks for the video!
Love this video. Thanks for the help
Great Video! I really appreciate your channel! I will make my next buys at your store front. :)
Very clear explanation. Thanks :)
You are great at teaching people.
I see now.. the advantage using current mirror, control the current flow, without become a voltage divider, as a resistor would be. Very nice example.
Cool implementation of this, I got to try it. Could use variable resistor to also set [variable] current which is what I'll try. Happy Memorial Day!
Happy memorial day 2 years to the day later. Cheers
I simply love your videos
Merci for this explanation.
Very nifty, I like the principle behind it.
Awesome circuit! Regards from Argentina 🙋
Really interesting, man! 😃
Neat little circuit! I'm going to have a play with it a bit later on once I'm done adulting for the day!
Such a cool topic!
Fun fact: These are used extensively inside ICs as resistors in ICs are huge, but transistors are small. They even have weird transistor-like elements with multiple collectors to mirror a given current made by 1 resistor multiple times to feed a known current to multiple parts of the IC.
great video
These are great.
That was good, thanks!
Very nice video Paul! Great subject! I wonder if the drop in current might be due to the circuit running out of working voltage - applied voltage?
I think it has to do with the Beta factor - individual device parameters affecting change. As more and more BJTs are added in the mirror to add multiple current mirror branches, this error increases. There are other designs which mitigate this response - check out the Wilson current mirror!
Most likely. There isn't enough voltage to run 3 leds and the bjt at 6mA.
Interresting Paul!
Can you make a video on how to make diy NCV detector? Nice video btw.
To be fair you should have connected the three leds in paralel to show the mirrored current is limited by R1 of Q1, still I enjoyed your class very much. Thanks.
Except that LEDs in parallel can be problematic as the one with the lowest turn on voltage will tend to rob the others of drive, although admittedly less likely if they are the same colour from the same batch.
Great video Paul, current mirrors are fun to play with. I have used them to get consistent brightness between different colored LED’s many times.
I am going to have to order a 555 shirt but the logic gate joke is so good that I may have to pick it.
I'm not sure what the current mirror would be used for; perhaps a constant current power supply? Would Q2 continue to conduct no matter the load , even a short?
Current mirror are used in a lot of circuits, and since you could set the output current they can be used as constant current sources. You could create a ramp wave generator with a constant current in a capacitor. They're also used in audio and operational amplifier, from the differential input stage to the output stage.
Thanks!
Nice video, did you try to remove the jumper cable connecting the collector and base at the reference transistor and measure the output current series to those leds, I would like to know the measured value. It will be great if you can post it once more. Tnx
Ingenious indeed
What If You put the leds in paralel?
Thank You.
No matter how you connect a load (or more) in the collector of Q2, the current through it is determined by Q1. Q2 acts as a repeater ("mirror")...
The three LEDs would share the 6mA
I liked your quote " close enough for government work"
Thank you. Though how does this compare to simply using a resistor to limit?
I come across a circuit I had to make recently where a MUCH more experienced and knowledgable friend suggested I had to current limit the "COR" out of a 2way radio which would drive via pnp I think it was the "PTT" of a second radio to make a repeater. Now he stated that I had to use a resister to limit the current because otherwise the radio would keep supplying as it's like a short until it burnt something. Now I always used resistors to lower voltage from say 12 to 5 or 3 for leds rather than current limiting. How would a resister which would sink that current reduce that current?
The difference of this circuit and a simple resistor is that it acts as a variable resistor for any load within its limits. If the load changes the current supplied is kept constant. No need to put a different resistor go keep the load functional as needed in terms of current supply. Just remember the "within limits" detail...
Nice!
I am from🇮🇳 india, a land of most ancient culture that is Hindu religion🙏🙏. I love❤❤ your voice and videos so much that I have no words to describe it. I have learnt many many things from your most educational and Inspireing videos. Please make a video on timer ic 4060.
Thank you for your kind words. Peace and blessings my friend.
Yes. I have difficulty with locins.
Is there a circuit you can use to actively control the current flow by means of a micro controller? Like an I2C variable resistor would be great!
Yes! LOADS! Just search for i2c potentiometer!
As Bob Lewis mentioned, pretty much every digital potentiometer IC will do the job, the I2C-enabled ones in particular.
1. With all those multimeters you have I was expecting you to hook 2 up at once on the board.
2. What was the current through q1 when you hooked up all 3 leds? Was it the same as through 12? If not, wouldn't this be a current limiter and not a current mirror?
I showed the current through Q1. As both the collector and base are biased with a fixed resistor, the current can not change. This circuit is a current mirror.
Where we use that! The current is very low
I agree, that there's a lot of things you should know, but if your instructor or reading materials don't COVER it, how are you suppose to know 🤔 I think you should put out a series called, "Things In Electronics, But May Not Have Learned" 🤔😎🤔
appreciated
Hummm neat, current mirror question: Can we use to set a as constant current source of 10mA on a LED do determine its forward voltage using another meter ? Like the 555 timer t-shirt :D
Yes you can or use the diodefunction on your multimeter.
Yes, you can certainly do that. Or, for example, you can sort leds according to their luminance when fed with constant current. This is what (serious) factories do for luminance matching of leds intended for back lights of their instrument displays...
I set everything up as is, checked over and over but I kept getting like 45 mA in second obe and always bang on 6.17 V in first?
Awesome
Could that be one way of controlling the brightness of a common cathode led display ?
Certainly yes!
Nice video, but it doesn't explain how it actually works. If you take out Q1, R1 will set the Ib2 to a fixed value, and Ic2 should be Ib2*Beta, and (theoretically) constant.
What is the purpose of Q1?
If you remove Q1, Q2 will act as a switch. It will be fully driven into saturation when loaded and in this case its collector current will depend exclusively on the load and the voltage applied to it, according to Ohm's Law (ignoring the Vce drop on it). The point is that in this circuit Q1 makes the regulation, whereas Q2 follows it or repeats it if you prefer i.e. it acts like a mirror...
So can you use this as a simple current limiter by replacing the 1K resistor with a variable resistor ?
Yes
Are both vccs must be from the same power supply? Or can the vccs be connected to different power supply?
No.
Hi Paul, I want to mirror what I tell people that are were/in the service. My heart felt Thank You for your service to our country.
Sir thank u very much.only one thing I understood is same current drawn for even more LED'S but what's the concept!!!🤔🤔🤔🤔
en.m.wikipedia.org/wiki/Current_mirror
Can you please always title your drawing with circuit name? I love learning these basic circuits and I love to screenshot your drawings to study later..
Thank You
Hello,
I have a question as well maybe its strange but I want to know what am I missing if you could help me. As far as my knowledge goes if you put resistors in series the current will be constant no matter what. Only the voltage will get changed on the loads. For example. If you would put 3 resistors instead of LEDs for simplicity. All of them same value, the current should be the same only the voltage should divide. But if you get LEDs or resistors in parallel the voltage should be constant but the current draw should be 3 times bigger?And if you put in parallel mode will this circuit still draw just 6mA? and if so, would this be some kind of constant current mode circuit? Or would 3 parallel LEDs draw 3x6mA = 18mA? and with that neglected the "mirror current".
Thanks for your videos the are really great,
I appreciate your work. Wish you a nice day.
With this circuit, whatever you connect as collector load on Q2, the maximum current through it will be the current that Q1 provides. Of course it works as a current limiter but the essence of the circuit is that it keeps the current stable and independed of the load magnitude. So your three leds in parallel will share those 6,2mA. (Equally or not is another story to be explained)...
@@user-pz6cx8zf2y
ohh okej, thank you for your answer. I was just confused because he mentioned that regardless to the load it will never draw more current, and to prove that he put 3 LEDs in series. And that confused me because even if he would put 10 LEDs in series it would still draw that max of 6,3mA because its not demanding more current if you put elements in series but it requiers higher voltage. LEDs would only be dimmer but still would only draw 6,3mA. Is my thinking right? But instead if he would put 3 LEDs in paraller mode he would prove that this circuit would never draw more than 6,3mA.
Thank you very much for your reply. It helped me greatly. I wish you a nice day.
How stable is a circuit? ( temperature drift )
Hey I'm an EE student and I was going to make a website of eclectic electrical engineering videos that I watch to help me understand the subjects for my fellow class mates or anyone interestes. Is it cool if I link to your videos on one of the pages or is that frowned upon?
Feel free
Like what you are doing but the term good enough for government work should be used any where else. With electronics it meant Mil-Spec which was the best things could be, not just good enough for the real world. Just a pet peeves. Keep up the good work.
Liked the video but still confused understand BOTH NPN transistor base current the same, but WHY does that mean the output current the same?
Also PLEASE in these videos give us poor non-techies the reason you would build/use this circuit.
PS marked the video with a like!
Well, keep in mind that minor differences in current sharing between the two bases will always be present, but this is not so important. If theoretically the transistors are absolutely equal in their working parameters there will be no difference in base currents. But this is ideal, not real. Now, the current will be the same because it is simply divided in two parts. One for Q1 and one for Q2. As said these sub currents are equal. When the system is powered, both transistors' bases are (equally) forward biased and their collectors are conductive. The point is that Q1 drops it's collector voltage because of its base bias. This drop adjusts the base drive to a level that stabilises it's collector current to a specific point depended on the collector resistor. Then Q2 "mirrors" Q1, i.e. it reacts like copying what Q1 does. That is all about it.
What kind of camera are you using?
Old Samsung Galaxy S4
I would have liked a diagram of the final circuit that was predawn and not hand-drawn during the video (I find that distracting). Then, I would have liked the flow of current being outlined, with this being the stage that the math and the theory is explained.
I understand the mirror action but I don't understand what applications would need this "non-stiff" supply ??
I thought circuits take only as much current as they are designed to take, i.e. u can't "force" more ... or they can't "demand" more than designed ?? (Ignoring fault conditions)
Any experts want to try & explain to a noob whats so "magical" about current mirrors ??
👍😎☘
Think of it as a type of current limiting circuit. Some components will take every miliamp and burn out, like LEDs. These must be limited.
@@learnelectronics .... thanks for replying... I guess I just don't see the versatility in the mirror... a led simply needs a current limiting resistor so thats easy enough, I have several reference books which go into great detail about current mirror design but are less enlightening on the apps.... 🙄😏
Looks like I need to do more research 🤔
would the current still be limited if you put the LEDs in parallel? oops! i saw the answer........
do you have breathing problem?
It is important to understand that the term "current mirror" indicates the INTENTION NOT THE RESULT.
given the circuit with arbitrary, that is UNMATCHED components the current mirror approximately works ; there will be measurable differences in the
ib currents and hfe of each transistor . As a result Ic1 will not PRECISELY equal Ic2.
given careful design of an integrated circuit the the differences are an order of magnitude smaller, but there are still differences:
It is impossible to determine the molecular structure of each transistor so ib, hfe still have small differences...what is FAR more difficult to control is temperature effects that have a direct influence on the transistor action ! ! !
I cannot understand why people just CANNOT get their head around the fact that IF you short out collector to base of ANY bipolar transistor it DOES NOT and CANNOT have ANY Hfe because it is no longer a transistor!!
The emitter current of Q1 IS the base current and the B-E junction operates as a DIODE!
So why use a transistor? More specifically why it should ideally be the exact same type for Q1 and Q2? It's so that the forward bias current of the Q2 Base is as close as possible to the forward bias current of the "diode" formed by the Q1 B-E PN junction.
Why? Sôo that as the current through the load increases from the base current into Q2, there will be an increase of the B-E Voltage of Q2. T nohis provides an increase in voltage at the Q1 base too which attempts to pull more current from Q2 base ... It's a negative feedback mechanism, nothing more.
The circuit is really a current limit, not a fixed current one. Changes in Vcc will result in changes in the current feeding to the two bases and will cause the output to change likewise.
Should the Vcc supply suddenly go over voltage by a good margin, then it is quite easy to overload the output considerably, possibly blowing Q2 AND/OR the load itself.
The actual current THROUGH Q2 is determined SOLELY via R1, Vcc and Q2 Hfe. (Q1 Hfe does not exist as it is NOT a transistor as wired). Consequently, unlike a proper current source the output current through the load is NOT independent of Vcc. Replacing R1 with a traditional current source WILL allow the output current to be kept stable AND independent of Vcc however. A minor circuit reconfig and three more components will do the trick.
The reason for using a current mirror in such a circuit is to remove any non-load resistance from the output current path. In IC's, where typically Vcc is expected to be pretty constant, internal heat from any series output resistance AND lack of space required for internal, series, "large" resistors helps HUGELY in the IC design.
In EXTERNAL circuits where space may not be at such a premium a much better performance may be obtained from a more traditional current source.
But make NO mistake! A SHORTED NP junction is NOT and CANNOT act in any way as a junction AT ALL therefore Q1 acts PURELY as a forward biased PN junction ONLY (by definition, a DIODE!). Those claiming that a "similar", almost identical result cannot be achieved should try replacing Q1 with a small signal diode (1N4148, BAX16 et al) to prove the point.
Bob Lewis, BTech MIET
“Or close enough to it for Government Work”
These should have been done in parallel.
Shorting collector to base on Q1 simply turns it into a diode. If anyone trying to figure out what's going on should just redraw the schematic with a diode from the base of Q2 and ground . It's just a very poor constant current circuit.
Τhe diode equivalence is misleading. There is real transistor action there. If there is no C-B bridge on Q1, no current will flow through it. With C-B bridged, there is current flow through its base while at the same time the C-E path is now conducting and draws current. This current reduces the base current, which is then kept at a constant level. Then this current is shared with Q2 base and Q2 "mirrors" the action of Q1...It is a very well known circuit for constant current generators.
@@user-pz6cx8zf2y in order for any current to flow there MUST be a potential difference. There can be ZERO potential difference between Q1 collector and base as the two points are SHORT CIRCUITED.
There is NO confusion or misleading here. The transistor Q1 becomes a PN junction which performs EXACTLY as a diode, closely matched to the Base to Emitter PN junction of Q2.
The idea being that the two PN junctions being biased through a single resistor must share their base currents thereby "sharing" the current through the bias resistor.
An increase in current through Q1 base emitter causes voltage negative feedback to lower Q2 base current and increase Q1 base current. THIS is what stabilises the output. The opposite is also true.
Usually however it's more common to use such a shorted collector base junction transistor in pairs and a resistor in between the emitter to ground connection of Q2. This forces a constant voltage across that resistor (the collector resistor would normally not be used) the current is then very accurately defined as Vd/Re usually taken as 0.7/Re.
This circuit then represents a negative constant current source or a positive current one. Changing the components to PNP transistors and the relevant new positions creates a positive constant output current source or a negative current sink.
@@boblewis5558 Ok Bob. Apart from what theoretically exists, you just rejected what you saw in the video. What more is there for me to tell? Just hit "current mirror" in Google and read about it...
@@user-pz6cx8zf2y I didn't say there wasn't a current mirror there. There IS. BUT it DOES NOT operate with TWO transistors. It works With ONE transistor with a "diode" to ground from its base made by shorting out Q1 C to B.
I'm talking about there being ABSOLUTELY ZERO collector current flowing INTO Q1. IT IS AN IMPOSSIBILITY. The internal collector to base voltage is ZERO ergo there can be no collector current. Don't believe me? OK simple, just set up a meter between Q1 collector and the junction of the bias resistor and the wire connecting to the base and tell us ALL exactly how many nanoamps (picoamps?) are flowing THROUGH the meter and INTO the collector.
Then measure the current flowing through the wire to the junction of the two bases. The latter is present the former is non-existent. Learn some basic laws like Ohms law and how a zero resistance (the wire link) and a PN junction reverse biased resistance manage to "share" ANY current? ALL THE CURRENT ALWAYS FLOWS THROUGH THE SHORT CIRCUIT!
The wire SHORTS OUT THE Q1 PN JUNCTION!
A SHORTED PN JUNCTION CANNOT FUNCTION IN ANY WAY AS A TRANSISTOR! IT IS IMPOSSIBLE! IT BREAKS EVERY SINGLE BASIC LAW OF ELECTRON FLOW AND THE PHYSICS OF A TRANSISTOR!
IF YOU DONT UNDERSTAND THIS STAY OUT OF THE CONVERSATION!
@@boblewis5558 You are right about the diode function of Q1 Bob. In fact I have seen this circuit only in text books but never in real form. Instead of that I have seen a variation of that, mostly using PNP transistor, supplying the constant current from the collector against ground. The configuration is the same, with the polarities reversed of course. So, the emitter goes to the positive supply through a properly dimensioned resistor and between the base and the positive terminal (where the emitter is connected) there are either two diodes in series or a zener diode (depending on the design). The cathodes of the diodes (or the zener cathode) are connected to the emitter. Then a single resistor biases the base against the ground. The magnitude of the constant current depends on the diodes (or zener) voltage and the emitter value. This circuit has much better behaviour in terms of stability and is the one I have experience with.
Apart from my mistaken thinking now, I admit it as you see, let me give you a simple advice. We, humans, all make mistakes. These are in my humble opinion the best source of knowledge, just because they are (usually) difficult to forget. What is annoying in your behaviour is your monstrous egoism, about which you should apply some control for your own good. Therefore, calm down, drink a coffee, and think that the sun shines for all of us. No matter if we know what current mirror is or not. Words like "stay out of discussion" do not sound very human to my ears. TH-cam is a public place for exchanging ideas. Not your property anyway. Try to stay human is my advice. Have a nice Sunday!
To everyone reading this comment: BUY one of Pauls Shirts and show if off this summer! You'll be glad you did.
6.3mA OR 5mA ... 😂 it d't matter - it's equal ))))
expert measuring techniks!
HOW did you get Ie1 = Ie2 ?!!!!!!
Are yours left and right resistances is equal ?!!!! what is the bull### are you flow?
For Government Work...lol
This circuit wastes half the power so not sure if it's practical anymore today given all the cheap alternatives....
If you wanna prove that current is stable you should add loads (LEDs) in parallel not in series 🙄🙄🙄
Where is the difference?
When you add resistors in parallel you are reducing the resistance and current goes up.
@@twentycm Exactly. But in this case, no matter what or how you load the collector of Q2, the current supplied will be the same,determined by the resistor on the collector of Q1. Therefore, where is the difference?
Πάρις Αζής True, but I mean it makes more sense to reduce the resistance rather than raising it to prove current is stable, as a matter of fact that circuit can be used as a current source. You are right either way should give the same result.
@@twentycm There is no "should" dear! It puts out the same current, no matter if you put a potentiometer as a load on collector of Q2 and play with it all day. The current will stay at the value preset (mainly) by the collector resistor of Q1. Finally you've got it!!
There geeky ,but you wouldn't be here if you didn't like geeky. I am out :) should have tshirt for that saying!
All these videos are great but WHY are you writing so small? PLEASE WRITE BIGGER!
Nice!