BJT Current Miror - Page 1
Forum section: electronics / beginners

author: Dat Tran
author_rank: Contributor
timestamp: August 24, 2018, 02:50:28 am
content: Anyone help me to explain this circuit, I don't know the role of R4 resistor? In my lessons, I didn't see it!This circuit is in TIDUE56 application note of Tihttp://www.ti.com/lit/ug/tidue56/tidue56.pdfThank in advanced!

author: JS
author_rank: Frequent Contributor
timestamp: August 24, 2018, 03:14:01 am
content: R4 is there to bias Q1, if all the current would be going to the bases it could easily saturate them or go out of control, the circuit would be highly dependent on Q1 HFE which is never good.Then you set a working point for Q8 setting it's emitter current, and Q2-Q7 will follow as all they bases sit at the same voltage and have equal emitter resistors. That's a rough explanation, better analysis is required if you want to know exactly how it does it, mainly around R3 and Q1, but there you have a kick start.JS

author: Dat Tran
author_rank: Contributor
timestamp: August 24, 2018, 04:08:27 am
content: Thanks JS, when i study at my school, I don't see anything same that, I learn about Current mirror not include R4 so I really trouble when i see it! but now, it is ok. thanks JS again!

author: JS
author_rank: Frequent Contributor
timestamp: August 24, 2018, 04:23:15 am
content: In theory isn't needed but in practice it's better if it's there. Text book circuits are usually not good practice but oversimplified ones, easier to get the head around but not what will end up in a product.While designing a product two or three major things have to be considered that usually are not considered in text books examples. Reliability, component tolerances and temperature ranges. In a text book you consider a single fixed value, that will change with time, temperature and between components. When you design a product and you need to manufacture thousands of them and all of them should last a long time, you need not to be worried about the tolerances in components or temperature variations making it work different than intended.As you go along you learn this things, add to your circuits books more and more components to do the same thing that you did with two components on first year. One thing that's changing the game is semiconductor manufacturers producing ICs for each application, containing all this considerations where you just need to connect a few external components and get the job done. When you get into a niche application you need to work as hard as usual with less experience as the common products you made were all integrated.JS

author: Dat Tran
author_rank: Contributor
timestamp: August 24, 2018, 11:22:14 am
content: Yeah, You are right! I'm a newbie in electronic and less experiment!

author: P_Doped
author_rank: Contributor
timestamp: August 24, 2018, 04:21:34 pm
content: I would give a slightly different answer for R4.Each of the current mirror legs share the same basic base voltage, so except for different collector voltages and the Early effect, they should have the same collector currents. For all strings but the controlling string, the collector current is the LED current.For the controlling string (leftmost), the base current needed for the follower between collector and base "steals" from the LED current, so there is a difference between the LED current and the collector current. The leftmost string should always have a slightly lower current than the LED currents in the rest of the legs (except for Early effect issues).Of course, this error is helped greatly by having that follower there. It's reduced by the beta of that transistor.Just like those small ballast resistors in the emitter legs of each of the mirrors helps to even out the effects of the LED I-V curves, this R4 helps to ballast the error between the controlling leg and the other legs. It adds a fixed amount of current which is somewhat larger than the sum of all the other base current contributions to the follower current, so the follower current's emitter current is not so sensitive to the actual base current, which can change with temperature, the number of current mirrors wired up, etc.). It does increase the magnitude of the error, but it lessens the variation in that error.I don't agree with JS on danger w/o this R4 causing the bias current to be somehow redirected into the bases of the other legs causing it to "saturate them or go out of control". That doesn't make sense to me.TL;DR R4 is there for reproducibility in results. It increases the error slightly between the left, controlling leg and the mirrored legs, but makes that error more constant over operating conditions.

author: JS
author_rank: Frequent Contributor
timestamp: August 25, 2018, 04:32:05 am
content: Quote from: P_Doped on August 24, 2018, 04:21:34 pmI'm not saying this circuit wouldn't work without R4, but from what I read I think you agree with me that without it the design would be more dependent on HFE of the individual transistors. In that case one string of LEDs will be able to draw much more current than other, and in particular, if q8 has much lower HFE or VBE, or another particular one much higher, it might cause a problem in the long term, having one string drawing much more current than the reference one.By no means I made a complete analysis on the circuit, but my first guess is around there, making the circuit more suitable under, real world, different transistors.JS

author: Kleinstein
author_rank: Super Contributor
timestamp: August 25, 2018, 09:05:32 am
content: The current sharing would still work without R4. The dependence on the HFE of the transistors would not be much different. Just the current through Q1 would depend on the Hfe. As usually HFE goes up with temperature the current through Q1 would go down and this way make the overall LED current to go up a little more. As the voltage over R4 also goes down with temperature, there is not really much gained from R4.

author: Dat Tran
author_rank: Contributor
timestamp: August 25, 2018, 06:20:11 pm
content: Thank all of you!Maybe to everyone understand right the role of R4 in this circuit, we need to solve and analysis this circuit in detail.We need to find out the relationship between the resistance of R4, hFE, Vcc and current through LEDs.