Current mirror fail? - Page 1
Forum section: electronics / beginners

author: Lucky-Luka
author_rank: Unknown
timestamp: September 20, 2019, 06:35:07 pm
content: Hi allI just wanted to realize a current mirror circuit.Something went wrong: the load current keep rising...It looks like there is something that I haven't understood of the theory behind the circuit.Can anyone help me?https://youtu.be/umwotfRn-owCheers

author: Zero999
author_rank: Super Contributor
timestamp: September 20, 2019, 07:25:16 pm
content: It would help if you posted a schematic, rather than a video.Were the transistors matched?They're obviously not in thermal contact with one another.In short, this is exactly what you should expect to happen. The transistors need have a similar hFEand VBEat a given current and be at the same temperature, in order for a current mirror to work properly. Adding emitter resistors may help and glue the two transistors together. The VBEcan be matched using the diode test function on a multemeter. Matchign the hFEis more tricky, but not as critical as the VBE.

author: Lucky-Luka
author_rank: Unknown
timestamp: September 20, 2019, 08:11:23 pm
content: HiQ1 Vbe is 0.68V and Q2 Vbe is 0.75V.You said "The VBE can be matched using the diode test function on a multemeter" but I haven't understood how I can actually do the matching in order to have a better current mirror circuit.Cheers

author: Zero999
author_rank: Super Contributor
timestamp: September 20, 2019, 08:52:34 pm
content: The VBEmust be matched without the transistors in the circuit.Set the multimeter to diode test. Connect the base and collector to positive lead and the emitter to the negative lead. Repeat the process with several different transistors until a matched pair is found.The transistors will need to be at the same temperature to work, so glue them together. 15mA might be too higher current, because it will cause more heating in the transistor driving the LED, than the one with the resistor, as it will have a higher voltage across it.Emitter degradation resistors can also help. Try putting a 470R resistor between each of the transistors' emitters and 0V.

author: Lucky-Luka
author_rank: Unknown
timestamp: September 22, 2019, 07:56:13 pm
content: Thanks for the tips.Glue them together? Isn't glue insulating?Cheers

author: Zero999
author_rank: Super Contributor
timestamp: September 22, 2019, 08:16:21 pm
content: Quote from: Lucky-Luka on September 22, 2019, 07:56:13 pmYes, compared to metal, most adhesives are thermal insulators, but they're much better thermal conductors, than air. I recommend cyanoacrylate, commonly sold as superglue, but be careful, it bonds human skin very well.

author: Lucky-Luka
author_rank: Unknown
timestamp: December 08, 2019, 08:26:32 am
content: Finally I've found time to improve my circuit as you suggested.I didn't use emitter resistors but nonetheless the circuit performance improved.Cheers

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 08, 2019, 10:08:47 am
content: Also helps to cascode the output side (Wilson current mirror, sorta). Then both current-setting transistors see the same voltage and current (give or take a factor of 1-hFE or thereabouts).Tim

author: magic
author_rank: Super Contributor
timestamp: December 08, 2019, 12:12:22 pm
content: Quote from: Zero999 on September 20, 2019, 08:52:34 pmFolk wisdom has it that transistors from one production batch tend to match to 3mV. I have actually tested some small (5~20) lots of a few different transistors using exactly your technique and to my moderate surprise I found it to be true, except for super cheap no-name BC547/BC557s where maybe 33% were outliers but still within 10mV between the extremes.So it would seem that most of the time static mismatch is not going to have significant impact. If I got my math right, the gm=Ic/Vt formula predicts that 2mV mismatch makes some 6~8% difference in collector current and 10mV is less than 1.5x difference and it takes 20mV to get 2x difference. LTspice seems to agree.Thermal tracking is the real problem because 20mV is a result of merely 10°C difference in junction temperature.

author: w2aew
author_rank: Super Contributor
timestamp: December 09, 2019, 11:58:24 pm
content: Also keep in mind that you're not accounting for the burden voltage of the ammeter. When you have the ammeter on the left side (where the current is set), the series resistance of the ammeter is added to the 1K resistor, which lowers the current. When you move the ammeter to the LED side, the reference current on the left is higher because the ammeter's resistance isn't in series with the 1K resistor anymore.

author: Zero999
author_rank: Super Contributor
timestamp: December 11, 2019, 07:02:22 pm
content: Quote from: magic on December 08, 2019, 12:12:22 pmYou're probably right. I haven't tried this with transistors but have found LM334s from the same bag are matched very closely. I made the circuit on page 8 of the data sheet and trimmed it to 50µA. I noticed the regulators from the same bag matched. The diodes will all from the same batch.http://www.ti.com/lit/ds/symlink/lm334.pdfQuote from: Lucky-Luka on December 08, 2019, 08:26:32 amGood, I'm glad you got it working.Yes, emitter resistors aren't completely necessary and they do help but do have their disadvantages, apart from more parts, the volt drop across the resistors reduces the maximum voltage across the load, before it stops working. In hindsight, 470R is overkill for 10mA, as it would cause a volt drop of 4.7V. I'd probably opt for 68R to 120R for a 680mV to 1.2V drop.

author: Yansi
author_rank: Super Contributor
timestamp: December 11, 2019, 07:10:15 pm
content: Or use a transistor pair dedicated for current mirror applications, such as BCV61, BCV62.

author: magic
author_rank: Super Contributor
timestamp: December 11, 2019, 07:22:26 pm
content: Note that the datasheet is silent on the question of internal thermal tracking and even the Early effect. They only guarantee 30% current match at modest 5V and 2.5mW power dissipation.

author: Lucky-Luka
author_rank: Unknown
timestamp: December 23, 2019, 02:11:48 pm
content: Quote from: w2aew on December 09, 2019, 11:58:24 pmIt doesn't have anything to do with the post but I just wanted to say that whenever I've got spare time I watch your videos and I love them.Cheers