Microamp constant current source - Page 1
Forum section: electronics / beginners

author: RichardM
author_rank: Frequent Contributor
timestamp: August 03, 2021, 12:26:05 am
content: HI All,I have an old transistor curve tracer and another I built that I was hoping to be able to test/calibrate with a low constant current source (say up to 500 microamps max). I have the option to apply an external bias/current to the transistor base on the curve tracers. I have collated a number of circuits that include the TL431 shunt or LM4040 precision voltage reference and have tried several on the breadboard. I admit I don't really understand some of the circuits, probably never willI have found some circuits don't appear to have the accuracy at such low values of current, the oscilloscope x-y output produces either a very thick line or loop. I notice the eevblog has a video using a Ref102 which I do not have but looks to be interested in > 1ma range.Anyway, I will keep playing and learning but if anyone knows of a simple circuit or has tried something similar I would love to know some details.CheersRichard

author: Zero999
author_rank: Super Contributor
timestamp: August 03, 2021, 08:23:30 am
content: I've used the LM334 with a temperature compensation diode in the past and it works quite well, but needs a bit of adjustment to get an accurate current.The REF200 is more expensive but can give 50 μA, 100 μA, 200 μA, 300 μA, or 400 μA out, accurate to 0.5%, with no adjustment.

author: RichardM
author_rank: Frequent Contributor
timestamp: August 03, 2021, 08:44:40 am
content: Thanks for the reply. I would be interested in knowing what your schematic looks likeI may order a Ref200, just need to think of another $40AU to spend to save postage.Richard

author: Zero999
author_rank: Super Contributor
timestamp: August 03, 2021, 08:50:04 am
content: Refer to figure 1, on page 8 of the TI data sheet.https://www.ti.com/lit/gpn/lm134

author: magic
author_rank: Super Contributor
timestamp: August 03, 2021, 09:03:52 am
content: Just build the common reference+opamp+MOSFET+resistor circuit using any voltage reference and resistor of satisfactory precision that you can get and some jellybean chip like OP07.https://www.ti.com/lit/pdf/snoaa46

author: Terry Bites
author_rank: Super Contributor
timestamp: August 03, 2021, 05:13:58 pm
content: If you want a simple sink or source then a MOSFET circuit is ideal but choose your mosfet and opamp with care or you'll lose a lot of compliance. They can be made adjustable over small spans. They are also prone to instability.I'd use this bipolar configuration with a differential amp and a buffer, the output is set simply by the input voltage + or - . Both of these current sources work in exactly the same way. The AD8221 circuit is the simplest to set up. All that matching business is inside the chip. There are only two programming variables Vset and Rsense. Io=Vset/Rsense. You dont need a precision resistor for Rsense, just a decent tempco. A single voltage referrence is and a pot will let you set anything from a 0 to 500uA. To change from source to sink, just swap Vset and input gnd. Better still make a bipolar referrence to set the ouput. Note that the maximum current is set by the output capabilties of the AD8221 (or other inst amp). The ouput voltage compliance depends on the power supply and the amplifiers. Read the full dirt herehttps://www.analog.com/en/analog-dialogue/articles/a-large-current-source-with-high-accuracy-and-fast-settling.htmlWith +/-15V a compliance of at least +/-12V is achievable. An additional benefit is that the gain of the IA can be changed to increase the output current. As shown the IA has unity gain, if you set the gain to n the current will be n times larger. See IA datasheets for gain setting.

author: RichardM
author_rank: Frequent Contributor
timestamp: August 04, 2021, 09:01:07 am
content: Ok, thanks. Which figure are you referring to specifically ?

author: RichardM
author_rank: Frequent Contributor
timestamp: August 04, 2021, 09:02:33 am
content: Excellent, thanks Terry. I will definitely try this circuit.

author: Zero999
author_rank: Super Contributor
timestamp: August 04, 2021, 09:40:44 am
content: Quote from: Zero999 on August 03, 2021, 08:50:04 amQuote from: RichardM on August 04, 2021, 09:01:07 amSorry, I meant figure 15 on the data sheet. It's the circuit with the diode, which has a negative voltage temperature coefficient, used to compensate for the LM334's positive temperature coefficient.https://www.ti.com/lit/gpn/lm134EDIT:I quickly searched the forum and found the exact circuit I used. It was designed for 50µA.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 04, 2021, 11:25:36 am
content: If you need a bipolar ref to drive the IA circuit, try this. Again it will work with many opamps and series referrences. For the diffamp version you will need to buffer the output from the pot..

author: RichardM
author_rank: Frequent Contributor
timestamp: August 05, 2021, 09:53:03 am
content: Quote from: Zero999 on August 04, 2021, 09:40:44 amNo apology needed, thanks for taking the time to help out.

author: RichardM
author_rank: Frequent Contributor
timestamp: August 05, 2021, 12:20:30 pm
content: The LM334 circuit is perfect for my needs. So simple and stable.