Current mirror as current sensor - strange behavior  - Page 1
Forum section: electronics / beginners

author: szabolcsx
author_rank: Contributor
timestamp: June 28, 2014, 05:22:18 pm
content: Hi everybody. I found a topology which I thought I could use in my power supply as current sensing element. Its a current mirror formed by four transistors. The circuit converts the voltage drop on R1 to ground referenced voltage which could be measured across R3. Actually it works, but it has a problem. If I don't touch the Q2 and Q4, the circuit doesn't turn on. I mean I measure 0 volts across R3. If I touch the transistors it works quite well. It is built on breadboard and on pref board too and I faced the same problem. Based on my experiments it has an accuracy around 2%. I haven't found any information about that behavior. Somebody could help me figuring out why the problem occurs and how to solve?

author: mij59
author_rank: Frequent Contributor
timestamp: June 28, 2014, 06:47:48 pm
content: Hi,You build a differential amplifier without biasing the transistors , non of the transistors can conduct.Connect a resistor ( lets say 4,7 M ) from the collector of Q1 or Q2 to ground.This function is available as a single chip solution, search for high side current sensor.

author: szabolcsx
author_rank: Contributor
timestamp: June 28, 2014, 06:59:48 pm
content: Yes, I saw that kind chips, but this will bi a part of my discrete power supply so I don't want to use chips yet. Actually a figured out by experimenting but I wasn't sure until now. I've used a 1.2M resistor but that introduced some error. So I think I will use a higher resistance one.

author: szabolcsx
author_rank: Contributor
timestamp: June 28, 2014, 07:24:59 pm
content: Actually I have an other problem. With no load the voltage across R3 is about 30 mV. It's not a big problem, but when I put a load and the voltage drop across R1 increases the voltage drop across R3 will be higher than 30 mV after the load is disconnected and it takes some time to "recover". Any ideas?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 29, 2014, 03:22:12 am
content: You've got a positive feedback circuit that depends on the current mirrors being each unity gain. A small imbalance in voltage goes into the Vbe, which requires exact temperature matching too.Tim

author: Andreas
author_rank: Super Contributor
timestamp: June 29, 2014, 09:32:18 am
content: Hello,I would use a LTC6101 for this purpose.With best regardsAndreas

author: SeanB
author_rank: Super Contributor
timestamp: June 29, 2014, 09:51:58 am
content: You probably will want to make a quick jig and use 100 transistors or so so you can match a few pairs for equal gain at the desired operation current. That way your errors will be a lot less. You probably want to group into a few bins and then take each bin and recharactarise at a different current to get pairs that both match and track. If you are really wanting to get the lowest then do again at a different temperature, keeping the devices in a cooler box between tests and a big ice block inside.

author: szabolcsx
author_rank: Contributor
timestamp: June 29, 2014, 10:53:57 am
content: There is an other thing to mention. Without the biasing resistor when the circut worked only if I tocuhed the Q2 and Q4, it was more accurate. I mean that with no load, I got only 3 mV across R3. So I should increase the resistance of biasing resistor?P.S.: As I said, I know that I can buy chips for current sensing but I want to avoid them for now because this circuit has training purposes too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 29, 2014, 10:15:17 pm
content: This type of circuit (with equal emitter resistors, not a Widlar mirror one side) is often used in ICs for bias generation, I think. The output side transistors can be made slightly larger, so the loop gain is greater than 1, a very large resistor provides initial bias (a few nA, perhaps), and series current limiting resistors allow one branch or the other to saturate, limiting current. Or more subtly, a JFET can be inserted in one leg, to act as a current limiting diode.I'm pretty sure I've seen this feedback structure before, anyway -- but I can't convince myself what advantage, if any, it has over just wiring the bias current into current mirrors and using it directly.Anyway, not very valuable for current sensing.It is true that, if you set up a dual-output current mirror (one diode-strapped PNP and two open-collector PNPs, all bases wired together), and apply a voltage difference between the two output transistor emitters, you will get a difference in current roughly proportional to the voltage difference. But the gain is proportional to absolute temperature, and exhibits a tanh characteristic so the linearity is useless beyond about 20mV (at room temperature). You also need to draw 3*Ibias from the sensed line, which may be inconvenient.Tim