Are there any low end limits to how low of a constant current you can make with a transistor ?

For a simple circuit like the following, is there a lower limit to the achievable constant current ? Would 100uA-1mA be asking too much ?

I'm not that concerned with (yet) about the temperature effects of the current (that might be a seperate question).

I've sim'd it, and everything appears to be in order, but are there any practical limits that a simulation will exclude ?


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Transistor beta falls off as current decreases, so at very low currents it would be impossible to set the current accurately using a set resistor as the base current would be on the order of the emitter current. That being said, according to the data sheet the 2N3904 has a minimum beta of 60 at 100uA, so I don't think that will be a problem in this circuit. \$\endgroup\$ – Bitrex Apr 12 '15 at 2:43
  • \$\begingroup\$ Yeah, what @Bitrex said. But I think 100uA puts you in a good zone for a wide variety of transistors. Once some time ago when I was worried about this, I found a transistor that maintained good beta at lower bias currents. If you are interested I can look it up for you, but I think you are fine at 100 uA for many transistors. \$\endgroup\$ – mkeith Apr 12 '15 at 3:48
  • \$\begingroup\$ @mkeith ya that would be great. Take your time. Every day I learn something new. Love it. \$\endgroup\$ – efox29 Apr 12 '15 at 4:02

The 2N5088 is my favorite for low collector current use. It's gain is specified down to 100uA - at 300! Given that gain rolloff is gradual, you can go considerably lower at the cost of less gain. Here is the data sheet:


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