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constant current source design

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Zero999:

--- Quote from: Arjunan M R on June 04, 2019, 04:45:00 am ---If I use matched pair transistors does i have to use emitter resistors?

--- End quote ---
You should be able to get away without them or using much lower values, especially if the transistors are well thermally coupled. Try it.

Arjunan M R:
I will just use 2N3904.
I am going to use a 49.9Ω emitter resistor.
When I was simulating it in LTspice I saw that if you  choose resistor with low resistance like you said(22Ω) the difference between the collector current of each transistor increases so i chose 49.9Ω.

Zero999:

--- Quote from: Arjunan M R on June 04, 2019, 01:45:55 pm ---I will just use 2N3904.
I am going to use a 49.9Ω emitter resistor.
When I was simulating it in LTspice I saw that if you  choose resistor with low resistance like you said(22Ω) the difference between the collector current of each transistor increases so i chose 49.9Ω.

--- End quote ---
LTSpice isn't very good at simulating this, because it assumes the transistors are identical and are at the same temperature.

Arjunan M R:
I have changed the temperature.

Tom45:
Since you don't need much accuracy, why not just use a simple 1 transistor current sink.

The circuit below uses a 2N3904 and 3 resistors. Testing shows that it works down to 0.2 volts.

Current was about 1 ma at 0.2 volts, up to about 1.2 ma at 25 volts.

If you don't have 5 volts available, just recompute the base bias resistor divider for whatever voltage you do have.

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