Perhaps this circuit is stating the obvious, but sometimes the obvious can elude us! Though not as good as many other constant current generators, it’s very simple and very inexpensive. The figure shows the basic idea. The circuit, which would usually be considered as an emitter-follower output stage, is ideally suited to bias current generators with a small compliance. It also could be used in other applications where its poor accuracy and temperature dependence aren’t a problem.
RB is chosen to provide the required output current, IG, and is calculated as follows:
This ignores base currents, etc.
RE is chosen so that at the maximum compliance voltage, the transistor current IT > 0. Stated in another way, the current IE is greater than IC, and therefore the transistor is always conducting.
where VG is the generator’s output voltage.
The variation in transistor current, IT, will directly affect the circuit’s performance. For example, a 10:1 variation in transistor current will cause about 10% variation in IG, and a 2:1 variation in transistor current will cause about 2.5% variation in IG. Assuming a constant VA.
transistor current ratio L =
Current generator variation M @
Equivalent resistance RG@
For example with a supply of VA = 15 V driving a circuit of compliance 1 V to 3 V, putting RB = 680 O and RE = 6.8k gives an output of about 1 mA, a transistor current ratio L of 0.7, a current generator variation M of about 1% and an equivalent resistance RG, of about 150k. Keep in mind that the circuit has -0.3% per °C temperature dependence.