Current Source

HOW IT WORKS

The circuit above maintains a current through a floating load RL (neither end is connected to ground.) The key to the circuit's operation is in placing a current sensing resistor RSENSE in the opamp feedback loop. The current delivered to the load RL is

I = Vin / RSENSE

How does the circuit work? Three simple functions help this circuit accomplish its goal. (Remember, no current flows into the opamp's input terminals.)

1. The opamp automatically adjusts its output (Q1's base drive) to bring its negative input equal to the positive input. This means that Vsense = Vin.

2. A simple application of Ohms law now tells you the current in RSENSE; I = VRSENSE / RSENSE.

3. Q1 delivers the current. And because Q1's currents are related by Ic ≈ Ie, the same current that is developed through RSENSE must also flow through RL.

http://www.ecircuitcenter.com/Circuits/curr_src1/curr_src1.htm