How to make a constant current source using an op amp circuit

1. Basic principles:

The principle of making a constant current source with an op amp is the addition and subtraction circuit of the op amp.

The circuit needs a reference power supply and a sampling resistor to determine the size of the output current. The potentials at both ends of the sampling resistor are compared and the sampling output is controlled to ensure that the voltage on the sampling resistor remains constant, thereby ensuring the constant output current.

2. Basic circuit:

The figure below is a typical constant current source circuit, the reference power supply Vref, and the sampling resistor RS.

3. Circuit analysis:

It can be seen that it is actually an adding circuit: its output Vo is the result of the superposition of two inputs Vref and Vo'.

V01=(Vref/2)*2=Vref

Vo2=(Vo'/2)*2=Vo'

Vo=Vo1+Vo2=Vref+Vo'

Then there are:

Vo-Vo'=Vref

Sampling the current in resistor RS:

I=(Vo-Vo')/RS=Vref/RS

If the current taken is very small, it satisfies:

IR<

In the actual circuit, as long as R>>RS is satisfied, IR<

Then there are:

Io≈I=Vref/RS

Therefore, the output current only depends on the reference voltage Vref and the sampling resistor RS, and has nothing to do with the output load.