What is an Op-Amp Constant Current Circuit?

[MD68]

Hi All,

Howdo I figure out how to make one of these work? I know the basics and formula, but things like Offset Voltage and voltage drops across various places confuse me. How do I know how to choose the bias voltage and the sense resistor value? How do I know what kind of op-amp to use in the first place? What are the tolerances? What happens if I put a load on to it that requires more voltage than the + rail voltage to keep the current constant? What happens If i put a direct short across it instead of a load?

Thanks

 

[FvM]

Howdo I figure out how to make one of these work? I know the basics and formula, but things like Offset Voltage and voltage drops across various places confuse me. How do I know how to choose the bias voltage and the sense resistor value? How do I know what kind of op-amp to use in the first place? What are the tolerances? What happens if I put a load on to it that requires more voltage than the + rail voltage to keep the current constant? What happens If i put a direct short across it instead of a load?

Some aspects are discussed in Horowitz, The Art Of Electronics, Chapter 4.07 Current sources.

For special questions, e.g what's the useful operation range of actice current sources, I suggest to play around with a cicruit simulator and be watchful.