The transmitter is the heart of the 4-20 mA signaling system. It converts a physical property suchas temperature, humidity or pressure into an electrical signal. This electrical signal is a currentproportional to the temperature, humidity or pressure being measured. In a 4-20 mA loop, 4 mArepresents the low end of the measurement range and 20 mA represents the high end.BAPI specifies the power to our current transmitters as a range, 15 to 24 VDC for a humiditytransmitter or 7 to 40 VDC for a T1K temperature transmitter. The lower voltage is the minimumvoltage necessary to guarantee proper transmitter operation. The higher voltage is the maximumvoltage the transmitter can withstand and operate to its stated specifications
If you are sensing a current loop transmission, you can use a resistor;If you are sending a current loop transmission, you can use a current generator. You can build one from an opamp + transistor, or with just four transistors + some resistors. Either way, it is fairly simple.
What exactly are you trying to emulate or achieve and in what enviroment?
So you are talking about a three wire or two wire sensor? Id assume three to keep it simple?