2-wire (“loop-powered”) transmitter current loops

It is possible to convey electrical power and communicate analog information over the same two
wires using 4 to 20 milliamps DC, if we design the transmitter to be loop-powered. A loop-powered
transmitter connects to a process controller in the following manner:

Here, the transmitter is not really a current source in the sense that a 4-wire transmitter is.
Instead, a 2-wire transmitter’s circuitry is designed to act as a current regulator, limiting current in
the series loop to a value representing the process measurement, while relying on a remote source
of power to motivate current to flow. Please note the direction of the arrow in the transmitter’s
dependent current source symbol, and how it relates to the voltage polarity marks. Refer back to the
illustration of a 4-wire transmitter circuit for comparison. The current “source” in this loop-powered
transmitter actually behaves as an electrical load, while the current source in the 4-wire transmitter
functions as a true electrical source.