4-20 milliampere transmitter

Claim 1

A transmitter circuit comprising an instrumentation amplifier adapted to receive the output voltage of a sensor in an inverted manner and having an output voltage proportional to its input voltage and having its output voltage supplied to a sense input on said instrumentation amplifier, a voltage setting means for establishing a predetermined voltage to a reference input of said instrumentation amplifier, an operational amplifier having an output voltage responsive to a difference between its positive and negative input voltages and having its positive input connected to the output of said instrumentation amplifier, with each of said amplifiers connected to a positive power supply via a positive power supply wire and to a common potential via a common potential wire, said circuit further including an output transistor having a source, drain and gate and with the gate connected to the output of said operational amplifier, an output resistor connected between said positive power supply and the source of said output transistor, with the voltage at the source of said output transistor supplied to the negative input of said operational amplifier, said voltage setting means including a band gap reference circuit and a resistor connected in series between said positive power supply and said common potential, with the band gap reference circuit connected to said positive power supply, and with the voltage between said band gap reference circuit and said resistor supplied to the reference input of said instrumentation amplifier, wherein an output current is generated on a current output wire at the drain of said output transistor which is proportional to the output voltage of said sensor, and whereby the output current drops to zero upon breakage of said positive power supply wire or said current output wire or said common potential wire.