Patent ID: 6384582
Filing Date: 2002-05-07
Classification: G05F

Abstract:
A controlled current source of a two-wire meas-uring instrument which generates a measurement signal in the form of an output current between 4 mA and 20 mA and which is controlled by a control signal derived from an output signal of a physical-to-electrical transducer, said controlled current source comprising:a first current path, which goes from a first terminal of a DC voltage source, to be connected from outside, via a diode, an emitter-collector path of a first bipolar transistor, a voltage regulator connected to ground, and a grounded current-sensing resistor to a second terminal of the DC voltage source; a second current path, which goes from the first terminal via the diode, the emitter-base path of the first bipolar transistor, a first resistor, a collector-emitter path of a second bipolar transistor, whose conductivity type is complementary to that of the first bipolar transistor, a second resistor, and the current-sensing resistor to the second terminal, with the current in the first current path and the current in the second current path adding to the output current; a feedback path for the output current which goes from the second terminal through a feedback resistor to a noninverting input of an operational amplifier, said noninverting input also receiving the control signal, and an output of the operational amplifier being connected to the base of the second bipolar transistor; and a transistor which renders the first bipolar transistor conductive after turn-on of the DC voltage source, said transistor having its controlled current path connected in parallel with the emitter-collector path of the second bipolar transistor and having its control electrode connected to a tap of a first RC section, containing a series resistor and a first capacitor, the series resistor being connected to the collector of the first bipolar transistor, and the capacitor being connected to ground.