Circuit arrangement for forming the difference and the sum of two detected signals

Claim 1

A circuit arrangement for forming the difference and the sum of first and second detected signals, comprising a first detector (D 2 ) and a second detector (D 3 ) connected to each other in back-to-back fashion for forming a junction (J), a first operational amplifier ( PI ) connected with its input to said junction (J) to directly form a difference signal from currents representing said first and second detected signals and flowing through said first and second detectors (D 2 -D 3 ), a second operational amplifier (O PII ) connected with its input to said first detector (D 2 ) for inverting and amplifying said first detected signal to provide an inverted and amplified signal, a third operational amplifier (O PIII ) connected with its input to said second detector (D 3 ) for supplying said second detected signal to said third operational amplifier (O PIII ), said second and third operational amplifiers virtually short-circuiting said first and second detectors (D 2 , D 3 ) to ground, and resistor means (R 4 ) connecting an output of said second operational amplifier (O PII ) to said input of said third operational amplifier (O PIII ) for forming said sum from said inverted and amplified signal and from said second detected signal.