A. c. measuring circuit having an amplifier with feedback path in which a synchronous switch output drives a d. c. meter

Claim 1

IN AN AMPLIFIER CIRCUIT FOR DRIVING A D.C. MEASURING UNIT FROM AN A.C. SIGNAL SOURCE, THE COMBINATION OF: AN OPERATIONAL AMPLIFIER INCLUDING AN INPUT TERMINAL, AN OUTPUT TERMINAL AND A NEGATIVE FEEDBACK CIRCUIT COUPLING SAID AMPLIFIER OUTPUT TERMINAL TO SAID AMPLIFIER INPUT TERMINAL, SAID A.C. SIGNAL SOURCE BEING CONNETED TO SAID INPUT TERMINAL; PHASE SENSITIVE DEMODULATOR MEANS COUPLED IN SAID FEEDBACK CIRCUIT, SAID DEMODULATOR MEANS HAVING AN INPUT CONNECTED TO SAID AMPLIFIER OUTPUT TERMINAL, AND FIRST AND SECOND OUTPUTS; A SOURCE OF A.C. REFERENCE VOLTAGE CONNECTED TO SAID PHASE SENSITIVE DEMODULATOR MEANS WHEREBY SAID DEMODULATOR MEANS IS OPERATED IN SYNCHRONIZING WITH SAID A.C. SIGNAL SOURCE; A PARALLEL NETWORKING COMPRISING A FIRST PATH CONNECTED BETWEEN SAID FIRST DEMODULATOR OUTPUT AND SAID AMPLIFIER INPUT TERMINAL AND A SECOND PATH CONNECTED BETWEEN SAID SECOND DEMODULATOR OUTPUT AND SAID AMPLIFIER INPUT TERMINAL, SAID DEMODULATOR MEANS PROVIDING A DIRECT CURRENT IN SAID FIRST PATH; AND A D.C. MEASURING UNIT CONNECTED IN SAID FIRST PATH TO PROVIDE AN OUTPUT INDICATION VARYING AS A FUNCTION OF THE COMPONENT OF SAID A.C. SOURCE SIGNAL IN PHASE WITH SAID REFERENCE VOLTAGE OF SAID DEMODULATOR MEANS.