Amplifying and holding measurement circuit

Claim 1

An amplifying circuit including holding circuit means coupled to the output of a signal amplifier for the signal desired to be measured, said holding circuit means comprising a sampling capacitor and a holding capacitor, first switch means for intemittently coupling said sampling capacitor to the output of said signal amplifier and thereafter sequentially coupling said sampling capacitor across said holding capacitor, a field effect transistor operational amplifier having its input connected across said holding capacitor, a suitable indicating meter capable of providing a continuous visual indication of the value of a signal being measured connected across the output of said operational amplifier, said holding circuit means further including a second switching device and a discharge resistor intermittently connected in series circuit relationship through said second switching device across said sampling capacitor sequentially intermediate each operation of said first switching device, a properly poled coupling diode connected intermediate the output from the signal amplifier and the first switching device whereby the coupling diode blocks any discharge current from the sampling capacitor back into the output of the signal amplifier and causes it to discharge only through said discharge resistor intermediate each sampling interval, scale changing network means comprising a plurality of scale changing resistors and first selector switch means for selectively connecting a desired one of the plurality of scale changing resistors in circuit relationshiP across the output of said signal amplifier for the signals desired to be measured, a fixed value scaling resistor and a variable scaling resistor, and second selector switch means for selectively connecting either said fixed or said variable scaling resistor in circuit relationship with said plurality of scale changing resistors whereby the circuit may be operated selectively in a normal absolute reading mode with the fixed value scaling resistor connected in circuit relationship or may be operated in a comparative percent of previous measured signal mode with the variable value scaling resistor connected in circuit relationship.