Abstract: A differential amplifier is arranged to amplify faithfully the voltage produced by an ungrounded voltage source to provide the amplified result as a voltage in relation to a reference (ground) voltage. Errors resulting from leakage resistances between the ungrounded voltage source and ground are avoided by providing an inverting amplifier responsive to the output voltage, which draws, through a resistance, a current from a terminal of the voltage source which is proportional to the output voltage of the differential amplifier. This causes a shift of the potential of the voltage source in the direction towards the reference voltage. Optimum results are obtained when the shift is such that the variations in the voltage source being observed are caused to become balanced in relation to the reference potential.

Abstract: A condition responsive element, disclosed as a negative temperature coefficient thermistor, operates in conjunction with a bridge circuit to provide a differential input to a special direct current amplifier system. The amplifier system utilizes a combination of positive feedback and negative feedback with the negative feedback always exceeding the level of the positive feedback under steady state operating conditions. The output of the amplifier means further is connected to the lowest energizing potential for the amplifier means through a network that includes a relatively fixed voltage dropping means so that changes in the negative feedback can be made to change the gain of the amplifier system with a minimum of offset or change in calibration of the condition responsive device.