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An instrument-type magnetic amplifier is operated with positive feedback to produce high gain. A negative feedback loop then trades this gain to achieve high effective input impedance. The resultant circuit presents negligible loading to a low-impedance thermocouple for use in temperature sensing and control equipments. Two circuits are analyzed to determine conditions for optimum performance. One circuit has a single-stage amplifier; the other circuit uses a two-stage magnetic amplifier. Amplifier operation is analyzed and the behavior is predicted from the resulting equations. Predicted performance compares favorably with measured performance. The analytical results are used to optimize amplifier parameters and to anticipate the best amplifier for a given circuit. A typical practical circuit is found to have an error of about 2 per cent when adjusted for an optimum gain of 262.