Abstract: A plurality of input stages having their outputs connected to provide differential currents to a load. When a bias current flows through a selected one of the input stages, the differential currents are provided in response to a differential signal provided to the selected stage. The input stages are each comprised of a pair of emitter-coupled transistors. Resistors are connected in series with the emitters of the transistors and the bias current is directly proportional to absolute temperature. The resistors may be adjusted to cause the offset voltages of the input stages to substantially equal zero at all temperatures.

Abstract: An improved metal detecting device of the beat frequency oscillator type is provided which incorporates a method for determining the side of the zero beat frequency at which a search oscillator is operating. The device includes a search coil conveniently mounted which is part of a search oscillator. A reference oscillator is also provided, the output of which is digitally compared to that of the search oscillator by a digital phase comparator. The digital phase comparator output is applied through a low pass filter to a slope detector circuit. The slope detector output signal is supplied to both an audio output circuit and a filter/comparator. The audio output is sufficient to facilitate location of permeable and conductive substances. The output of the slope detector also contains information which is extracted by the filter/comparator and translated to an audible signal by an oscillator and the audio output circuit.