A driver circuit to drive an M-lead in a tie trunk between telephone branch exchanges is disclosed. The driver continues to provide for a current increase as the resistance of the load on the M-lead decreases, until at a particular point maximum current is achieved. After this point, as the resistance of the load continues to decrease, the driver circuit decreases the load current, such that power dissipation in the driver is kept at a safe level for a device. The V-I characteristic of the driver circuit is such that for any given value of load resistance, the circuit has only one possible point of operation which will eliminate any possibility of latch-up. Simplicity and precision are also achieved by this circuit. The accuracy of the performance characteristic is limited only by the tolerance of the passive components used.

A voltage-current converting circuit having an output switching function, comprises an operational amplifier having a non-inverting input connected to an input terminal, a resistor having one end connected to a reference potential line and the other end connected to an inverting input of the operational amplifier, first and second PNP transistors having commonly connected emitters connected to the inverting input of the operational amplifier, and an electronic switch for alternatively connecting an output of the operational amplifier to either a base of the first transistor or a base of the second transistor. With this arrangement, a current output is supplied from a collector of the transistor receiving the output of the operational amplifier through the electronic switch.

A feedback circuit for passing uniformly constant current relative to cha storing devices is disclosed. An open circuit is disposed between two terminals in a feedback loop and these terminals are interconnected through the charge storing devices to close the open circuit, when the uniformly constant current is to-flow for the purpose of either charging or discharging such devices. The feedback loop is included in a voltage regulator which in the preferred embodiments, is comprised only of an operational amplifier and a resistance.

An implant for stimulating the vagus nerve receives commands over a radio frequency link. The implant disables a voltage convertor used in providing the stimulation signal at periodic intervals to monitor for communications attempts. The implant provides radio frequency marker pulses before providing stimulation signals of certain frequencies. During the providing of stimulation signals the implant monitors the voltage overhead present between the voltage source and the load and adjusts the overhead to a desired level. The implant changes the voltage applied to certain internal components during stimulation periods to allow full power operation during those intervals and reduced power levels at other intervals. The implant is reset by the placing a magnet in a given location in conjunction with the application of a radio frequency transmission for a given period.

A method and system for regulating current flow through a core sample from a subterranean formation being tested for electrical impedance, including a variable-frequency current source, means for passing such current through the core sample, means for measuring such current flow, and means for regulating the current flow through the core sample so that it is dependent upon the amplitude of the current flow from the current source and is independent of non-linear variations in core inpedance responsive to the current flow through the core sample.