High speed integrated switching circuit for analog signals

Abstract

A high current capacity junction field effect transistor, or JFET, is provided with certain special circuitry so that it may be rapidly switched on and off, and is protected against fault currents. The gate electrode of the large switch JFET is controlled by a circuit including four additional JFET's of somewhat smaller size, arranged in two pairs, with a transistor interconnecting one of the pairs of JFET's. The first pair of JFET's permits the rapid charging of the sizable gate capacitance of the switch JFET as it is being turned off, with the second series connected pair of JFET's of smaller current carrying capacity serving to continue the current flow, and maintain the gate electrode of the switch JFET at the desired 'pinch-off' voltage, When the switch JFET is being turned on, the transistor is initially energized by the differing current capacities of the two transistors making up the second, current limiting air of JFET's mentioned above, to rapidly discharge the large switch JFET capacitance. Fault protection for the switching circuit is achieved by various techniques including the use of a current limiting JFET, and other components placed to block heavy fault currents which could otherwise occur when the analog voltage applied to the source and drain electrodes of the large JFET exceed the potentials of the positive and negative power supplies of the integrated circuit.