Electronic switch maintaining a predetermined state independent of supply voltage variation

In a Schmitt trigger circuit having supply voltage furnishing means (8, 11,) for furnishing a supply voltage, an input amplifier element (T.sub.1) connected to said supply voltage furnishing means, said input amplifier element having an input control circuit and an output control circuit, said output control circuit having a conductive and a nonconductive state in the absence and presence, respectively, of a switching signal applied to said input control circuit when said supply voltage is substantially equal to a nominal supply voltage, said output control circuit switching uncontrollably between said conductive and nonconductive state when said supply voltage decreases below a critical supply voltage substantially less than said nominal supply voltage, and an output amplifier element (T.sub.2) connected to said input amplifier element and said supply voltage furnishing means, said output amplifier element having a conductive and nonconductive state when said input amplifier element has said nonconductive and conductive state, respectively:

• apparatus for maintaining said output amplifier element in said nonconductive state in the absence of said switching signal independent of the value of said supply voltage throughout a predetermined supply voltage range extending from substantially zero supply voltage to said nominal supply voltage, said apparatus comprising first biasing means (9, 10, 12, R.sub.1, R.sub.2) connected to said input control circuit of said input amplifier element, for furnishing a DC voltage adapted to maintain said output control circuit in said conductive state in the absence of said switching signal throughout at least a first predetermined supply voltage range extending from said nominal supply voltage to said critical supply voltage;
• and second biasing means (R.sub.3, R.sub.5, R.sub.6) connected to said output amplifier element for maintaining said output amplifier element in said nonconductive state in the absence of said switching signal independent of said state of said output control circuit throughout a second predetermined supply voltage range extending from said critical supply voltage to substantially zero supply voltage, whereby uncontrolled switching of said output amplifier element during a supply voltage shutoff is prevented.