Push-pull transistor amplifier with driver circuitry providing over-current protection

Claim 1

An amplifier comprising: an interconnection point adapted for coupling to a load circuit; first, second and third bipolar transistors of a first conductivity type, each having base and emitter electrodes with a base-emitter junction therebetween and having a collector electrode, the emitter electrode of said first transistor and the collector electrode of said second transistor being connected to said interconnection point, the collector and base electrodes of said third transistor being respectively connected to the base electrode of said first transistor and to the base electrode of said second transistor; means for applying an operating voltage between the emitter electrode of said second transistor and the collector electrode of said first transistor; a source of substantially constant quiescent biasing current; a variable conduction device with a variable conduction path between first and second electrodes thereof and with a control electrode, the conductance of said conduction path varying in response to a control signal applied to said control electrode, said first electrode of said device being connected to said source of substantially constant quiescent biasing current and being direct coupled to the base electrode of said first transistor and said second electrode of said device being direct coupled to the base electrode of said second transistor; first and second linear resistive elements, said first linear resistive element being connected between the emitter electrodes of said second and said third transistors; and a semiconductor junction connected in series with said second linear resistive element between the base and emitter electrodes of said first transistor with said junction poled for simultaneous conduction with the base-emitter junction of said first transistor.