Electronic ballast having emitter coupled transistors and bias circuit between secondary winding and the emitters

Claim 1

An electrical ballast for energizing fluorescent lamps comprising: first means for converting an alternating source voltage into a substantially direct current voltage; second means for converting a direct current voltage into an amplified sinusoidal voltage having a relatively high frequency that is generally inaudible to humans, said second means including first and second power transistors, each having a collector, a base and an emitter, said emitters being coupled together; a tank circuit including a primary winding of a transformer connected to said collectors, said tank circuit being resonant at said relatively high frequency; a secondary winding of the transformer having its opposite ends connected to said bases and being inductively coupled to said primary winding, said secondary winding serving to feed back a portion of the voltage developed by said tank circuit; and bias circuit means coupled between the opposite ends of the secondary winding and said emitters, said secondary winding and said bias circuit means serving to alternatively bias the base-emitter junction of said first and second transistors with a substantial portion of the feedback voltage developed across said secondary winding, whereby when said direct current voltage is connected across said base and said emitter of said first transistor, said amplified sinusoidal voltage is developed across said primary winding; at least two fluorescent lamps connected in a series configuration across said primary winding; and capacitive means having a relatively high capacitive reactance at the frequency of the alternating source voltage for isolating said fluorescent lamps from said second means for converting such that should said lamps be inadvertently connected to ground, electrical stress will not be transmitted to said second means for converting.