Push-pull amplifier circuit with stable class AB operation

Claim 1

In a push-pull amplifier means having an output terminal, a first and second input terminal, a first and second power supply terminal, a first and a second transistor of opposite conductivity type, said first and second transistors having respective collector-emitter circuits connected in series and connected to said first and second power supply terminals, said first and second transistors further having respectively, a first and a second collector coupled to said output terminal, and respectively, a first and a second base coupled to said first and second input terminals, said push-pull amplifier means further having means for maintaining the sum of the base-emitter voltages of said first and second transistors substantially constant, said means comprising voltage-to-current converter means having a third transistor for converting said base emitter voltage of said first transistor into a first current, and current-to-voltage converter means connected to said voltage-to-current converter means for converting said first current to a first voltage equal to said base-emitter voltage of said third transistor, said current-to-voltage converter means comprising a fourth transistor substantially indentical to said third transistor, said fourth transistor having a base connected to said base of said second transistor, the improvement comprising a differential amplifier having a fifth transistor having a fifth emitter, a fifth collector, and a fifth base and a sixth transistor having a sixth emitter, a sixth collector, and a sixth base, said push-pull amplifier means further comprising a constant current source connected to said first power supply terminal, unidirectional conducting means connected between said constant current source and said second power supply terminal, a common emitter line connected to said fifth and sixth emitters, a first current source arranged in said common emitter line, a second and a third substantially identical current source furnishing a sum current substantially equal to said first current connected to respective ones of said fifth and sixth collectors, means coupling said fifth base to said current-voltage converter, means connecting said sixth base to said unidirectional conducting means, the voltage across said unidirectional conducting means constituting a reference voltage for said sixth base, and means connecting said fifth and sixth collectors to respective ones of said first and second bases.