Speed control apparatus for DC motor

A speed control apparatus for a DC motor comprising:

• a first transistor having a collector connected through a first resistor to a first end of a power source and having an emitter connected through a second resistor to a second end of the power source,
• at least one second transistor of the same conductivity type as said first transistor having a base connected to the base of said first transistor, having a collector connected through said DC motor to said first end of the power source and having an emitter connected through a third resistor to said second end of the power source,
• a differential amplifier including a pair of transistors of the same conductivity type, said transistors having having emitters connected in common and joined through a constant current circuit to said collector of the first transistor, a first one of said pair of transistors having a base connected to the collector of said second and a second one of said pair of said transistors having a base connected through a reference voltage source to the collector of said first transistor,
• a current mirror circuit including an additional pair of transistors each having a collector, the collectors of said additional transistors being connected to respective collectors of the transistors of the differential amplifier at respective junction points,
• a third transistor having a control electrode connected to one of said junction points and an output electrode connected to the bases of said first and second transistors to control their base currents in response to changes of collector current of one of the transistors of said differential amplifier, and
• a speed adjusting variable resistor connected between the collector of said first transistor and the collector of said second transistor.