Control circuit of an electric motor comprising an output transistor of NPN type

Claim 1

A circuit for controlling actual speed of an electric motor, the circuit being provided with first, second, third and fourth terminals, the first and second ones being respectively connected to positive and negative terminals of a d.c. power source, the first and third ones being coupled to the electric motor of which actual speed is supplied to the fourth one in the form of an electric frequency signal, wherein the improvement comprises: first means connected between said first and second terminals for supplying a stabilized voltage which is stabilized in reference to a potential fed to the first terminal and which is arbitrarily varied relative to ground; second means connected to the first means and responsive to said frequency signal representative of said actual speed of the electric motor for generating a difference signal for adjusting the actual speed of the motor to a desired value; third means responsive to said difference signal for amplifying said difference signal and applying the amplified signal to said motor as a control signal, said third means comprising an output transistor of npn type provided with a base, an emitter and a collector and a level shift circuit coupled between said output transistor and said second means, said level shift circuit being resposive to said difference signal to provide a level shifted d.c. signal to the base of said npn transistor, said level shifted d.c. signal being independent of any variation of said power source, said emitter and collector of said output transistor being respectively connected to said second and third terminals.