The excitation circuit according to the invention controls an excitation current (lexc) in an excitation winding (2) of an alternator by means of an on-board network voltage control loop (Vbat+, 3) of the vehicle, and comprises a first MOS power transistor (9) which is connected between a first positive supply terminal Vbat+ (6), and a first excitation terminal (7), and is controlled by the control loop by means of a first control circuit (10), a second MOS power transistor (11), which is connected between the first excitation terminal and a second excitation terminal (8) which is connected to a ground terminal (5) and acts as a free wheel diode, whilst being controlled by means of a second control circuit (12). According to the invention, the circuit additionally comprises a control loop (15) of a drain-source voltage (Vds) of the second transistor.