Abstract: Amplifier units for controlling valves, in particular proportional solenoid valves, comprise a number of components such as an end stage, a power supply, a control electronics and possibly a closed loop control including signal inputs for desired and actual values. The control electronics generates a desired positioning characteristic for each type of valve. According to the invention, the control electronics is replaced by a programmable component including a read-only-memory in which the amplifier is associated to a particular type of valve by loading the read-only-memory with valve specific data which specify a particular type of valve. The invention allows to manufacture and to stock non-programmed amplifiers of identical structure. To make an amplifier unit ready for use a particular program is loaded to suit the amplifier to the valve type.