Patent ID: 7197268

Claim:
A printer apparatus comprising: a photoreceptor; a magnetic roll configured to attract development material including toner to a development zone adjacent the photoreceptor; an electrode positioned adjacent the development zone and configured to induce toner transported by the magnetic roll to be released in the development zone by generating a magnetic field induced by a current flowing through the electrode; a current source coupled to the electrode and supplying a current thereto for generating the magnetic field, the current source having a signal generator inputting a signal to an input winding of a transformer exhibiting a parasitic capacitance capable of being modeled by an equivalent parasitic capacitor coupled between a hot terminal of an output winding of the transformer and ground; and a current measurement circuit comprising: a simulation capacitor having a capacitance proportional to the parasitic capacitance of the transformer, the simulation capacitor having a first electrode and a second electrode, the first electrode being coupled to the hot terminal of the output winding of the transformer and the second electrode being coupled to a first node; a second sense resistor coupled to the first node and to ground so that the current flowing through the simulation capacitor flows through the second sense resistor; a first sense resistor coupled to a second node through which a current having a component representative of the output current of the current source and a component representative of the parasitic current flows; a differential amplifier coupled at an inverting input to the first node and at a non-inverting input to the second node; whereby the differential amplifier supplies an output signal proportional to the output current of the current source.