The present invention introduces an arrangement for enhancing the performance of an electronic circuit comprising a phototransistor (Q). Either a common-collector or a common-emitter connected phototransistor (Q) has a main resistor (R<；sub>；L<；/sub>；), and at least one external bias resistors (R<；sub>；L2<；/sub>；, R<；sub>；L3<；/sub>；, R<；sub>；L4<；/sub>；), each in parallel to one another. The microcontroller may directly control the voltage outputs or act via respective switches (S1, S2) regarding each respective resistor. When the electronic circuit with the phototransistor (Q) is switched on, at least one of the external bias resistors (R<；sub>；L2<；/sub>；, R<；sub>；L3<；/sub>；, R<；sub>；L4<；/sub>；) are switched on. The voltage output rise time is short, and when the bias has been set, the external bias resistor(s) are disconnected functionally. This means that during the actual measurement with the electric circuit, only the main resistor (R<；sub>；L<；/sub>；) is used in the connection.
