Current sources with Howland topology made with operational amplifier are often used for sensors that requires constant current polarization, for bioimpedance measuring, actuators driving systems and so on. Theoretical determination of overall performances for this circuits is generally known. This paper presents a practical method for real output impedance determining of such current sources based on performing two simple measurements. The results is according with the measurements on the practical circuit. Using the TL084 circuit the frequency cut-off was 250 KHz but using one other circuit as THS3201 the cut-off frequency was 20MHz.