Abstract: The conductive state of a transistor in a semiconductor integrated circuit is determined by irradiating the transistor with a radiation beam and measuring changes in load current, thereby indicating whether the transistor was conducting or non-conducting prior to irradiation. A correlated double sampling method is employed in measuring changes in load current. A load resistor in series with the device under test is capacitively coupled to a differential amplification means including a plurality of differential amplifiers with buffers connected between successive amplifiers. A system clock is stopped at a predetermined time period prior to irradiating the transistor. A bypass switch shunts the load resistor until the clock is stopped.