The invention is a simple amplifier circuit for a current transformer to measure very small AC currents, as required for ground fault detection. The circuit presents a very low impedance to the current transformer output and provides high amplification of the current transformer signal. Concurrently and with no additional components, the inventive circuit supplies a bias current to the current transformer to improve its performance and provides output signals which can be used to detect disconnection of the current transformer or a short circuit between either of the current transformer terminals and ground.