A possible alternative to read a radiation sensor is to use a current amplifier, a circuit that must have a low input impedance and a high output impedance. Consider the scheme shown in Fig. 3.57. The circuit is very similar to a source follower, with two important differences. First, the gate of the input transistor is kept to a steady potential, which is an AC ground, hence the name common gate amplifier. Second, the sensor signal is applied in the form of a current to the source of the input device M1. The load resistor RL senses the output current and converts it back to a voltage. We derive now the input and the output impedance of the circuit and its current gain.