A simple high-speed low input impedance CMOS current comparator is presented. The circuit uses improved Wilson current-mirror to perform subtraction. Negative feedback is employed to reduce the input impedance of the circuit. HSPICE is used to verify the circuit performance with standard 0.5 µm CMOS technology. Simulation results demonstrate propagation delay of 1.02 ns, average power consumption of 0.9 mW, and input impedance of 137 Ω for 0.1 µA input current at the supply voltage of 3 V.