The invention provides a clock sensor that can be operated at very low current and power. This sensor can also detect an input clock signal with a very small amplitude to detect the presence of clock inputs. According to one embodiment of the invention, a clock sensor comprises an input circuit configured to receive an input clock signal; a biasing circuit configured to receive the input clock signal and to provide biasing voltages; and a switching circuit configured to receive the biasing voltage and in response to the input clock signal, to provide a switching signal for switching an output circuit to generate an output clock signal. The input clock signal may have a small amplitude and the output clock signal has a full amplitude.