According to an embodiment, an amplifier which amplifies a first signal to output a second signal includes the following elements. The comparator compares the first signal with a third signal to output a fourth signal. The delay circuit delays a fifth signal by a delay time to generate a sixth signal. The first capacitor is connected between a voltage source and a first node that provides the third signal. The second capacitor is connected between the first node and a second node that provides the second signal. The first switch is connected between the second node and a constant current source, and is controlled by the fourth signal and the fifth signal. The second switch is connected between the first node and the second node, and is controlled by the fifth signal and the sixth signal.