A comparator has a differential pair circuit and a current control circuit. The differential pair circuit has first and second comparator transistors, and is arranged to compare a first input and a second input according to a clock signal to generate a result indicating whether a difference of the first and second inputs exceeds an internal offset. The current control circuit is coupled in series with the differential pair circuit, and configured to provide unequal abilities of drawing currents for the first and second comparator transistors. An amplifier circuit is also disclosed, having a differential pair circuit, a current control circuit, an amplification circuit and a reset circuit.