PURPOSE: To securely remove an undershoot signal by treating retiming of a unipolar signal obtained after differentiating a rise and fall input signals to identify a voltage.

CONSTITUTION: If an input bipolar signal A has undershoot, a comparator 2 identifies voltage at + threshold voltage and a comparator 3 identifies voltage at - threshold voltage, respectively. On the other hand, a differential transformer 4 extracts only the rise and fall of the input bipolar signal A that is input in a differential transformer 4 to create a signal D. The signal D is treated for voltage identification by comparators 5 and 6, and after that passes a logic summation circuit 7 and delay circuit 8, resulting in a signal E. A signal B is treated for retiming by the signal E at a flip-flop 9, resulting in a signal G from which an undershoot signal has been removed.