OUTPUT BUFFER CIRCUIT

PURPOSE:To suppress overshoot and undershoot in an output signal sufficiently by activating 1st and 2nd output circuits to vary an output signal quickly at a change in an input signal, and activating only the 1st output circuit having the same output impedance as the impedance of a signal transmission line in the steady-state. CONSTITUTION:When an input signal I1 changes from a VDD level into a '0' level, a transistor(TR) P11 is turned on and a TR N11 is turned off. Since each gate level of TRs P12, N12 changes to a '0' level via a NOR gate 15 of an auxiliary drive circuit G11, an inverter 19, a NAND gate 20 of an auxiliary drive circuit G14 and an inverter 24, the TR P12 is turned on and the TR N12 is turned off. Thus, the signal transmission line is rapidly charged by the TRs P11, P12 and the output signal rises quickly. Then a high speed operation by low output impedance drive is attained and also overshoot and undershoot of an output signal O1 are reduced.