An input buffer is a part of a semiconductor memory device which compares the level of the external input signal with the level of a predetermined reference voltage and determines whether the external input signal is of a high level signal or a low level signal. The determination result is then outputted by the input buffer to an internal circuit of the semiconductor device. That is, the internal circuit included in the semiconductor device receives the external input signal through the input buffer that has compared the level of the external input signal with the level of the predetermined reference voltage. There are several types of input buffers, but a type of input buffer that is implemented in a differential amplifier configuration having a current mirror is typically found in a synchronous semiconductor device.
FIG. 1 is a waveform diagram of an input signal VIN and an output signal VOUT in a conventional input buffer.
As illustrated in FIG. 1, when a reference level of an input signal VIN (that is, the mean value between a high level and a low level of VIN) inputted to an input buffer is lower than a preset reference level, that is, for example, the reference level of the input signal VIN VDD/2−ΔV is less than the preset reference level VDD/2 as shown in FIG. 1, the high level durations X of an output signal VOUT outputted by the input buffer become shorter than the low level durations Y thereof. As such, whenever the high level durations X of the output signal VOUT outputted by the input buffer end up being short due to lowering of the reference level of the input signal VIN, this could cause malfunction in the internal circuit of the semiconductor memory device that operates with the output signal VOUT.