Patent ID: 7675351

Claim:
A frequency output circuit comprising an output circuit that outputs a rectangular wave signal, and an open-collector transistor whose collector is an output terminal, the collector being connected to a pull-up resistor connected to an external power supply, the output terminal outputting a frequency signal with the same frequency as a frequency of a signal outputted from the output circuit, and the frequency output circuit further comprising: a current limiting circuit for limiting a current that flows through the output terminal such that a high potential and a low potential of an output waveform outputted via the output terminal are smoothly switched, wherein the current limiting circuit comprises: a differential pair circuit having a first transistor with a resistor provided to the emitter thereof, and a second transistor with a resistor provided to the emitter thereof; a third transistor that forms a current mirror circuit with the open-collector transistor, the third transistor being connected to the collector of one of the first transistor and the second transistor; and a constant current source configured to distribute currents to the first transistor and the second transistor through the resistors that are connected to the emitters of the first transistor and the second transistor respectively, based on a difference between the input signal inputted to the base of the first transistor and an input signal inputted to the base of the second transistor.