1. Field of the Invention
The present invention relates to a delay circuit, a testing apparatus, and a capacitor. More particularly, the present invention relates to a delay circuit which generates a desired delay time by changing the junction capacitance of a field effect transistor.
2. Description of the Related Art
FIG. 1 shows a conventional delay circuit 300. The conventional delay circuit 300 has a first buffer 302 which shapes the wave form of an input signal and then outputs the resultant shaped signal, a path 306 through which the output signal transmits, a first capacitor 312 which adds capacitance C to the path 306, a second capacitor 314 which adds capacitance C′ to the path 306, a first switching device 308 which electrically connects or disconnects the path 306 with the first capacitor 312, a second switching device 310 which electrically connects or disconnects the path 306 with the second capacitor 314, and a second buffer 304 which shapes the wave form of the signal that has transmitted through the path 306 and outputs the resultant shaped signal. A control unit not shown in the drawing controls the switching devices 308 and 310 so as to change the capacitance added to the path 306. In this way, the control unit not shown in the drawing delays the signal that transmits the path 306 by a desired length of time.
The conventional delay circuit 300 achieves a fine delay resolution by selectively adding either the capacitance C or the capacitance C′ which differs slightly from the capacitance C. However, in the conventional delay circuit 300, the channel capacitance of the first switching device 308 differs from that of the second switching device 310, and the wire capacitance of the wire which connects the first capacitor 312 with the path 306 differs from the wire capacitance of the wire which connects the second capacitor 314 with the path 306. These capacitance differences influence the capacitance added to the path 306. As a result, the desired fine delay resolution which is designed to be achieved by utilizing the fine difference between the capacitance C and the capacitance C′ has been very difficult.