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Diode Logic Gates

Diode Logic Gates The diode logic gates are the historically first and simplest circuits that implement the Boolean functions OR, AND, AND/OR, OR/AND. These diode elements are peculiar in that they neither amplify input signals, nor are able to execute the NOT function. Now, let's look at the way the OR element is executed. The circuit shown indicates that the high voltage level at the output of diode element OR is set, when one or two inputs are exposed to high voltage levels, which results in opening of diodes VD1 or VD2 (or both of them). If both the inputs are exposed to low voltage levels, the diodes are closed, the current does not flow in the load current and the output voltage is virtually equal to zero. The element's output usually has parasitic capacitance Cp = 25-100 picofarad, which is why the duration of the output edge is extremely low, as the capacitance quickly charges from the source of input signals through the low forward resistance of the open diode, while the fall duration is large, as the diodes are closed and the capacitance is discharged through the resistor Rh. That's why the following inequality is satisfied for the OR diode circuits: edge duration is substantially shorter than the fall duration. Now, let's look at how the AND element is executed. We can see that the high voltage level at the output of diode element AND is set only when the two inputs are simultaneously exposed to high voltage levels, which causes diodes VD1 and VD2 to close. The output signal edge duration depends on the charging time of the parasitic capacitance Cp through the high resistance of resistor R. If one of the inputs, say, the right one, gets exposed to the low voltage level, diode VD1 opens. This involves current flowing from the power supply across the following circuit: resistor R, open diode VD1 and input signal source. Low voltage level is set at the output. The output signal fall duration depends on the time of the Cp parasitic capacitance' discharge through the low forward resistance of the open diode. That's why the edge duration of the output signal in diode circuits AND is substantially greater than that of the fall.