Let say we have two diodes having a common cathode as output and anodes as input. There is also a clamp down resistor at common cathode to ground, could someone explain its utility in all conditions.
Basically, diodes need a minimum current flow in order to be 'ON' through forward biasing of its junction. If the Diode-OR logic has any input as 'high', then the resistor is needed to provide the diode current path. If both inputs are 'low', then the diodes are cut-off and the output, which is the input to the next stage, needs to be pulled down to ground to ensure output in 'low' state. Otherwise, the input to the next stage will be left floating (since the diodes are cut-off) and the logic level sensed is likely not going to be a steady 'low' level. Thus, the value of the resistor is selected to be a compromise depending on the output impedance of the earlier stage and input impedance of the next stage. Too low a value will draw larger current from the earlier stage while too high a value will not ensure 'low' input for the next stage.
Source(Vcc) would be loaded, only by providing resistance, to reached zero, in forward bias, weather it is diode or transistor(not), from no current, one level, in logic circuits, without resistance logic could not be obtained......
If both anodes are below the ON voltage of the diodes, the resistor should be pulled to ground. If one or both of the anodes are above the ON voltage, then resistor will be pulled up to anode voltage minus ON voltage of diode.
Without a resistor to ground, there will be no reference and it would float to some value based on any other circuitry connected to the cathodes when the anodes were both below the ON voltage of the diodes
Basically, diodes need a minimum current flow in order to be 'ON' through forward biasing of its junction. If the Diode-OR logic has any input as 'high', then the resistor is needed to provide the diode current path. If both inputs are 'low', then the diodes are cut-off and the output, which is the input to the next stage, needs to be pulled down to ground to ensure output in 'low' state. Otherwise, the input to the next stage will be left floating (since the diodes are cut-off) and the logic level sensed is likely not going to be a steady 'low' level. Thus, the value of the resistor is selected to be a compromise depending on the output impedance of the earlier stage and input impedance of the next stage. Too low a value will draw larger current from the earlier stage while too high a value will not ensure 'low' input for the next stage.
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