# Diode logic doubt (digital circuit)

• Engineering
Hello guys,

I'm not getting the output of the diode logic circuits:

If both inputs are 1, then there is current across the diodes then the output is 1, but shouldn't it drop because of the resistor R?

If both inputs are 0, then why is there any kind of output?

I'm really lost,

berkeman
Mentor
Hello guys,

I'm not getting the output of the diode logic circuits:

If both inputs are 1, then there is current across the diodes then the output is 1, but shouldn't it drop because of the resistor R?

If both inputs are 0, then why is there any kind of output?

I'm really lost,

If either input is 1 (or if both are 1), then forward current through the diode will cause a voltage drop across the resistor, hence the output is also a 1.

If neither diode has a forward current (both inputs are 0), then the resistor pulls the output voltage down to ground, and hence the output is a 0.

Does that make sense?

Hi berkeman,

If either input is 1 (or if both are 1), then forward current through the diode will cause a voltage drop across the resistor, hence the output is also a 1.

If either input is 1, then current will flow through the diode and there will be a voltage drop across the resistor. Ok I get that... but I don't understand the link between that and the output being 1

If neither diode has a forward current (both inputs are 0), then the resistor pulls the output voltage down to ground, and hence the output is a 0.

If both input are zero then no current will flow through the diodes. I don't get why the resistor pulls the output voltage down to ground

berkeman
Mentor
Okay, maybe the mixing of voltage levels and logic levels is what is causing the problem. Think of the "logic 1" inputs as 5V voltage supplies, and logic 0 inputs as grounding out the inputs. When you apply 5V to either input, that causes a current to flow through the diode and through the resistor to ground. There is about a 0.6V drop across the diode, so that means that there is a 4.4V drop across the output resistor. With normal logic levels, that 4.4V still qualifies as a logic 1.

If there is no voltage source driving the resistor through the diodes, then there will be no voltage drop across the output resistor, and that will be seen as a logic 0.

The output node comes before the voltage drop across the resistor.

If it helps, consider ideal diodes. So if the input is 5v, the diodes are replaced with a short. Now your output and input nodes are the same node and you have...

out/in =5v -----/\/\/\/\/------Gnd