Understanding the Role of Resistance in an OR Gate Using Diodes

[jd12345]

http://www.tutorvista.com/content/physics/physics-iv/semiconductor-devices/logic-gates.php
I don't know how to post a pic so please go to this site and look at the OR gate

why is resistance R connected to the point C. I don't understand its use? I think the OR gate would function without it too

IF A=1 and B=1 both diodes are conducting and a potential is created at the point Y
If A =0 and B = 0 both diodes do not conduct and no potential is created
So what's the use of that resistance?

 

[wukunlin]

One reason is you need a R because your diodes would otherwise get fried.

The other reason is that you typically use the output of a logic gate to drive another circuit. R represents this following circuit.edit: and I forgot to write the most important reason. In order to measure a output signal you've got to have a load (in this case R) to measure across. Without the R you are either measuring the ground, 0V with diodes blown to pieces, or if the end isn't connected to anything you simply wouldn't have a circuit

 

[willem2]

One reason is you need a R because your diodes would otherwise get fried.

The other reason is that you typically use the output of a logic gate to drive another circuit. R represents this following circuit.


edit: and I forgot to write the most important reason. In order to measure a output signal you've got to have a load (in this case R) to measure across. Without the R you are either measuring the ground, 0V with diodes blown to pieces, or if the end isn't connected to anything you simply wouldn't have a circuit

Why would your diodes get fried and how does the resistance R prevent this?

I don't really get your other answers as well.

If you leave out the resistor, the output won't be connected to anything. This can be okay if use it to drive the coil of a relay, or a LED in series with a resistance, but:

Some inputs, like an opamp will have a very large input resistance, and the positive charge at the output might stick around for a long time, and you're at the mercy of all kinds of electromagnetic noise.

For some inputs you must be prepared to sink a current to get the output to 0V.

 

[cmb]

The resistor pulls the output low if there are no inputs on the diode.

Imagine you had a signal into one diode without the resistor. Then the signal drops off the diode; the output should go low too but the output will remain high - that part of the circuit is still charged up so it needs some path to ground to discharge whatever charge is on the output.

This could be charge simply by virtue that it is a conductor, but also bear in mind that that output might be connected to the gate of a FET, which is capacitive. Also, the diodes will act as small capacitors so if one is 'on', it will charge the other up. That charge is dissipated by the pull-down resistor.

 

[pmacias]

I can explain the role of resistance in an OR gate using diodes. In an OR gate, the output is high (1) if either of the inputs is high (1). This is achieved by using two diodes and a resistor in the circuit.

The resistance R connected to point C serves as a pull-up resistor. It ensures that when both inputs A and B are low (0), there is still a small current flowing through the circuit. This current is necessary to maintain a stable voltage at the output point Y. Without the resistance, the output voltage might be unstable and fluctuate between high and low, resulting in incorrect logic operations.

In addition, the resistance also helps to limit the amount of current flowing through the diodes. This is important because excessive current can damage the diodes and affect the overall performance of the OR gate.

In summary, the resistance in an OR gate using diodes serves to stabilize the output voltage and protect the diodes from excessive current. It is an essential component in ensuring the proper functioning of the logic gate.