Diode OR Gate - the role of resistor

dobry_den

Hi! What would happen if the resistor in the attached diode OR gate removed (connection with the ground would be preserved)?

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berkeman

It ensures that the output is low when both of the inputs are low. Without the resistor, the output voltage would be undefined in that case.

dobry_den

What do you mean by the word "low".. like when the inputs are of zero potential, when they're grounded? In that case the output would be also grounded and therefore defined... Or am I mistaken?

ranger

In digital logic, a logic zero or low would occur when the input is somewhere in the range of 0 - 2V. Its probably less than 2V, that varies depending on technology being used. For TTL devices, I believe it 0V-0.8V for a logic zero.

The output is always grounded via the resistor i.e. the output is voltage drop across the resistor and not taken directly from ground.

dobry_den

ok, thanks a lot. Another problem occurs when I am about to calculate the current in the output - should I take the output branch and the resistor-ground branch as connected in parallel?

ranger

Yup. The current through the resistor is not the same current going to the load connected to the output. The voltage however is the same.

dobry_den

Ok, that makes it clear. Thanks