resistors in a logic gate

Discussion in 'Homework Help' started by Voltboy, Jan 10, 2007.

1. Voltboy Thread Starter Active Member

Jan 10, 2007
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im just learning about circuits and when i saw that they use resistors in a logic gate i was confused, why u need resistors if its enough with transistors??

2. Dave Retired Moderator

Nov 17, 2003
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Can you post up the logic gate where you have seen this? It largely depends on the technique used to implement the logic gate; for example traditional nMOS logic gates (a precursor to CMOS logic) utilise a resistor in the pull-up path. The current path to Vdd would be through the pull-up resistor providing a logic '1' at the output dependant on the nMOS configuration in the pull-down. Unless the resistors you are referring to are for protection purposes.

A circuit diagram of an example would make an explanation easier.

Dave

Jan 10, 2007
197
0
4. beenthere Retired Moderator

Apr 20, 2004
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Hi,

It seems that you are surprized to find that the inner workings of ic's are not magic. In every case, the functions are done by collections of transistors and/or fet's. Since the semiconductors functions as saturated switches, resistors have to be present to limit current.

5. Voltboy Thread Starter Active Member

Jan 10, 2007
197
0
but then how to know how many Ohms to add, using Ohm law?? beacause i dont know how much the current must be limited

6. Dave Retired Moderator

Nov 17, 2003
6,960
172
Without a circuit to comment on this question is like asking how long is a piece of string. The current capabilities of a transistor vary considerably and knowing how much the current must be limited by is dependant on, not only the configuration of the transistors in the logic gate, but also the transistors used to implement the gate. For example, current flow in CMOS logic (argueably the most popular implementation - correct me if I'm wrong) tends to only be significant during gate transistions, i.e. in steady-state conditions there is no (read as negligible) current flow from Vdd to ground, and the inputs and outputs tend to drive capacitive loads, hence there is negligible current flow from this perspective. If we are looking at current limiting for the inputs and output to the logic gate (i.e. external to the gate design) then this is a distinctly different question than why they use resistors in logic gates.

Dave

7. Voltboy Thread Starter Active Member

Jan 10, 2007
197
0
Thanks alot 