Calculate Fan-Out Resistance of N Gates in Parallel

[gs408]

Hi, pretty basic question, I'm looking through some notes and it says:

The combined resistor value of N gates in parallel is: Rtotal = (1/N) Rgate
The load resistor decrease as we connect more input gates, let us calculate the logic-1 level with a load resister of 10k Ohms. Using Ohm's law:

Vout = 3.5v * (10,000 / N) / [ 1000 + (10,000 / N) ] > 1.7v
( where Vsource = 3.5, Rsource = 1000 and the threshold for indeterminate values is 1.7v )

we get: 1700N < 18,000, so the maximum number we can connect is ten or so.

What does the 10k load resistor refer to? Is it the resistance of one gate? Or the combined resistance of multiple gates? Just wanted to clarify.

Thanks.

 

[berkeman]

Hi, pretty basic question, I'm looking through some notes and it says:



What does the 10k load resistor refer to? Is it the resistance of one gate? Or the combined resistance of multiple gates? Just wanted to clarify.

Thanks.

Yeah, I think they are just saying each gate's input "resistance" is 10kOhms, so as you parallel up N of them, you end up with 10kOhm/N.