# Determine input voltage given resistances

1. Oct 3, 2015

### orangeincup

1. The problem statement, all variables and given/known data

Calculate the expected input voltage for each set of resistances below. For the CMOS gate, assume the input resist of the gate is infinite.

2. Relevant equations
i=v/R
Voltage divider rule v*R1/(R1+R2)=v1

3. The attempt at a solution
1)For R1=1K and R2=infinity, would V simply be 0 since the resistance is infinity? I'm mostly confused with how the CMOS gate and the infinity resistance will effect the voltage.

2)For R1=1k and R2=10K
v1=5V*1K/(1K+10K)= 0.45V?

I assume for all the rest, the values with infinity I'd calculate as in step 1, and the values with two specific numbers I'd calculate as I would in step 2?

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2. Oct 3, 2015

### Staff: Mentor

Check your voltage divider equation: The input voltage is developed across R2, not R1.

Redraw the input circuit without R2 (an infinite resistance is equivalent to an open circuit):

What's the input voltage?

3. Oct 3, 2015

### orangeincup

Oops,

1)The input voltage would just be 5V then wouldn't it?

2) v1=5V*10K/(1K+10K)=4.545V?

4. Oct 3, 2015

### Staff: Mentor

Looking good!