Determine input voltage given resistances

[orangeincup]

Homework Statement

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

Homework Equations

i=v/R
Voltage divider rule v*R1/(R1+R2)=v1

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?

 

[gneill]

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

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.

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

What's the input voltage?

 

[orangeincup]

Oops,

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

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

 

[gneill]

Oops,

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

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

Looking good!