# Attenuator resistor formulas ?

1. May 8, 2012

### fran1942

Hello, I am trying to work out the resistor values for an 'O' attenuator.
I am using the attached formula to work out resistor 3's value.
My values to insert into this formula are:

Input impedance (Z1) = 50 Ohm
Load impedance (Z2) = 600 Ohm
a =1.995

However when I substitute these figures I am getting a negative number result.
Obviously I can't have a negative Ohm resitor, so how do you interpret that ?

Thanks for any help

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Last edited: May 8, 2012
2. May 8, 2012

### Staff: Mentor

Your circuit diagram looks like that of a $\Pi$ attenuator, not an O-pad.

There's a minimum attenuation that is determined by the source and load impedances. Have you checked to see whether or not your "a" value meets this criterion?

3. May 8, 2012

### fran1942

sorry, I attached the wrong image.

What does it mean when I get a negative value from one of the resistor formula calculations (as I am in this case).
Obviously I cant have negative Ohm resistors ?