Attenuator input and output impedance

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oldjack

Member
Joined 2011
I know this subject has been on many previous threads but I am trying to find out how to calculate the input and output impedance of an attenuator based on a simple type potentiometer, pi type and a pi type with a series resistor at the inlet to the pi network. Its the theory I am after.
These resistor layouts appear on various switched type attenuators.
I think I understand how to work out the input impedance but not completely sure. I am looking at using a switched type attenuator using two 2 pole switches 6 way with pcb mount terminations to give course and fine volume control.
 
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oldjack

Member
Joined 2011
for pi attenuator I add the series resistor with the second resistor to ground and then take parallel sum, ie for r1 (series), r2 first resistor to gnd, r3 2nd resistor to gnd,
input imp = r2 || (r1+r3).
for potentiometer I take input impedance as r1 section of pot where total r = r1 + r2.
 
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oldjack

Member
Joined 2011
Have attached attenuator layouts with my best guess of input and output impedance
 

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S

sreten

R.I.P.
Joined 2003
oldjack said:
Just to clarify what you are saying; is it that the load impedance will be much greater than Rin meaning Rin can be neglected.
I can understand the theory for Rout from your formula except for 5, why does R2 have no effect.
Click to expand...

Hi,

Rload can be neglected assuming its very high, it usually isn't.
Assuming Rsource is very low, an ideal voltage source is safer.

For 5 Rout = R1 + R2||Rsource and if Rsource is zero ....

rgds, sreten.
 
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