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How to design a digital switch to match 50 Ohm or 75 Ohm RL?

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Member level 2
Aug 5, 2006
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I have a question on a analog design project.

In this project, I need to design an amplifier that can match 50 Ohm or
75 Ohm output impedance by using a digital switch.

I'm considering using a source follower output stage. By changing the
bias current of the source follower, I can change (switch) the gm, and
therefore 1/gm to match the 50 Ohm/ 75 Ohm output impedance.

Is there any good ways to switch on/off the current source of the
source follower by cascoding a on/off MOSFET?

Or is there better ways to switch between 50 Ohm and 75 Ohm?

Thank you!

option 1:
as you said, to modulate the bias current of your source follower, but I suggest the best way is to design a source follower at 300ohm, and duplicate 6 of them in parelle to match well. then you can choose to turn on 4 of them to achieve 75ohm or 6 of them to achieve 50ohm.

option 2 :
1st design a source follower like you did, target the output impedance at 50ohm.
then, put a poly resitor under the cascode device source, in serial, at 25ohm, and you can use a mos switch device (make it big) to short the both end. so turning on the mos switch, you got 50ohm, turning it off, you got 75ohm

note that bias current change with temperature and vcc, so does Gm. It is very hard to maintain a constant output impedance across corners. this has been a major headache for us. if you really need accurate impedance, you need a lot of compensation techniq to do the job.


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Re: How to design a digital switch to match 50 Ohm or 75 Ohm

thanks mdcui

could you explain more on option 1?

how to turn on/ turn off the MOSFETs?
It would be desirable if DC bias change is minimum and the frequency response (due to loading effect of the switching branches) is maintained.

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