Path: utzoo!utgpu!cunews!software.mitel.com!grayt
From: grayt@Software.Mitel.COM (Tom Gray)
Newsgroups: sci.electronics
Subject: Re: CMOS Analog Switches
Message-ID: <6922@nst>
Date: 14 Mar 91 13:37:04 GMT
References: <5292@umbc3.UMBC.EDU>
Organization: Mitel. Kanata (Ontario). Canada.
Lines: 37

In article <5292@umbc3.UMBC.EDU> hack@umbc5.umbc.edu () writes:
>
>      I am tring to build a audio amplifier and I wish to have
>it electronically switched (as few mechanical switches as
>possible).  What I was concidering is using CMOS bilateral
>switches to handle this.  What I am concerned is they might
>produce a great deal of distortion to the signals.  Now I know
>that audio signals are typically 20-20kHz, still I don't know
>if that would be a problem.
>
>     I would like to know if there is a better way, or if
>the CMOS chips are right for the jobs which ones are better
>for this type of application.
>

The problem you will find with these divices is not the variation in
impedence with frequency but the variation in impedence with
amplitude. The on resistance of these devices can very from 
3 ohms for large signals to 300 ohms for small signals.
You will have to compensate for this variation in resistance
or there will be significant intermodulation distortion in 
your amplfier.

Some sampling gate designs, I have seen close a feedback path
around the switch to null out any variations in the
on resistance.

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