[sdiy] switching audio with mosfets

[Ian Fritz]

Isn't the usual solution to use two devices in series, with the protection 
diodes in opposition?  Zetex has a design note on this, DN3, you should be 
able to find.

   Ian


At 05:25 AM 5/2/2007, Heiko van der Linden wrote:

>Hi All,
>
>I am currently trying to work out a method to switch an audio signal
>with a mosfet transistor. The signal is 10Vpp centered around ground.
>I want to be able to use high frequencies with the mosfet switch both in
>the frequency of the processed audio as the switching frequency.
>I tried some simple experiments with n-channel mosfets and the problem I have
>encountered so far is that the substrate diode starts conducting during
>the negative cycle of the signal. So just by simply driving the mosfet gate
>with a constant Vgs will not work. The constant Vgs drive was done by
>using a summing amplifier to measure the voltage at the source and add the
>Vgs voltage to it. This works nicely and keeps the VGS constant during
>the positive cycle of an audio signal to very high frequencies (500kHz).
>Unfortunately during the negative part of the cycle the substrate diode
>conducts thus short circuiting my signal.
>
>I have come up with some solutions:
>1) buy a 4-terminal MOSFET like the micrel MC94030. This is a p-channel device
>that has the substrate terminal available separately. The solution would be
>to bias the  substrate to a voltage that is more positive than will be 
>encountered
>by the mosfet switch. Vice versa a n-channel device could be used in
>an opposite way. link: http://www.micrel.com/_PDF/mic94030.pdf
>2) use (reed) relays (too slow switching speed)
>3) level shift the audio signal so that it has only positive or negative
>polarity and then process it with the mosfet switch. Then level shift it
>back to the original polarity.
>
>I have some questions:
>The micrel device seems interesting. They have a low Rdss(on), have
>the substrate terminal available and work at high frequencies. I have tried
>to find a place that sells these at small quantities but cannot find it. Does
>anyone have a clue where I can find these devices ? I tried digikey, newark,
>arrow, farnell, etc. It appears that this device is only available at 
>large volumes...
>Does anyone know another type of mosfet where I can access the substrate
>terminal ? Both p-channel or n-channel would work for me as well as discrete
>as array types... I know that it's possible to get VHF mosfets with the 
>4th terminal
>but those are a bit pricey and seem overkill for what I want to do (I 
>don't need
>a lot of power).
>
>Does anyone know other solutions to my audio switching problem or is it
>not very feasible ? To sum up what I am looking for: a audio switch for
>bipolar audio signals that has a low on-resistance (preferably below 1 Ohm)
>which can be switched at high speeds (20 - 100kHz). The off resistance
>should be very high (>10MOhm - GOhm).
>
>Hope that my mail is not too incoherent...
>
>Heiko
>
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