[sdiy] switching audio with mosfets

[Heiko van der Linden]

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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