[sdiy] MPF102 sub

[David G Dixon]

Sorry for the top-posting, but...

I concur exactly with what Ian is saying.  4391s are great for VCOs but they
don't always work in other applications, especially where variable pinchoff
is an issue.  In those cases, I generally find the 2N5485 to be more
reliable.  It's also available in SMD (MMBY5485) for those with Euro
aspirations.

Also, I simply can't find ANY discrete MOSFETs without the protection diode.
The 2N4351 is a classic in this regard, but it is practically unobtainium,
and if you do find it you might expect to pay $5 or more for it.  Forget
that!  This closes the door on a whole pile of interesting switching
options, and really leaves one with no choice except to use analog switches
-- those of the DG variety are indeed a very nice way to go for most
applications, and they come in a large variety of configurations.  However,
just be careful not to order the "low-voltage" variety accidentally.  These
usually have an "L" after the number, like "DG418L" and they won't work for
synth-level signals.  The Vishay DGs in particular are generally less
expensive, but some types only seem to be available in the low voltage
configuration.


> In days of old the 2n3819 was used as a near equivalent to 
> the MPF102.  It is still manufactured (Central Semi) and is 
> available from Mouser.
> 
> Other FETs available from Mouser are:
> J105, J107, J109, J111, J112, J113, J310
> BF256
> 2n4392, 2n4393
> 2n4416
> 2n5460
> 2n5484, 2n5485, 2n5486.
> 
> Things to remember:  Just because a certain substitution can 
> be made in one project doesn't mean it can be made in all 
> projects.  When you purchase a bunch you usually get pinchoff 
> voltages near the "typical" value, which may be much smaller 
> than the max value.  If you need a low pinchoff you can 
> always hope that works, but in general you should measure the 
> pinchoff voltage to be sure.  Do not substitute a MOSFET 
> unless you know what you are doing.  Most n-channel types are 
> enhancement-mode devices, meaning that the bias requirements 
> are different.  Additionally, nearly all have protection 
> diodes, which means they cannot switch bipolar signals. Plus, 
> they have large input capacitance unless you track down the 
> few devices that are suitable for low-power switching. This 
> capacitance can put huge switching transients on the signal, 
> so that for discharging a VCO cap the output is very messy.
> 
> I've pretty much decided that for future designs I'm going to 
> use the IC switches (dg211, etc.)  Even though they are 
> expensive, there are no worries about pinchoff voltage 
> variations, and I doubt they are going to vanish anytime soon.
> 
> Ian
> 
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