[geeks] (a) FETs? (b) current Wien bridge?

[gsm at mendelson.com]

On Thu, Jun 04, 2009 at 11:41:42PM -0400, der Mouse wrote:
>(a) I'm a dabbler in electronics, and find myself wanting to play with
>FETs.  (I'm used to BJTs, but - see below - would find some FETs
>convenient to have.)  As I understand it, there are basically four
>kinds of FET ({n,p}-channel {depletion,enhancement}-mode), but I don't
>consider my understanding anywhere near complete, so I know if there
>are any other aspects I should be considering, and in any case I don't
>know any numbers for small general-purpose ones.  Loosely put, I'm
>looking for FET versions of the 2N3904/2N3906, if that helps -
>through-hole parts costing a few cents each when buying quantities in
>the few-hundred-to-a-thousand range.  Suggestions?
>
>(b) I wanted to build an oscillator that ran in the neighbourhood of
>400KHz and produced a moderately clean sine wave.  This is not
>conceptually difficult, but most of the circuits I found for such
>things are based on opamps, and I found the only opamps I have at ready
>hand are 741s, whose gain drops with frequency, reaching 1 somewhere
>near 1MHz - fine for the sort of audio stuff I've been playing with in
>the past, but not for this. 

Considering the first 741's appeared in 1968, they are really very 
old technology. There are lots of op amps around. You should IMHO look
at the various circuts used in ham radio.

You could also ask around at a local ham radio club, there is always one
guy who could help you.

For small signals, I've used MPF102's which have been around forever.
Mouser sells Fairchild ones (Fairchildren?) for $.10 each for one
and 7.6 cents each for 100. I think they may have a minimum order.

For an RF input device, there is always the 3n211 or it's RCA counterpart,
but I think they have both been replaced with something else. 

You could also look up the power FETs used in switching power supplies, they
are cheap and may do exactly what you want.

If you have a transistor tester, you can buy a "grab bag" of assorted 
transistors and sort them yourself. You'll have to do some research to find
ones that actually contain something of use, the days of walking into a
Radio Shack and buying a bag like that are long gone.

Some people who wrote books on the subject are the late Forest Mims III
(who wrote those little "mini notebooks" Radio Shack used to sell), the
late Doug DeMaw who was a ham and wrote lots of articles and books on
building radio and audio related things, and Don Lancaster, the author
of those "cookbook" books, who is still very much alive and will answer
your emails. All of whom, I've managed to keep books by over two two life
changing moves and several decades.

Geoff.
-- 
Geoffrey S. Mendelson, Jerusalem, Israel gsm at mendelson.com  N3OWJ/4X1GM