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

[wa2egp at att.net]

-------------- Original message ----------------------
From: der Mouse <mouse at Rodents-Montreal.ORG>
>
> (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.  So I started to build a circuit with
> discrete parts, and looked into oscillator circuits.  The most
> plausible circuit I found was the Wien bridge, but it is based on a
> voltage amplifier, and the BJTs I have are current amplifiers, not
> voltage amplifiers (see item (a)).  So I set myself the task of working
> out a current-amplifier analog to a Wien bridge.  After an hour or so
> of scribbling, I have something that should exhibit negative
> resistance, but I haven't managed to convert it into anything
> practical, much less anything practical which should oscillate.
> Pointers, anyone?

I think a circuit that you might be interested is a twin T oscillator.  It produces a sinewave output that is pretty stable.  It is basically an amplifying device (usually a transistor) with a feedback circuit that consists of two "t" shaped circuits, one  pair of resistors with a capacitor to ground and the other a pair of capacitors with a resistor to ground. Usually used in audio ranges, it could go up to the frequency you're interested in. 

Bob