Emitter follower/Source follower math questions!

[EW57]

Greetings, how does one go about calulating the correct emitter/source resistor needed for a emitter/source follower?  What would the effect be of using a resistor value higher or lower than the "optimum" value?

Also, if a guy wanted to tie two jfets together sharing a source resistor, how would that resistor value change (double, halve, or remain constant)?

http://www.thermionic.info/misc/CFMixer.pdf

Thanks!

[edvard]

Just make sure the resistor you use dissipates enough current to come within the device's rated characteristics.
For example, a 2n5457 JFET has a maximum Idss of 5 mA, so Ohm's law says with 9V power, the source resistor should be no less than 1.8k (9/.005=1800).
I believe the same might hold true for transistors, but somebody else might chime in.

For the record, I use 10k on all my source-followers and never had any problem.

[soggybag]

That's a strange looking circuit. I wonder if you could make a tremolo, or a pseudo ring modulator out of two FETs in that configuration?

[edvard]

Yes, it's a pretty strange-looking circuit at first, but once you understand what's happening, it becomes clear that it would make a wonderful back end for a "blend" control.
I always had too much bleed from the standard blend circuits, this would fit the bill just fine.
Ring mod or tremolo? Hmm... probably not. At least not in an obvious way.
It looks like this circuit mixes together two signals but with no cross-talk, which is nice.
With any kind of modulation, the LFO is actually interfering, not getting mixed in, so I don't know if this circuit is going to be helpful in those kinds of circuits.

The source resistor for such an arrangement would stay the same value, but you would want to be careful about how much power you run through it.
Both devices will see the same resistance to ground, but remember it's also two devices you have to dissipate current for.
Depending on how much current each one wants, you may have to upgrade to 1/2 watt resistors, but for most garden variety JFETs, it's probably not necessary.

[EW57]

Thanks for the input!  If I wanted to turn off one channel, my best bet would be to short the gate to ground, rather than leaving it hanging correct? Thanks!