On Sun, 02 May 2010 23:43:14 -0500, flipper wrote:

On Sun, 02 May 2010 17:08:30 -0700, John Larkin
wrote:

On Sun, 02 May 2010 18:39:11 -0500, flipper wrote:

On Sun, 02 May 2010 14:59:19 -0700, John Larkin
wrote:

snip


Why not just do this? No adjustments needed.

John



I dunno. Give me some component values and we'll see.

Make the upper resistor whatever it needs to be to get +10 on the
gate. Make the source resistor whatever you like to get your preferred
idle current.

Actually, I was wondering if you had a magic FET but that's now moot.


Just off hand, the 100k makes input impedance rather low and if that's
a MOSFET then gate capacitance is a problem.

Depends on what you drive it from. 100K is not going to load many
driver circuits.

It's not all that great in the tube world and AA5 radios normally have
470k to 1M grid leaks but that's solvable. The trick here is to make a
separate reference, not tied to the gate, and then take gate to it
through an equivalent 470k to 1M 'gate leak'.


If you delete the source bypass cap, Cin goes way down. At that point,
drive levels will be closer to what a toob would need, and linearity
is better.

First thing I looked at was just like what you drew but bandwidth
stunk so I moved on. Source feedback is the trick to it and looks like
there's enough to make it work.

Of course, drain impedance is high, so speaker damping is poor. Some
people like that. Local NFB is easy (just connect the upper resistor
to the drain) and helps that situation.

I think adding some local drain feedback might be a little more
complicated than that because bias would go away with large output
swings.

Anyway, a 'plain Jane version might look something like the
attachment. A zener, rather than the resistor divider, keeps hum off
the gate. Or one could filter the divider.

[snip]

What will Cdg feedback do to an uncascoded circuit such as yours?
Limit the high end?

...Jim Thompson
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