Klaus suggested BC847. Is this the same as BC547?. Also, is there a
negative bais voltage on the emitter of NPN or is the emitter
connected to ground? So basically the circuit is


COAX 75ohm Line
video source------------------------------+----video receiver
|
|
|
R=75
|
|
C
TTL-------R=1k---B BC547
E
|
|
-V=????

By the way, I notice that when I connected a -5v to the source of the
MOSFET, I did see a DC variation. I had to connect the source of the
FET to GND in order to stop the variation. It is interest that in
your analysis that you commented that the MOSFET arrangement will have
"...significant DC shifts". Maybe the DC shifts are still in the
MOSFET arrangement with the source connected to GND, but I don't see
the shift under the GND connection condition.

Caesar

On Sun, 19 Feb 2006 11:33:04 -0800, "John Jardine."
wrote:


"LRCR" wrote in message
.. .
So John, you're saying that if I can bais the FET' Drain to Source
with the 5V while it is OFF, this will help to lower the Drain to
Source Capacitance.


Caesar
[...]

Yes. (but ........
As arrangement seemed so intriguingly "simple", thought I'd run the spice
and oh-so casually figure out a working arrangement. A whole 3 hours!
later, figured the FET idea is a time waster.
Now been driven home to me the significance of the Ciss,Coss,Crss value
interactions under this linear-switching condition.
Horrible little things are constantly on the move, resulting in significant
DC shifts, signal loss variations and harmonic distortion. Tease apart one
complicated interaction and another strolls in to fill it's place.
Tried various bias arrangements but at 20MHz, the FET harmonic distortion
(5% at best), the loss of output voltage (-10% at best) and variable DC
offset shifting with signal level suggested I was on a loser.

Eventually just fitted a normal NPN small signal transistor like Kraus K
suggested (I used a BC547 and 1k base R)).
Magic ... . Distortion mostly 3rd at the 1% level. No mysterious DC shifts
and NO signal loss.
(confirmed on a spectrum analyser and tracking gen).

I enjoyed that, Ta!.
john