Tam/WB2TT wrote:

I have never seen clipping. These things are supposed to limit in
cutoff, not saturation. As the signal build up, the conduction angle
gets smaller and smaller until the device runs out of gain. That is
another way of saying that the DC value of the gate voltage gets more
negative the bigger the amplitude. This works out automatically with
a JFET. You need about 10K - 100K DC resistance from gate to ground.
Using a bipolar transistor is not a good idea.

I have now changed it to an MPF102 that I've had laying around for many
years. It works great, thanks.
:-)

"Anthony Fremont" wrote in message
...
Tam/WB2TT wrote:

I have never seen clipping. These things are supposed to limit in
cutoff, not saturation. As the signal build up, the conduction angle
gets smaller and smaller until the device runs out of gain. That is
another way of saying that the DC value of the gate voltage gets more
negative the bigger the amplitude. This works out automatically with
a JFET. You need about 10K - 100K DC resistance from gate to ground.
Using a bipolar transistor is not a good idea.

I have now changed it to an MPF102 that I've had laying around for many
years. It works great, thanks.
:-)

Glad it worked out. By the way the feedback path is through the capacitive
network between source and gate. That would be more obvious in the
configuration that uses a tapped inductor, but works the same way. Leaving
out the diode was the right thing to do; it just adds to the noise. I don't
know what kind of stability and linearity you need, but if that is
important, do not use the common type of ceramic capacitors that are meant
for bypassing. They are lossy, and their value varies with applied voltage.
Use mica, NPO ceramic, or Mylar and similar for larger values.

Tam

Tam/WB2TT wrote:
"Anthony Fremont" wrote in message

I have now changed it to an MPF102 that I've had laying around for
many years. It works great, thanks.
:-)

Glad it worked out. By the way the feedback path is through the
capacitive network between source and gate. That would be more
obvious in the configuration that uses a tapped inductor, but works

I had to play around with this quite a bit to get it running near 10MHz. It
seemed to be more picky about the cap ratios than their actual values.

the same way. Leaving out the diode was the right thing to do; it
just adds to the noise. I don't know what kind of stability and

I figured that it would cause horrid clipping the way it was installed. I
thought maybe the designer intended it to be the other way around to protect
the transistor from reverse voltages on the gate, hmm.... always something
to keep you guessing. ;-)

linearity you need, but if that is important, do not use the common
type of ceramic capacitors that are meant for bypassing. They are
lossy, and their value varies with applied voltage. Use mica, NPO
ceramic, or Mylar and similar for larger values.

I don't really "need" anything in particular, it's just an exercise to try
and learn something. I just used the ceramic caps because they were handy.
I didn't have a pile of NPO caps laying around. ;-) I d

Time to add a buffer now since the scope probes load it down so badly now.
I'm getting 1.1Vpp into the apparent 5M load of two scope probes. It's
probably pulled a ton off frequency as well. Let's see.....(removes one
probe).....yep, 90KHz rise in frequency after taking one probe off.