Unlike gain, stability is a DC-to-daylight phenomenon.
Meaning...the LPF presents the "Right" impedance to the amp output in
the passband, but consider: the LPF is a gross mismatch in the
stopband. Evidently the output of the amp is only conditionally
stable above the LPF cutoff, and the stopband impedance of the LPF
lies in the unstable region.
This kind of thing can be quite common when not considering stability
across all possible frequencies that the amp has a gain1.


On 26 Sep 2004 12:11:07 -0700, (Bill N2CQR
MOHBR) wrote:

I've built a little RF amp (for 20 meters) using a single MTP3055V
MOSFET.
Class AB, binocular core RF transformer in the output. I drive it
with
a homebrew 20 meter DSB rig.

Here is the problem: The amp works fine IF I take the low pass filter
out of the circuit. I'm using a groundplane antenna with a T network
tuner.
I can adjust it to make it look like 50 ohms. Without the low pass
filter,
I get a very nice clean output (looks good on a scope, and SWR is low,
so I don't think it is putting out spurs or harmonics).

But if I include the LP filter in the circuit, the amp takes off,
turning
into an 18 Mhz oscillator!

I checked the filter with a sig generator. The cutoff freq is where
it is
supposed to be.

I've had this kind of problem before, with other amps.

Any suggestions?

73 de Bill M0HBR CU2JL N2CQR
http://planeta.clix.pt/n2cqr