RadioBanter - View Single Post

September 27th 04, 01:47 AM

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

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).

Suggestion, try a low pass to the antenna and a highpass to a 50ohm
load. then the amp[ sees 50 ohms for all loads and also the high pass
section will absorb harmonics. The pow pass and the highpass share a
common port (the one looking toward the amp). This is documented in
EMDRF and other places.

I'd also look at the layout and maybe try loading the drain choke with
a resistor in the 40-100ohm range. The choke may be resonant at some
nearby frequency.

One of the problems with MOS power FETs is there is substantial drain
to gate capacitance and if the gate is not loade or the drain enough
results and feedback can be problematic. You can also try loading the
gate to ground with a resistor in series with a cap (try 50 ohms and
..01uf) or a series combo of 100ohms and .01uf from drain to gate to
add negative feedback.

Allison