Bill N2CQR MOHBR wrote:

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

I have found that the best way to build a stable amplifier is
to try and build an oscillator. To build a guaranteed to start
first time, every time oscillator, just try to build a stable
amplifier :-)

Follow good HF/VHF practice with the circuit layout.

Negative feedback can sometimes help (or make the problem worse).

Pay careful attention to power supply decoupling. The very
high LF gain of RF transistors can cause parasitic oscillation
at the resonant frequency of an L/C or R/C decoupling circuit.

Try the usual parasitic suppression tricks. A low value resistor
or ferrite bead in series with the FET gate. A ferrite bead
in series with the FET drain. A resistor in parallel with the
drain load inductor or transformer, as suggested by Allison.

Sometimes it can be easier and faster to start again with
a different layout than to debug a 'bad' circuit.


73, Ed. EI9GQ.



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