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g579 said:1. Some part of the circuit were not correctly modeled.
2. If all parts were correctly modeled, then some stray (capacitive or inductive) coupling between input and output of amplifier was not taken into account.
g579
flatulent said:If you used the part models that came with your simulator this is less of a potential source of the problem than layouts issues.
Your circuit should be made on a card with a ground plane. The component leads should be kept short. The amplifier should be laid out in a straight line across the board instead of looping around. (This makes the distance from the input to the output large) Another potential problem is coupling through the power supply. Make sure that there are bypass capacitors on each power connection to any part of the circuit.
flatulent said:Post your circiut diagram and simulation results. What frequncy is it oscillating? Does the frequency change with supply voltage?
mr_ghz said:Hi eladg,
as flatulent wrote, I think it would be a good idea if you upload a
schematic, or (better) the layout of your amp. At which frequency is
your amp designed to work? Often the problem is the grounding of the
amp. For higher frequencies even short vias (15mil) can have a high
inductance which can cause oscillations.
An amp is changing it's S11 when powered or not. Is your input-matching
network designed for the correct bias?
Bye
flatulent said:It looks like a case of injection lock oscillation if it is the same frequency as your drive signal. It also looks lie the parasitics are what are causing the problems. Try laying a piece of lossy ferrite over the circuit. Also try lowering the test signal level to see what results.
flatulent said:Prayer is best done at the design stage and not at the test stage.
Try cutting out all but one stage and see if that works properly.