Simulated RF amplifier stability, but with a filter?

Hi All,

This has been really bugging me: When I design and simulate an RF amplifier (LNA) for a class project, it shows points of huge instability (below the desired passband 'K' is *way* under 1 ). But when I add a bandpass filter to the input of the unstable simulated amplifier, the instability is "gone" (the K is now way above 1). My question is this: Is the instability really gone just because the simulator cannot see the "real" S-parameters of the unstable amplifier through the bandpass filter? Is the filter just masking the amplifier's instability when, in reality, the LNA is still highly unstable, with or without the filter? If so, is there a way to "see" the "real" simulated K when a filter is placed at the input of an amplifier?

Any help is most appreciated!



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** K is a measure of the *potential* for an RF amplifier to oscillate.

A K value less than one means that oscillation possible, with the right combination of input Z and load Z.

** The BP filter eliminates a whole *range of values* for input Z from the game - so eliminates the main cause of possible oscillation.

This link might be of interest:

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...... Phil

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Phil Allison


Thanks Phil. That makes sense to me. So I guess if a small-signal amplifier is simulated to be unstable out-of-band, and you add the desired bandpass filter in front of it, and K now looks good (K

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