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!