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Beyond Maximum Stable Gain?

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fuseless

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I was plotting constant-gain circles on the Smith chart the other day for a potentially unstable RF transistor. I recently read on-line that for a potentially unstable transistor the maximum gain that can achieved is the MSG or maximum stable gain. MSG is simply the ratio of the magnitude of S21 over the magnitude of S12. For my transistor, the MSG in absolute gain 162.5 or 22.1dB as shown on the attached Smith chart. The on-line article also said that "one should never try to tease more gain from the transistor than the MSG". I have the output stability circle plotted and the unstable region marked as UR on the Smith chart. Notice that some of the 22.1dB constant-gain circle is located pretty far away and outside of the output stability circle's unstable region. For the heck of it, I plotted gain circles all the way up to the ludicrous values of 50 and 100dB to see what would happen. The 29dB gain circle is dangerously close to but still outside the output stability circle's unstable region. Only when I jumped to 50 and 100dB did the gain circles just align with the output stability circle. Apparently, gain seems to be infinite with respect to a potentially unstable transistor.

What is it about MSG that makes it the final word on stable gain when, I'll say, the 23 or even the 26dB gain circle seems safely out of the output stability circle's unstable region and seems like there would still be some safe terminating impedances, although not many, available...right?001.jpg
 
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