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Will band pass filters provide stability for the circuit ?

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akhilpaulv

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Hi

If i add band pass filters both input and output of the amplifier. Will the amplifier is stable for entire region except the pass band ?? . Adding BPF will reduce S12 and S21 in out band , will this lead to reduce the stability problem and provide more gain??

-Akhil
 

Problem is that transistors S11 or S22 may be >1.
If S11 of filter is highly reflective (close to 1), then some signal may bounce between filter and transistor leading to unwanted oscillations.
Depending on transistors S11 S21 S12 S22 there may be other ways for instability to occur.

I usually do simplified analysis by drawing a plot: distance between filters S11 and center of stability circle of transistor vs frequency.
What we want is to keep external elements reflection coefficients in stable regions. Although such analysis may be not enough, but it is good starting point before going to further analysis, such as transient simulation.
 

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