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Cascade 2 unconditional stable LNA and then kf less than 0

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I'm trying to cascade 2 LNA together to achieve higher gain. Both LNA are unconditional stable (kf>1 and b1f>0), but when I cascade them together, they are unstable (kf<1). Does it mean the cascade of 2 unconditional stable LNA is not 100% unconditional stable?
 

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Stability Factor K is valid only and only Single Stage Amplifiers.Even though you connect 2 identical Unconditional Stable Amplifier in Cascade configuration, it doesn't mean that overall system will absolutely be stable.
For system stability, you should check NDF ( Normalized Determinant Function ) that is available in Keysight ADS and Cadence AWR Design Environment. Using this method is quite complex but it will give you concrete insight.
 

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Stability Factor K is valid only and only Single Stage Amplifiers.Even though you connect 2 identical Unconditional Stable Amplifier in Cascade configuration, it doesn't mean that overall system will absolutely be stable.
For system stability, you should check NDF ( Normalized Determinant Function ) that is available in Keysight ADS and Cadence AWR Design Environment. Using this method is quite complex but it will give you concrete insight.
Thank you for your reply. What I don't understand is unconditional stable means the amplifier will be stable at any load condition. why I cascade another amplifier, which input impedance is R+jX, could cause unstable condition?
 

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Thank you for your reply. What I don't understand is unconditional stable means the amplifier will be stable at any load condition. why I cascade another amplifier, which input impedance is R+jX, could cause unstable condition?
Both Input and Output Impedance may result Instability.Therefore you have to know the interaction between the amplifiers.
 

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