The question of stability factor K in LNA design

[MW+RF=Netpig]

The question of stability factor K in LNA design(BJT transistor):
1.when the stability factor : K<1 ,we need to stability design to make the K >1,
the usually methods are: add R-C,R-L,L-C circuit parallel connection at output of the transistor.
Who can tell me why?
2,Anybody can narrate the match tech in LNA design?

thanks

 

[vfone]

Five methods for LNA stabilization:

- The first one consists of resistive loading of the input. This method, although capable of improving the stability of the circuit, also degrades the noise of the LNA and is almost never used.

- Output resistive loading is preferred method of circuit stabilization. This method should be carefully used because it effects are lower gain and lower P1dB point (thus IP3 point).

- The third method uses collector to base resistor-inductor-capacitor (RLC) feedback to lower the gain at the lower frequencies and hence improve the stability of the circuit.

- The fourth method consists of filter matching, usually used at the output of the transistor, to decrease the gain at a specific narrow bandwidth frequency. This method is frequently used for eliminating gain at high frequencies, much above the band of operation. Short circuit quarter wave lines designed for problematic frequencies, or simple capacitors with the same resonant frequency as the frequency of oscillation (or excessive gain) can be used to stabilize the circuit.

- The final stabilization method can be realized with a simple emitter feedback inductor. A small inductor can make the circuit more stable at higher frequencies. But if the source inductance is increased, the K-factor at higher frequencies eventually falls bellow 1. This effect limits the amount of source inductance that can safely be used.

 

[MW+RF=Netpig]

thanks a lot