Queries on checking stability of transistor

[narayani]

I want to check the stability of the transistor using the available transistor S-parameters, am using the Avago ATF54143 transistor to my work.

I have checked the stability of the transistor using the following conditions

K > 1, and B1 >0

If above these conditions are satisfied simultaneously, then we can say that transistor is unconditionally stable. Otherwise it is unstable.

Other than this, can we check the stability of the transistor using Smith chart. How we can check stability of the transistor using Smith chart drawing Software in ADS 2011.10.

Can you help me.

 

[D.A.(Tony)Stewart]

1. Unconditionally stable (K > 1 & B > 0).
2. Conditionally stable (potentially unstable) (K < 1 & B1 > 0).
3. Unstable (K < 1 & B < 0).

The stability parameters can be evaluated as a function of the Y-matrix for both the transistor admittance and the feedback network admittance to find the optimum Real and Total admittance from the s parameters.

RL feedback is often used for stabilization where
Yf = 1/( Rf + jωLf)

Smith Charts can be used , but someone else can assist better.
Also K and Gmax contours can be plotted with incremental values plotting Rf vs f.
Then check K and B1 of the transistor with and without the stabilization network. vs f.

 

[BigBoss]

A important remark...
Even the stability metrics show the circuit is unconditionally stable, the circuit must be examined again in term of stability by doing a EM simulation of the PCB where the circuit will be lay on.Because a small parasitic effect coming from PCB layout may change the stability from unconditional to unstable.
I have seen very frequently this phenomena in my past experiences.