Expert: Cezar L. Palconet - 12/24/2009

Question
QUESTION: Dear Sir,

  for LNA design when we draw stability circles on smith chart then we need center and radius of stability circle ...my question is that when we get center in magnitude and angle form then while plotting that center on smith chart why do we take angle of reflection? why not angle of transmission?

ANSWER: Hi Ain

At any point along a transmission line, waves passes in both directions forward and reflected,  If we add up all the forward-traveling waves to get a total resultant forward wave complex amplitude, and vice versa for the total resultant reflected or backward wave complex amplitude, we can divide the total backward wave amplitude by the total forward wave amplitude to derive a complex dimensionless number gamma which is related to how much is reflected at the end of the line and how far away the reflecting point is.

SMITH chart lets us relate the complex dimensionless number gamma at any point along the line, to the normalized load impedance zL = ZL/Zo which causes the reflection, and also to the distance we are from the load in terms of the wavelengths along the line.


regards.

---------- FOLLOW-UP ----------

QUESTION: Dear Sir,

     I still didn't understood that what it has to do with stability circle? why we are taking reflection angle for stability circle?


Regards

ANSWER: Hi Ain.

The reflection coefficients plays a significant role in the stability of any amplifier, where K is greater than 1 and Magnitude is less than 1, most high frequency amplifier are designed under match load conditions, taking into consideration the transmission, will only provide information on forward to load characteristics.

This practice is common when using S parameters in amplifier design.


regards.

---------- FOLLOW-UP ----------

QUESTION: Dear Sir,


  Why low noise amplifier is importatnt? whats its practical application? Whats its imporatance?

Answer
Hi Ain,

Any device used for active amplification, introduces noise due to non linearity, so the main signal and the noise is amplified in addition to the noise introduced by the amplifier.

Devices need to be low in noise so as to introduce the smallest possible noise to the amplified signal, the practical application is very obvious, with very small noise the signal could be replicated more faithfully.


regards.