Transistor |s21| less than 1 and still can serve as amplifier?

[al_dddd]

Hi all,
Could it be that tha transistor's |s21| is less than 1 (50 Ohms) and still the MAG is high (say 20-22 dB)? How comes?

 

[Terminator3]

Maybe you are looking at wrong s parameters, for different biasing conditions / operation point?

 

[biff44]

Of course, provided that S11 and S22 magnitudes are large, and you design a low loss matching network for both.

Back in the day, it was COMMON for a S21 magnitude to be pretty poor! We got along somehow.

 

[al_dddd]

Maybe you are looking at wrong s parameters, for different biasing conditions / operation point?

Just took transistor sith dc blocks and feeds and plotted s21 and MAG... s21 is beliw 0db while mag is about 18dB. Same bias of course.

 

[BigBoss]

Just took transistor sith dc blocks and feeds and plotted s21 and MAG... s21 is beliw 0db while mag is about 18dB. Same bias of course.

MAG is depending on the Stability Factor.If K<<1, even dB(S(2,1))=1 MAG(dB) can be >>1.
K factor is important in definition of MAG.

 

[al_dddd]

MAG is depending on the Stability Factor.If K<<1, even dB(S(2,1))=1 MAG(dB) can be >>1.
K factor is important in definition of MAG.

So that means that while transistor is unstable the mag could be positive while the s21 negative? Mag btw is not well defined for negative k since it has sqrt (k-1) term...if I remember correctly.

- - - Updated - - -

Of course, provided that S11 and S22 magnitudes are large, and you design a low loss matching network for both.

Back in the day, it was COMMON for a S21 magnitude to be pretty poor! We got along somehow.

Can you elaborate a little? Just give an intuition of that- s11 and s22 are large and the gain is large despite poor s21...

 

[al_dddd]

Push it up...