Tjuby wrote on Oct 10th, 2010, 4:40am:It seems AC-analysis is available.
I have only used S-parameter analysis earlier,
so I do not know how to use AC analysis to investigate the problem.
See
http://www.designers-guide.org/Forum/YaBB.pl?num=1240334878/18#18Tjuby wrote on Oct 10th, 2010, 4:40am:Yeah, that was what I feared. Meaning, such an analysis is just.. not very helpful.
I can only verify that each single stage is, by itself, unconditionally stable.
Tjuby wrote on Oct 10th, 2010, 4:40am:I mean that, since I cannot use rollets or mu factor with simulated S-parameters for the entire cascade.
I have not actually been able to show, theoretically, that the cascade is unstable without the isolator, but stable with the isolator.
This is the core of my problem,
I am currenly at a loss of how I am supposed to conduct any simulation to show how the isolator helps to stabilize the cascade.
If a cause of unstability is due to amplifier's coupling via power supply line or ground,
you can not take such effects into your simulation,
since you treat each amplifier as two-port S-parameter block.
Can you characterize each amplifier as three-port S-parameter block with power supply as 3rd port.
pancho_hideboo wrote on Oct 10th, 2010, 4:09am:Tjuby wrote on Oct 10th, 2010, 4:02am:Is it possible that the only thing that was needed, was to increase the electrical distance between the amplifiers to change the phase output?
No, you are misunderstanding.
Even if you do so,
phase rotation at output does not change.I mean reducing feedback from output to input by increasing the electrical distance between the amplifiers.
Sorry, I was wrong.
If you increase the electrical distance between the amplifiers, phase rotation at output will large.
This could make cascaded system more unstable.
I mean reducing feedback from output to input by increasing the electrical distance between input and output of cascaded system.
For example, don't place three amplifiers in "U" figure or "L" figure.
Tjuby wrote on Oct 10th, 2010, 4:40am:But then, since the physical distance will be increased as the isolator is quite large.
Do you mean a physical size of "Isolator" ?
Tjuby wrote on Oct 10th, 2010, 4:40am:Will this also serve to increase the electrical length and reduce this feeback effect ?
No.