Maker Pro
Maker Pro

Simulated RF amplifier stability, but with a filter?

B

billcalley

Jan 1, 1970
0
Hi All,

This has been really bugging me: When I design and simulate an RF
amplifier (LNA) for a class project, it shows points of huge
instability (below the desired passband 'K' is *way* under 1 ). But
when I add a bandpass filter to the input of the unstable simulated
amplifier, the instability is "gone" (the K is now way above 1). My
question is this: Is the instability really gone just because the
simulator cannot see the "real" S-parameters of the unstable amplifier
through the bandpass filter? Is the filter just masking the
amplifier's instability when, in reality, the LNA is still highly
unstable, with or without the filter? If so, is there a way to "see"
the "real" simulated K when a filter is placed at the input of an
amplifier?

Any help is most appreciated!

Thanks,

-Bill
 
P

Phil Allison

Jan 1, 1970
0
"billcalley"
This has been really bugging me: When I design and simulate an RF
amplifier (LNA) for a class project, it shows points of huge
instability (below the desired passband 'K' is *way* under 1 ).


** K is a measure of the *potential* for an RF amplifier to oscillate.

A K value less than one means that oscillation possible, with the right
combination of input Z and load Z.

But
when I add a bandpass filter to the input of the unstable simulated
amplifier, the instability is "gone" (the K is now way above 1).


** The BP filter eliminates a whole *range of values* for input Z from the
game - so eliminates the main cause of possible oscillation.

This link might be of interest:

http://www.daycounter.com/Calculators/Complete-RF-Amplifier-Design-Analysis-Calculator.phtml



....... Phil
 
B

billcalley

Jan 1, 1970
0
"billcalley"




** K is a measure of the  *potential* for an RFamplifierto oscillate.

A K value less than one means that oscillation possible, with the right
combination of input Z and load Z.

 > But


 ** The BPfiltereliminates a whole *range of values* for input Z from the
game  -  so eliminates the main cause of possible oscillation.

This link might be of interest:

http://www.daycounter.com/Calculators/Complete-RF-Amplifier-Design-An...

......   Phil

Thanks Phil. That makes sense to me. So I guess if a small-signal
amplifier is simulated to be unstable out-of-band, and you add the
desired bandpass filter in front of it, and K now looks good (K<1
across a wide bandwidth), then the entire circuit will be
unconditionally stable when built. (I was just worried that the
filter was simply "masking" the instability of the circuit by
preventing the S-parameters that determine 'K' from being seen by the
simulator's input port, and therefore giving me a false K>1
indication).

Thanks again,

-Bill
 
Top