Power amplifier stability

i am working on the main amplifier(class ab) of a doherty configuration power amplifier. the amplifier is stable for all values above 13MHz, below which there's a sudden dip and the value of Kf becomes extremely negative (-2.5E5). The PA is designed for 2.6GHz. for higher frequencies it is stable. will the amplifier be considered stable or not. B1f values are stable. What exactly is the criteria of judging a circuit's stablity.

The s-parameters of the transistors/active devices are defined for lower frequencies below 100MHz ?
Because simulators make extrapolation for the values which are not defined at those frequencies.It might be an extrapolation error..

thankyou very much..my supervisor really appreciated your response and it has been a great help!

Padoh, wich kind os stability are you speaking of?
If you are referring to dynamic stability (against the trend to oscillate) I like to mention that in this case the operating frequency does not matter.
If a circuit with feedback is unstable - it oscillates, independent on the signal frequency that is applied.

Padoh, Yes your amplifier is stable.
Criteria is, if the amplifier gives sufficient phase margin at the operating frequency, it is perfectly stable. That means your Operating frequency should always be less than UGB or UGB/10 most of the time.
Why you worry about lower or another non-operating frequency?

Hi varunkant2k,

may I correct the above citation of the stability criterion?
* At first, it has to be applied to the LOOP GAIN of the circuit under consideration
* Secondly, the criterion must NOT be applied to the "operating frequency" (in fact, this frequency does not matter at all).
Instead, the phase margin is evaluated at the cross-over frequency of the loop gain magnitude (0 dB).

LvW

Yes LvW, I agree that PM@UGB criteria should follow.
But you must be agree that operating frequency should fall under UGB of amplifier. else amplifier may not pass it.
UGB, generally gives minimum gain where the least PM required to sure of stability (I agree with this requirement). But if your operating frequency is less than UGB, close loop gain will certainly be higher than unity and system will have sufficient phase to get damped. (this is what I mean was with this sufficient phase margin).
At UGB frequency, the phase margin makes it sure that system response will die down any oscillation available,and will not let the system fall in regeneration mode (positive feedback).
-----------
LvW, I understand your point. Its correct , intermediate freq can not define stability.

- - - Updated - - -

Padoh, how you are defining stability?