stability factor at out of band?
How Much is stability factor critical at out of band of An amplifier?
is it necesseray to be stable?
It can make a design very difficult.
What is the practical for an amplifer?
I have seen in some internaly matched power Amps ( fujitsu) that s parameters have measured only in band, so we could stability only in band. waht about outside of band? they may oscillate or not?
A good amplifier design needs full-band unconditional stable. Because once the amplifier oscillates at any frequency, no matter in-band or out-band, the design is failed. I agree that it is the most difficult technology to make a amplifier full-band unconditional stable. You can find very rigorous theory for designing an amplifier with very good RL, NF, flat Gain, Linearity, efficiency etc. in any common text book, but text book hardly tells you how to make amplifiers full-band stable, because it's utterly experience dependent.
Then how to deal with full-band stability? I'd like to share my experience here. Assuming the designer is able to make amplifier full-band conditional stable, and I think this could be relatively simpler. In design phase we can improve the stability by yielding the input and output impedance in a safe range. Because in the case of conditional stable, the amplifier only oscillates at some special source or load impedance, if we can use a network to avoid the source/load impedance to locate in those area, the circuits will be unconditional stable. For example, cascadeing two conditional stable network might result a unconditional stable network (although problems will occur at other frequencies); and a lossy network can obviously improve stability; tuning the position and parameters of the biasing circuits (usually lossless in-band but lossy out-band) potentially improve stability as well. Be sure that the design is at least unconditional stable at lower frequency, and then in the debugging phase, we probably can use absorb material to suppress the oscillation at higher frequency.
I hope someone else can also share your experience on amplifier stability, thank you!
Yes, amps need to be unconditionally stable. But I would add a subcategory of fixed usage designs. Lets say a mixer drives and amplifier, followed by a bandpass filter. Here the broadband input and output impedances are known and fixed. If out of band the bandpass filter looks like a high impedance, and the amplifier is unstable for low output impedances, then you are still ok.