Amplifier unilateral assumption

Hiall, the unilateral assumption is validated with the U factor which indicate the difference in the outcoming gain. But there could still be a big difference in optimal source reflection and load reflection coeff even though U remains small. How will the choice between unilateral and bilateral design affect other parameters like S11,S22 or bandwidth?

Any advice is appreciated.:)

Please tell me which technology you are using (integrated, pcb) and at which frequency

My experience says that for microwave design the unilateral assumption does never hold. So I would not recommend using this approximation in this frequency range.

It's a discrete amplifier, working at X band. But even if I turned the frequency down a little bit in the simulation, say S band, it's still not working.

That's normal because the device is not unilateral in this frequency range. But no issue: Read any good textbook on microwave amplifier design and you can learn the methodologies involved. I recommend "Microwave Transistor Amplifier" from Gonzalez.

You can use conjugate match at input and output but be careful that the stability is maintained and stability (K-factor) should be ok for ALL frequencies, not just your operating range.