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?
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.