Why do I get a negative Re(Zin) while performing SP in ADS?

Please refer to the pix attached for the simulated circuit.

the real part of Zin I got is -52.450@1GHz, -22.039@2GHz.

I'm wondering why does this happen.

Best Regards.

Perhaps the ckt is not unconditionally stable.

Why does that happen? So that microwave engineers can get paid! It is a good thing.

Hi,
I think you have biasing problem.
check your dc bias poins.
your transistors aren't in active region.

Regards

Hi,

I surely have checked the bias conditions, they're definitely in the active regions.

I apprecitate if you have a shot at this circuit with SP simulation.

Thank you.

I agree with g579: it's the stability problem.

Rgs.

Ruri