Calculation of Breakdown Voltage
I think there are two things you should be checking when using the transistor model for your BJT in ADS. First, you should check if the vendor model for the transistor you're using takes into account the breakdown electric field of the BJT, which will be some sort of Zener or avalanche diode from the base to ground in the SPICE model. Second, since you have a "large signal" input you should check if the model you are using is a large signal or small signal amplifier model. Your transistor will behave quite differently with \$V_{EB} \gg V_T \$.

In general you can check the maximum ratings of the BJT you're using for base-emitter breakdown voltage in the datasheet.