Dear All,

   I am design a fully differential Low noise amplifier, which should be connecting with the next stage mixer. Because I want to design the monolithic LNA and mixer, so I don't consider the input impedance matching between LNA and mixer.

   When I use testbench 1 as shown in figure1, at the output I use ideal transformer (2:1) to realize the diffferential to single end, then add the port(10kohms) and load capacitance(100fF) as the input of mixer. It gives the good results.

   However when I use the test bench 2 as shown in figure 2, at the output I connect the output of LNA to the biased MOS transistor directly, but the frequency response of the LNA is totally changed. I don't know the reason.

   Which one gives the correct results, and what is the difference between the two test bench.

   Thank you very much!

This is testbench 2