Low noise transistor s2p parameters vs cascade vs cascode

[skatefast08]

Wouldn't a true LNA have at least 2-stages to increase the gain before being modulated in a superheterodyne receiver? I am trying to design on ADS a low noise amplifier but with 2 more stages. First of all, wouldn't you also put a 2nd stage transistor to increase the gain in an LNA, or how would you do it? maybe you would just look for a single transistor that can give you enough gain with a very low noise? I have seen LNA cascode designs before also. Anyway my main point is If I put two common emitters/drain of the same transistors in cascade using s2p file (lets say I configured my gummel_poon model to replicate the s2p file at the specific frequency instead and used this). Would this work correctly if created a 2nd stage in cascade? also would the gummel poon model work correctly if put in cascode, since one is common base/gate, and the other is common emitter/drain would those two configurations throw off the s2p parameters and wouldn't be accurate? or would it be accurate?

 

[vfone]

Most of the LNAs for superheterodyne receivers don't need 2 stages. An LNA needs to have low noise (NF < 1dB) and gain greater than 10dB, but not greater than about 15dB, because possible intermods in the first mixer.
May need to do a system analysis to see where you stay with system gain, system linearity, and system noise figure.
If use a BJT for the LNA, a single stage in CE configuration will do the job.

 

[BigBoss]

Cascode configuration does not improve the NF because NF id depended only Optimum Noise Impedance that is presented to the first transistor. Therefore a single stage, lowest noise LNA with a reasonable Power Gain will be sufficient.

Note : Cascode configuration is used where a wider bandwidth and improved isolation are needed.