Cascoded amp.

Usually, we stack cascoded transitor (CG) on top of input transistor (CS) to improve the S12, also bandwidth. But is it so that cascoded topolgy gives better gain than the simple CS amplifier?

Can anyone tell HOWand WHY?

The most important thing we use cascode transistor is to surpress miller effect.

Miller effect was a big deal back in the '70's, when transistors were huge. Now that the geometry of RF transistors is so small, there is much less Ccb to worry about. So you can get away without worrying about it most of the time. You would never be able to get a discrete version of a cascode amp to work with modern microwave transistors, since it would oscillate wildly.

how about the linearity of cascoded amp compared to the common source amp?

You guys' right. With cascoded transistor, drain voltage of CS tansistor could be more steady than that without cascoded transistor. this can on one hand suppress miller effect, and on the other hand improve linearity.