Question about Common-source and Common-gate structure

[abcyin]

Hi, all

I just found that the gain of Common-source and common-gate structure is almost equal to each other, it's about gm*RL, regardless of the sign of the gain.

And because of the body effect, the gain of the CG structure is a little larger than the CS one, Even the bandwidth of CG is always larger than the later, But why we always use the common-source structre instead of the common-gate structure?

the second question is that why don't we use the CG structure to form operational amplifier?

Thx in advance!

 

[solidstate]

think about input impedance for a second...

 

[abcyin]

you mean a smaller input resistor and capacitor will show a larger bandwidth for the common-gate structure?

And...if the amplifier is in series with other blocks, then the former stage's gain will be reduced by the input impedance of the common-gate stage, is it right?

 

[PaloAlto]

solidstate means that for CS the input is purely capacitive, while CG has very low input impedance