Is power gain S21 or (S21)2?

As I know, [S21]2 is the power gain.
But in some IEEE papers for LNA design, they call S21 the power gain directly.
Is there anything wrong?
Thanks.

i think it is simple . most of the paper give the S21 in dB value which is the power


khouly

it depends on the S21 referes to, if it stands for voltage, then the power gain is (S21)2, if it stands for power, normally in (dB), S21 shows the power gain.

S21 is defined as voltage transfer function.
The translation between power and voltage gain depends on the reference impedance. If the load is the same, such as 50ohm, then the voltage gain is same as power gain when presented in dB.

Be aware that there is both voltage and current S parameter. However in Microwave design, we always refer to power S parameter.

Then there is the further confusing factor that S21 is between 50 ohm (or other specified) real impedances. When used in a circuit, the source and load impedance seen by the transistor is frequently some other value that improves the circuit gain or circuit noise figure.

There are so many kinds of S parameters?
I think it is just like Z parameters, unique for a certain network, and could be measure.
But it seems indeed that the db result has counted it for voltages in HSPICE.
I just caculated a S11 of a circuit.
Though I thought it would be caculated as 10logS11, it should be 20logS11.......

Thanks.

You are correct. S parameter is for voltage. But it could be translated into current/power easily if the impedance of the VNA is same between source/load. This is normally true for VNA.