Quote:


Yes, if the output and input are well matched to the same impedance (50 ohms) then S21 equals the voltage gain from the input port to the output port.

Quote:


Well, I guess it depends on what you consider to be the "actual gain?"

Power gain definitions are used because the impedances have a significant impact on actual voltages and currents when operating at high frequencies, but the definition of "real" power is a more consistent measure of the input and output signals relative to the real part of the source and load interface to the circuit under test. There are different power gain definitions according to how the circuit is intended to be used or what performance parameters are most important in a given system design.

I would think that "available power gain" is most relevant to actual voltage gain when the real part of the output and input port impedances are the same. Available power gain is defined as the available output power delivered to the load divided by the available power of the source, this implies that the output and input are conjugate matched. Available power gain is still relevant when the real part of the input and output load impedances are different, you just need to account for the impedance transformation (just like a transformer) as part of the voltage gain (output impedance> input impedance) or gain loss (output impedance <input impedance).

Hope this helps.

Hi,

Quote:


They all have different meanings and uses. For example, Power Gain is not much use if you want to find the stop-band rejection of a low-pass filter. However, if you want to calculate the filter's effect on the efficiency of a PA, Power Gain is more meaningful. If you study the definitions and try a few circuits the meanings should become clear. The concept of available power is very meaningful.


regards,
Aaron

The available gain definition, RFICDUDE provided might not be correct.
In my understanding the available power gain (GA) definition is the available power delivered to the load (in conjugate matching) dividing to the available power from the source (in any matching condition). So the max gain is the available gain when the source is conjugate matching. The transducer gain (GT) the ratio of power delivered to the load dividing to power available from the source. S21 is transducer gain. When both input/output are conjugate matching:
S21=GT=GA=Gmax
In high frequency design, you always use Sparameter to characterize you block. S21 or GT is the real gain of your LNA when you terminate it in a 50 measure system.