I am trying to understand S parameters on RF transistors.
I chose the BFP840ESDH6327XTSA1 as my RF transistor, to operate it at 10 mA, 12GHz. The design should be optimal for simultaneously matching impedance and maximum gain. Lets say that the input/output loads are 50 Ω.
I am unsure as to how can I spot the S11 values from the Smith chart that the datasheet provides. For example, for the S11 I have the below figure from the datasheet. S11 is on the input side, where I have a 50 Ω source, so the input (source) is on point A.
My question is: How do I find the S11 parameter from this graph?
In the meanwhile I am posting my ideas since I am doing it as a homework project:
Now, point A is not being crossed by any line, so my ideas to find the S11 there are:
- Either the graph does not have information of S11 for a 50 Ω load (most probably?) so I need to match the impedance of 50 Ω that I have on the input to one of the drawn lines on the graph (green line) so that can I change my input impedance from 50 Ω to a known impedance and use this S parameter.
- I need to "walk" on the resistance path -blue line- (and select A2 point as S11) OR on the imaginary part -red line- (and select A1 point as S11), which both do not make sense to me.