LF: Class E PA design

[Andy Talbot <andy.g4jnt@gmail.com>]

There was a discussion a while ago about ththe PA design used here

http://www.gw3uep.ukfsn.org/100W_QTX/100WTX/PA_pa.gif    and Alan amde the comment, I think that teh series capacitor at 1uF looked suspiciously large to be part of a tank circuit for a Class E transmitter.

Well...   I've just been playing around with my spreadsheet for Class E stages with a built in series-L shunt-C match at the output. http://www.g4jnt.com/DownLoad/classe_match.xls

For a range of transformed impedances there are two configurations that you can end up with.    The reactance of the series L part of the match can be absorbed into the tank coil, increasing its value, OR,   it can be absorbed into the series C DECREASING its value.  Using this latter solution, there will (usually)  come a point with suitable choice of tank Q where the reactance of the matching L exactly equals that of the tank capacitor, making this disappear.   

That is almost certainly  what has happened in GW3UEP's design.  There are also a range of values where Xc tank is less than Xl match so it can never be absorbed, and only the one solution is possible - that of adding it to the tank inductor.

To do this you have to first get a working network with some arbitrary Q chosen, and a valid output match for your optimum power output etc. Then adjust the tank Q value while watching the resulting two possible values shown as C2 on the spreadsheet.  As Q is altered, one will move toward infinity then disappear completely.

Andy  G4JNT