EMRFD Message Archive 11880

Message Date From Subject
11880 2015-11-10 13:05:25 l_moline Matching networks for RF Power Amplifiers

A friend and I have been trying to study doing calculations for matching network components for some QRP transmitters. We have calculated collector impedance using the Vcc and power output. The question I have is how do I determine the reactance and resistance of the collector impedance to use the calculators available on line for matching networks.

For instance we know the transistor has a capacitance between collector and emitter which is sometimes grounded. Also if we use a choke in the collector it will cause the collector to have an inductive reactance to ground. This seems that there would be a capacitor in parallel with a choke from collector to ground and a resistance representing the resistance part of the collector impedance.

I have looked and have not found out how to come up with the resistance and reactance values to put into the calculator, Can someone tell me where to find this calculation? Thanks!

11881 2015-11-10 18:24:52 kb1gmx Re: Matching networks for RF Power Amplifiers
Your looking for a calculation and there is none.  You need to read the datasheet 
and lacking RF data then make measurements or realistic assumptions.

The key parameter is the impedance you  need to present to the collector to 
get the expected power.  Part of that is the collector voltage, the other part 
is the effective load resistance.  

Now you do have the choke assuming your not using a transformer.  You also have
the collector (or  drain) capacitance.  Those have to be absorbed into the impedance
matching circuit or otherwise dealt with.  Most of those parts also have impedance 
usually 4-10 times the load impedance in practice.

In every case it will be different and for the same power approximately similar.
There are few hard rules as each transistor is different to some extent 
even from the same batch.  Change to a MOSfet (Tmos, LDmos, etal) and 
the game changes again.

If it helps your delivering power into a load.  For a given voltage there is a 
load resistance that will absorb and transfer that power.  If the load 
resistance is too high then you cannot deliver the desired power.
That basic part is the same at DC or 2ghz.


11882 2015-11-13 09:43:58 timshoppa Re: Matching networks for RF Power Amplifiers
As a starting value, for a class C power amplifier, the collector impedance will be two times DC collector voltage divided by DC collector current.

The collector voltage and collector current are taken from the actual design although obviously if you don't want to burn up too many final transistors, you choose to operate somewhere in the max collector voltage and current on the device spec sheet.

Replace the "two" in the formula above, by 1.5 or 1.35 for class AB1 stages.

Replace "collector" with "plate" or "drain" for tubes or MOSFET's :-)

You are right, there are stray capacitances and inductances in the device itself. Some of them you have found already as simple numbers on the spec sheet and these numbers can sometimes for example simply be subtracted from the input C of a pi network to get a good starting value. In general these can be taken into account by the S parameters and calculations and this is very fundamental e.g. to UHF or microwave design. Or (more likely) in HF construction, they are accounted for during construction by tweaking for max power out (Aka max smoke!)

Tim N3QE