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# RF PA design questions

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#### gremlin

##### Newbie level 5
rf pa design

This is the first time for me to design a RF PA. I did some internal research, and I also read the famous book "RF circuit design" by Chris Bowick. I've found the chapter on PA design in this book to be useful. but I have a question after reading this book.

According to the book, when the transistor's input and load impedance are published, then the input and output are conjugately matched. An example was also given. When the input and load impedance are not published, then the collector capacitance (given in data sheet) and optimum load resistance (=(Vcc-Vce(sat))^2/2*Po ) are used to match the output.

Now I am trying to design a FM transmitter at 50 MHz using 2N3553. The data sheet does not have input and load impedance. So I can only use the latter method. The question is what do I do to match the input when there is no information available? Why the manufacturers do not publish the input and load impedance? I have a network analyzer in the lab. Maybe I can measure the input impedance and match it. Is that the only way to design the PA?

Andy

how to do pa loadline match

First, the optimum load impedance is an impedance concerned with "theoritical load line slope" around operating point of the transistor. But you can not achieve optimum RF power by using this technique because of capacitances and lead inductances of the package etc.. Indeed, this is a real valued resistance..
But in reality, optimum impedance is complex. In order to find this impedance , the amplifer should be either simulated by aid of nonlinear model of the transistor or measured by aid of sliding tuner or load pull techniques..
I think, it's quite easy to find the nonlinear model of 2N3553 ( in SPICE format or other formats..). If you use a nonlinear simulator with HB feature, you can try to find optimum impedance for PA..

Conjugate matching produces the maximum gain and not the maximum efficiency or maximum output for a given transistor.

Transistor manufacturers aim their products for a specific market and make sample amplifiers which they optimize for the needs of that market. They will tell you the load value which is totally different from the small signal output impedance.

2n3553 model

Thank you all for the replies. Now I understand better. I do not have full license nonlinear simulator, but I downloaded the student version of Ansoft Designer. I don't know if it includes the nonlinear simulator. Anyway, I think I will start by trying the load-pull technique. I have not done that before, but I have read it from books years ago. It's time to go back to the book again. Will the result from load-pull testing give me information to match the input?

pa impedance simulation

gremlin said:
Thank you all for the replies. Now I understand better. I do not have full license nonlinear simulator, but I downloaded the student version of Ansoft Designer. I don't know if it includes the nonlinear simulator. Anyway, I think I will start by trying the load-pull technique. I have not done that before, but I have read it from books years ago. It's time to go back to the book again. Will the result from load-pull testing give me information to match the input?

Ansoft SV does not include nonlinear simulator option.
But you can demand a time limited license from AWR for MW Office , it includes a very accurate HB simulator.I recommend you..( Agilent also offers demo license for a limited time ..anyway )
For load pull measurement techniques, I offer you to read some AN of Maury Microwave on their website..
And finally, if you read the book of Andrei Grebennikov " RF and Microwave Power Amplifier Design" and Steve Cripps " RF Power Amplifer for Wireless Communications", you'll get all necessary information about the power amplifiers.

But don't forget, loadpull technique is applied only with nonlinear simulators and verified by complex measurements. The simulation and measurement can show somehow differences because of temperature, biasing and deviation from typical model..etc.

Thanks for the advices and recommendations. I visited the Maury Microwave website, and the app notes are very useful. I was also reading some articles by Josehp F. White in mwrf.com on amplifier design. I looks like I have confused myself on the different gain matching methods. I will have to go back to the books and clear up my concepts before working on the PA again.

Thanks.

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