wyattmengy
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Hello everyone, I'm new to this forum and excitingly this is my first post at here!
I'm now working on design a Doherty power amplifier. I have read through the chapter on Doherty amp in RF Power Amplifiers for Wireless Communications by Cripps and everything makes sense, except two things:
1. Referring to Fig.1 (the two figures are from a past thesis report), I understand that in a Doherty amplifier, RL=Ropt/2, and the characteristic impedance of the quarter wave Tline, RTL, has to be equal to Ropt, so that as the input power goes from the -6dB backoff point to the maximum, the load impedance (RIN) seen by the main amplifier will decrease from 2*Ropt to Ropt. But I got a question that how to calculate Ropt for a FET? According to Cripps, Ropt is the optimum load resistance that the FET needs to see to output maximum voltage and current swing (hence maximum power). So I thought if I finish a load pull test, Ropt will just be the real part of Zopt come from the load pull test. But if you look at Fig.2, in this report, the load pull result is Zopt=8+j*19, but RIN in Fig.2 is not 8 but 64 ohms. The corresponding description is on page 39 to 41 of the report attached. The shunt inductor he used was 2nH. So by paralleling a 2nH inductor with a 64 ohms resistor, there will be an equivalent input impedance of 9.6+j22.9 ohms, which is fairly close to to his original Zopt=8+j*19, but I still don't understand why did he choose 64 ohm as RIN, but not directly 8 ohms from the load pull result Zopt?
After all, does Ropt equal to the real part of Zopt obtained from load pull test, or it is calculated using other method? If so, how should I calculate it in ADS?
2. For performing load pull in ADS, do I need to first conjugate match the input of the FET? I found that in ADS' design guide, all load pull tests are done without any input match. This makes me wonder what's the correct order of designing matching networks for a PA: load pull for the output matching and then conjugate for the input matching, is this correct?
Thank you guys so much!
I'm now working on design a Doherty power amplifier. I have read through the chapter on Doherty amp in RF Power Amplifiers for Wireless Communications by Cripps and everything makes sense, except two things:
1. Referring to Fig.1 (the two figures are from a past thesis report), I understand that in a Doherty amplifier, RL=Ropt/2, and the characteristic impedance of the quarter wave Tline, RTL, has to be equal to Ropt, so that as the input power goes from the -6dB backoff point to the maximum, the load impedance (RIN) seen by the main amplifier will decrease from 2*Ropt to Ropt. But I got a question that how to calculate Ropt for a FET? According to Cripps, Ropt is the optimum load resistance that the FET needs to see to output maximum voltage and current swing (hence maximum power). So I thought if I finish a load pull test, Ropt will just be the real part of Zopt come from the load pull test. But if you look at Fig.2, in this report, the load pull result is Zopt=8+j*19, but RIN in Fig.2 is not 8 but 64 ohms. The corresponding description is on page 39 to 41 of the report attached. The shunt inductor he used was 2nH. So by paralleling a 2nH inductor with a 64 ohms resistor, there will be an equivalent input impedance of 9.6+j22.9 ohms, which is fairly close to to his original Zopt=8+j*19, but I still don't understand why did he choose 64 ohm as RIN, but not directly 8 ohms from the load pull result Zopt?
After all, does Ropt equal to the real part of Zopt obtained from load pull test, or it is calculated using other method? If so, how should I calculate it in ADS?
2. For performing load pull in ADS, do I need to first conjugate match the input of the FET? I found that in ADS' design guide, all load pull tests are done without any input match. This makes me wonder what's the correct order of designing matching networks for a PA: load pull for the output matching and then conjugate for the input matching, is this correct?
Thank you guys so much!
