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## Re: [Amps] Transceiver Output Impedance

 To: amps@contesting.com Re: [Amps] Transceiver Output Impedance peter.chadwick@Zarlink.Com Sun, 9 Mar 2008 10:46:44 +0000 mailto:amps@contesting.com>
 ```This is an interesting question, and there are two schools of thought. One says that you get maximum power when you have a conjugate match, and the other says you get maximum power when feeding an optimum load impedance. The first school of thought (which includes Tom Rauch and Professor Belrose) have measured the output impedance of a tubed SSB transmitter and got an answer of about 50 ohms The second school of thought (Warren Bruene) argue that if you get maximum output power when conjugately matched, then efficiency can never be greater than 50%, and Class C stages work at much higher efficiencies. To which the first school suggests a non-dissipative resistance in Class C, but that one passes me by. If the PA stage is about 50% efficient, then you would expect to get something like a conjugate match. Look at the curves for the 6146, and you see the plate resistance is about 2000 ohms, which is roughly the correct load impedance, efficiency is about 50% and the correct load is something like a conjugate match.. Consider a transistor PA. It has a very low collector resistance, but wants a load given by Vcc squared divided by 2 Pout. Here we start getting to the point where other resistances start to play a part - emitter ballasting, bond wire, even the connecting tabs. This modifies the practical case. In general, I think most linear PA stages do look something like a conjugate match because they are 50% efficient, give or take a bit. Finally. Suppose a 12 volt 1 amp power supply. For load change between 0 and 1 amp, the voltage sags by 1 millivolt, so the internal impedance is 1 milliohm. If conjugately matched, it would try to deliver 12,000 amps! So maximum power in that case is not when conjugately matched - just as the electricity supply network isn't. If you try, you blow fuses - we had a chap in college who did just that. Except it was on 1200 amp phase of the 415 volt three phase supply! 73 Peter G3RZP ---------- Zarlink Semiconductor Limited is a Company registered in England and Wales under number 00705031 with its registered office at Cheney Manor, Swindon, Wilts SN2 2QW, England. This email is confidential and may contain information that is privileged and exempt from disclosure by law. If you have received it in error, please contact the sender immediately by return email and then delete it from your system; you should not copy it or disclose its contents to anyone. Emails are not secure and cannot be guaranteed to be error free as they can be intercepted, amended, lost or destroyed, or contain viruses. Anyone who communicates with us by email is taken to accept these risks. _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps ```
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