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If you want to obtain the maximum power transfer between a generator and a
load, and if all the pieces behave like ideal linear components, then you need to match the impedance of the two. But this is not what you are doing here. First, you want to obtain the maximum power out without burning up you output device, and second, a class-C output stage isn't a linear circuit! What this means is that in general your transmitter, like your wall socket, can deliver more than it's rated power if you put the right load on it. Normally you'd expect the final amplifier to be fairly "stiff" - the output power should be inversely proportional to the resistor you put on it. Your circuit doesn't act exactly like that, even if it doesn't follow the same curve for a generator with a linear impedance. I suspect that with your circuit the 7-pole filter is evening things out quite a bit. There's also a good chance that the "strange results" you see with a load below 10 ohms are the final amplifier going unstable and oscillating, or your output transistor heating up and changing characteristics on you. So what it boils down to is that it is very important that your output stage _sees_ the right impedance, but you shouldn't expect the output stage to _deliver_ the same impedance that it needs to see. As long as you're getting power out and your output transistor isn't letting all the smoke out then you're fine. "Gary Morton" wrote in message ... Over the Xmas break I constructed a simple 2 transistor QRP transmitter using the Tuna Tin II design. Using a power meter and a 50 ohm dummy load it appears to show around 200mW, a little less than the article suggested. Then again I had substituted the final stage 2N2222 transistor for a beefier (size wise) BFY51. I'm not too sure what exact difference this will make. In the article it says that the 200 ohm output impedance of the final stage is transformed down to around 50 ohm (using a 2:1 turns ratio untuned transformer wound on a toroid). I have added a 7th order low pass filter to reduce the harmonics. I am interested in verifying the output impedance. If I understand the theory, when the output is terminated with a resistor which matches the output impedance then the power transferred to the load will be maximised. I set about loading the output will a series of resistors and measuring the peak to peak voltage across them in order to calculate the r.m.s. voltage and hence the power dissapation. Resistors of value less than 10 ohm gave strange results. load r.m.s. power (V*V/R) voltage 10 1.272 0.162 15 1.767 0.208 27 3.005 0.334 33 3.253 0.321 39 3.676 0.346 47 4.066 0.352 56 4.384 0.343 68 4.666 0.320 100 5.303 0.281 The numbers work reasonably well and point towards 50 ohms-ish. The power value looks too high - so I might have made a mis-calculation somewhere! Can anyone pass comments on the above? I'm interested in where the 200 ohm figure comes from. Any suggestions as to other methods of measuring the output impedance? regards... --Gary (M1GRY) |
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