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I'm struggling to understand the implication of a poor large signal reflection coefficient at the output of a power amplifier.

I designed a power amplifier and checked the large signal S22 (for the optimum load impedance for maximum power) as the small signal S22 doesn't make any sense when the output swings rail to rail.

Even the large signal S22 isn't coming great (close to -2dB.) I expected to get a good large signal S22 as maximum power is transferred to the load (which implies Zload = Ztransistor*.)

Where can I be wrong in my assumptions? Is there anything to worry about here because I am getting poor LS S22 with maximum power delivery? If the impedance of the load isn't well controlled can it lead it reflections that will damage the device?

Vysakh K
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  • Is it actually causing you a problem? I have dealt with RF power amplifiers that have a very low output impedance, one being the 'Sierra', about 5:1 VSWR IIRC at 80 MHz. It was great for stress testing load protection switches against arcing as they opened, as you could tune it up into a lambda/4 air-line to deliver a high voltage at the switch. So, what's your frequency range, power level, and use case, and why do you think you have a problem? – Neil_UK May 11 '22 at 08:49
  • Hi Neil, what problems can a poor reflection coefficient can cause in this case? This is for 6GHz. I understand that a good reflection coefficient ensures good power transfer in small signal case. Why is it not met here? – Vysakh K May 11 '22 at 09:39
  • The question is, what problems is it causing for you? Do you get the power output you want? The efficiency? Is the amplifier output blowing up? Do you get the frequency flatness you want? If yes to all, then you have no problem. Poor output match **can** cause problems in any or all of these areas, but if it doesn't, then you don't have a problem. Link to what amplifier you are using, and spell out the specification you expect for the system it's embedded in, then we can see whether there's a problem or not. – Neil_UK May 11 '22 at 09:49
  • Hi Neil, I will be connecting this power amplifier to an antenna whose impedance is not that well controlled. So I am worried about the reflections damaging the device. – Vysakh K May 11 '22 at 14:22
  • A bad match on both ends of a cable will cause higher voltages/currents to be seen at the ends. These may or may not break your amplifier, some amplifiers are specified to be safe into bad loads, some are fragile. They certainly will cause non-flatness, more severe as the cable gets longer, which may or may not harm your system performance. If either is a problem (a) choose a better amplifier (b) insert a circulator in the line to remove the reflections (c) insert just enough attenuation in the line to improve the return loss. Different cost/performance tradeoffs for the different solutions – Neil_UK May 11 '22 at 15:16
  • For a constant source impedance, the maximum power is transferred to the load when load impedance is the conjugate match of the source impedance. However, for a constant load impedance, the maximum power is transferred to the load when the source impedance is zero. This is an important difference. We tend to like amplifiers to have a good S22 for other reasons (flatness, damage via voltage magnification, ease of accurate measurement), but for good power output, lower is better. – Neil_UK May 13 '22 at 19:41

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