RF Power Amplifiers power combining

Hi,

Thanks for the replies. Things clearer now.

@chuckey : Could you please explain the below further:

if one module has less output then its neighbours it will soak up power

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The outputs of the four amplifiers must be fed to a common point. This point can have any impedance that you want, for convenience 12.5 ohms would seem good. This then needs a matching network to get back to 50 ohms for the aerial. The whole RF path must have an equal phase delay right to the combiner point. This is so the RF from each module is in phase with its neighbours. Likewise the RF amplitude of each module must be balanced, because if one module has less output then its neighbours it will soak up power, so output power is then reduced and cross-modulation increases, and the overall distortion products will be higher then expected.
Frank

**broken link removed**

For higher power, you'll mostly use combiners that incorporate an impedance transformation and have all-side 50 ohm matching. E.g. Wilkinson dividers or 90° or 180° hybrids.

I don't however understand your "second approach". The available output power of a PA is mainly limited by the thermal and in part the maximum voltage/current ratings of the output transistors. You'll achieve maximum output power by operating the transistor with optimal impedance matching, but you can't arbitrarily increase the output power.

Thanks for the replies. I understand the point made here about using the combiners. The replies only clarify it further. My question though is:

Let's say we have a PA whose otuput is 1 KW, can I feed this 1 KW to the next PA to get a further amplification to let's say 2 KW? I assume this is not a good idea because of the thermal limitation (as mentioned by FvM). I am trying to understand do other facotrs like fan-in, dynamic range of input also play an important role here? In other words, if at all I decide to work on an N-stage amplifier as described, what all would limit the no. of stages I can have?

@chuckey: Could you please explain this further:

the RF amplitude of each module must be balanced, because if one module has less output then its neighbours it will soak up power, so output power is then reduced and cross-modulation increases, and the overall distortion products will be higher then expected.

Click to expand...

Thanks.

rfndmw said:

Let's say we have a PA whose otuput is 1 KW, can I feed this 1 KW to the next PA to get a further amplification to let's say 2 KW?

Click to expand...

No, this does not work.

1. if you have a PA module that is rated at 1kW, its an amplifier, yes? so its input power will be less then this, say, 300 watts? So what would happen if you feed 1kW into an input stage rated at 300 watts?. What the typical configuration is, would be one PA module as a driver feeding say 4, 5 ,6? PA amplifiers that feed the output.
2. You still have not let on what frequency you are using or if you want a linear amplier or a class C.
3. If you have two PA amplifiers, one of which is delivering "full" power and one less. then sitting at the collector of the low power amplifier, when the current reduces, its voltage increases( due to its own RF) but the voltage also increase due to the incoming excess RF voltage from the full output PA. This increase in voltage will then produce unwanted effects.
Frank

Every PA a rated maximum saturated output power. If a PA has a rated saturated output power of 1kW, you probably can't get much more out of the device. Also, you need to provide a driver to the PA. In order to get more power, you must use power combining techniques. For example, you have 0dBm input power. The input gets fed to a driver amplifier with a power gain of 40dB. At the output of the driver, you will have 40dBm. The 40dBm of power gets equally divide into two different output. After the the power divider/coupler, you have 37dBm of power at the two outputs. Those two output will get fed to the PAs, with a power gain of 23dB. The output of the PAs will have 60dBm individually. With another power combining topology, you will get a single output of 63dBm which is equal to 2kW.

Hope this helps.

Microwave 123