EMRFD Message Archive 6232

Message Date From Subject
6232 2011-04-30 22:37:08 Ashhar Farhan ultra-linear power amplifiers
is it possible that we can build a QRP class-A power amplifier that
has low enough harmonic distortion to be usable without the standard
half-wave filters? the reason why i ask this is, this could eliminate
the need for half-wave filters. i have seen it to be easily achievable
at lower power levels of less than 10 dbm. what goes wrong with life
at 37dbm?

- farhan
6233 2011-05-01 02:15:02 Lasse Re: ultra-linear power amplifiers
Yes it is possible... but hardly worth it!
We have at the lab a 3-watt class A amplifier made by ENI.
The power consumption is roughly 70 W ! Addmitting we pay a lot of
ineffencies for the extreme broadbandness
10 kHz to 1GHz but still you are facing something like 20% efficiency
for a more "normal" class A...
You need 20-30 watt to get those 5 watt output.

/Lasse SM5GLC
Ashhar Farhan skrev 2011-05-01 07:37:
> is it possible that we can build a QRP class-A power amplifier that
> has low enough harmonic distortion to be usable without the standard
> half-wave filters? the reason why i ask this is, this could eliminate
> the need for half-wave filters. i have seen it to be easily achievable
> at lower power levels of less than 10 dbm. what goes wrong with life
> at 37dbm?
>
> - farhan
>
>
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6234 2011-05-01 05:52:33 kb1gmx Re: ultra-linear power amplifiers
6257 2011-05-06 10:03:57 vbifyz Re: ultra-linear power amplifiers
Actually, there is a way to make the PA both ultra-linear and efficient. The method is called predistortion: the PA amplitude response is calibrated and then the input is dynamically scaled to yield the output that is exactly proportional to the desired value.

With the advent of SDR, this technique is now within the reach of homebrewers. It requires quite a bit of programming, but should work with the existing Softrock RXTX hardware (maybe with the mod that feeds back TX to RX for the initial response calibration).

73, Mike VE3YXA