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Re: LF: TXing 2200m WSPR

from [Andy Talbot] [Permanent Link][Original]
To: [email protected]
Subject: Re: LF: TXing 2200m WSPR
From: Andy Talbot <[email protected]>
Date: Sun, 24 Dec 2017 18:06:39 +0000
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The other thing about that circuit that is wrong is the power out specification for the turns ratio / loading resistance.

For a single ended PA, Pout max = Vdd ^2 / 2 / RL.   With a 9:1 (3:1 turns) transformer RL = 50 / 9 = 5.6 ohms
So for a 13.8V suply (ignoring Vsaturation) that means  an absolute maximum power output possible of 13.8^  / 2 / 5.6 = 17 Watts into a true 50 ohms load on the transformer output.

In practice the device will saturate with a volt or so across it, lowering the voltage swing and keeping power out to a lower value.   If the peak swing is 12V, Pmax now drops to 12^2/2/5.6 = 13 watts

 Riccardo, I assume you mean 1:4 turns ratio,  or 1:16 impedance transformation.   That Means RL = 50/16 = 3.1 ohms and at 12V swing gives 23 watts out as you state

I made a push-pull PA using a similar  type of FET, but used a 28V supply and 1+1:2 transformer.  It gives 40 - 50 Watts over the frequency range  100kHz to 2MHz


Andy  G4JNT

On 24 December 2017 at 17:33, Riccardo Zoli <[email protected]> wrote:

Hi Paul, Stefan 

my first PA for MF and LF, based on a IRF520, was similar.
However, I used a most appropriate 1:4 output transformer instead a 1:9. With 13.8V, a 50mA bias current is more than adequate for AB class. You could obtain about 20-22W max on output.
Obiviously, more supply voltage it means more power output.

Merry XMAS & HNY!

All the best


73 de Riccardo IW4DXW


Il 24/Dic/2017 13:30, "DK7FC" <[email protected]> ha scritto:
Hi Paul,

Am 23.12.2017 23:36, schrieb N1BUG:
For anyone trying to follow along, this is my current version:

http://n1bug.com/ASB_LF_Amp-20171223.jpg

The circuit looks good to me now. Maybe it helps to use a low pass filter in T configuration instead of a PI config. The T config starts with a series L. The PI config starts with a parallel C. That could make a significant difference for the harmonics.

73 and Merry christmas, Stefan


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