[Amps] Solid state amp

[Manfred Mornhinweg]

Hi Tom,

replying to your message from last year:

> I am working on a solid state amp project.  After a few false starts,
> and damaged parts, I am trying a new approach and not pushing the
> parts anywhere their limits.

As they say, you can very well push them to their limits - but not 
beyond them! ;-)

The trick is to know where exactly each of the limits is. That's why 
most people prefer to keep some safety distance to them.

> I have a 300W boards purchased online from rfsource.  They accept
> either BLF278 or MRF151G parts.  The boards arrived (from Greece) and
> appear well made.

I'm looking at that company's website. I assume you bought the PHV30S 
boards?

> A BLF transistor was installed on the first board with a 48V supply
> and Idq of 1A.  The board would put out about 100W and then the drive
> power needed to push it further would rise dramatically.   The
> current would not rise with the extra input power and power out
> divided by DC power in looked fine. 

That sounds like the gate bias is being pushed down by the RF drive. 
Since I can't find a schematic for that board, I don't know if there is 
a problem with the design, or perhaps you made some mistake during assembly.

 > The board has a 3db pad in
> front of the amp and I could feel the resistors getting hot so I
> suspected the input matching as it simply uses a 9:1 transformer.

It's entirely normal that the power resistors in an attenuator circuit 
get hot, when doing their job!

> After a lot of experimenting with input matching yesterday, I could
> get the first board to look good.  Efficiency was typically about 50%
> at 150W and got better as frequency increased from 1.8  to 28Mhz.
> Output power and gain looked OK.  However, the same approach did not
> work on the other BLF278 board.

Check the DC bias as you apply drive. If it goes down, there is a problem.

> There were two MRF151G's left from a prior experiment and I put them
> on two other boards.  They performed noticeably different than the
> BLF parts, but similar to each other.  At 150W, the gain was over
> 20db (including the pad) but the efficiency was very poor.  It was
> just under 50% on 160, but dropped steadily as frequency rose to
> about 20% on 10.   I presume this will get worse when I have real
> antennas and not a dummy load.

How are you doing the necessary ultra low impedance ground connections 
between the MOSFET's flange and all the different crucial ground spots 
on the board? Even one nanohenry of stray inductance in those 
connections is a problem! I understand that with the boards you bought, 
the idea is to connect the flange to the PCB just through the heatsink 
and the mounting bolts. This is a poor method. I would absolutely add 
some copper straps from the flange bolts, soldered to the PCB ground.

Does that PCB have a ground plane on the backside, or is it single sided?

Many people believe that in a push pull circuit the grounds are 
unimportant, because the signal is balanced. This would be true if the 
circuit operated in class A! But in class B, or AB, or C, the signal is 
NOT balanced at all. At any given time during most of the RF cycle, the 
drain current flows only through one device (one half of your Gemini 
MOSFET) and through ground. So the correct ultra low inductance 
connections between the flange and all relevant parts of the circuit is 
as important in push pull circuits as it is in single ended ones!

The gate circuitry has some more balance, because of the relatively high 
capacitance of the gates, often further aided by low value swamping 
resistors. But still it can often help to have a low impedance between 
the driver circuit center point, and the MOSFET's flange.

Maybe the reason for the unpredictable performance of your boards is 
simply a varying impedance in the flange-to-board connections, depending 
on exactly how you mount and connect the MOSFET!

Manfred

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