LF: Amplifier Topology

["Talbot Andrew" <ACTALBOT@dera.gov.uk>]

Hello all -

 

Mal and others have been asking for details of the high power amplifier I described earlier.  At the moment only the QRP 650W version has been tested and the 2-3kW design concept was a best otline estimate based on my tests of that breadboard.

 

As this sort of design is not for the faint hearted I am not just going to give out circuits that anyone could try building from (at this stage anyway).  Just think of the damage 5000uF of capacitors charged to 340V would do if shorted.  That is nearly 300 joules of energy, equivalent to dropping a 5kg sledge hammer from the 6m second floor of a building.  I blew 4 IRF840s on the breadboard due to various (now obvious) reasons, and each time they went short with an almighty band that set ears singing and me into a state of shock !  This design has 5 times that stored energy.

 

The outline concept given is enough to get experienced constructors going.  If you need more info than that, then you're not ready to be building such a beast! !!!!

 

Here's the outline again :

 

Drive stages, 2.Fc input, divide by two, BPSK phase modulator or other on-off gating, HP4426/7 driver chip. NO RAMPING OF DRIVE TO TRY TO AVOID CLICKS, don't even attempti t.   Could try using the invert/non invert HP4428 driving the drive transformer in push-pull (double the primary turns) - gives more power to drive 4 FET devices together.

 

4 x IRF450 or IRF460 in full - bridge driven as per the Decca transmitter output stages with BYV29-500 catch diodes.  Current spike soak, 1-2uH CT choke on each pair of devices, damped with 20 ohms   Look at hte LF experimenters manual for the Decca Tx design.

 

Tank circuit Ql = 6, caps rated at 3kV, use series parallel to take the current / voltage. I nductor wound with Litz wire - use Rayners formula to estimate the turns needed and measure from there.

 

Impedances and transformer taps calculated as per standard techniques 

Vcc = Vpeak.  Vsine peak = Vsq peak * 4/pi.   Pout = Vcc^2 / 2.Rl

 

PSU  Mains - 20A bridge rectifier, 5000uF capacitor(s)  output 340V DC, (obviously with soft start and surge limiting). 

Plenty of input filtering, lots of PSU rail decoupling at wide frequency band - all the usual etcs. of PA design

 

One of my aims over the Winter Solstice enforced holiday season is to build up a website wherec ircuits and software can be placed.  However, don't hold your breath,b uilding more interesting RF projects and software will probably get in the way, so  probably won't happen :-(

 

Andy  G4JNT

 

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