Sunday, December 16, 2012

PA-47 - A QRP KiloWatt

I have not posted anything for the last few weeks, I have been a little bummed out.  I started work on an interesting Project, which is a "QRP KiloWatt", a 5 Watt Amplifier. It will be used with my Propeller Beacon Project (see many previous posts) and maybe my other QRP projects.

Sziklai Pair
What renewed my interest in building yet another PA, was a new (new to me) configuration of transistors - called a "Sziklai Pair", or sometimes referred to as a "Complementary Darlington". The configuration is actually quite old, and was invented by George C Sziklai  (pronounced as: "SICK-LIE", which is the best guess as per the web). I had not seen or used the configuration before.

To me, the interesting spec is that the Sziklai Pair can produce nearly Rail-to-Rail output voltage - which is exactly what is needed for a FET PA driver. FET PA's require high signal voltages (i.e., +6 or 7 volts to drive the gate) which is much more than the typical 0.6 volts for a normal transistor. Transformers can be used to produce the high drive voltages for FET PA's. But, I did not want to do that, especially if two simple transistors can do the task. I have built PAs before and always had messy drive circuits and problems.

I configured my planned Sziklai Pair Driver and FET circuit within LtSpice. It took a while to get the configuration and front-end bias right for my desired "class C" operation. But in the end, it is much simpler than I had imagined. Resulting in a circuit with very few components. LtSpice suggested I should get about 4.7 Watts at 10.140MHz from 12.6V DC when driven from my Propeller (Prop) Beacon, The Prop only provides about 2.5 P-P Volts at its output pin (or only about 12 mWatts drive).

Excitedly, I transferred the LtSpice configuration into DipTrace Schematic Capture, and from there created a DipTrace PCB layout. The circuit includes; SOT-23 2N3904 and 2N2906 for the Sziklai Pair, a IRF510 for the PA (standing up to mount to a heat sink) and a TR Relay and its support circuit. As usual, I wanted the circuit board to be as small as my abilities allow. The PCB was going to be only 1.5 x 1.0 inches, using SOT's and mostly 0603 parts.

While designing the circuit, I noticed that I had used mostly "47" valued parts, for example 4.7K resistors and 47nF capacitors. And, therefore I decided to name my new QRP KiloWatt - the "PA-47". Just for fun, and with very little work, I was able to convert all parts to 47 values. Besides, LtSpice said it would produce about 4.7 Watts - so what else could I name it?!!

The circuit was so simple I decided NOT to try to breadboard it, I just quickly pressed the DipTrace button and ordered the PCB's (that is way too easy).

For about a week and half, I waited for the PCB's to return from the Manufacture.

PA-47 - PCBs as Received
PA-47 - PCB Under the Microscope
Finally, the PCB's arrived, I excitedly loaded a board. But - DANG!! - I noticed I had used the wrong SOT foot print for the two Sziklai Pair transistors!! Maybe I pressed the DipTrace Order button too soon (the button is just too easy and handy). The two driver transistor's Emitter and Bases were exchanged on the PCB - DARN, DARN and DANG!
2N3904 SOT-23
as  MMBT3904

OK, now what, . . . . if I turn the transistors up-side-down, "dead bug style", the PCB foot print would work. All I had to do was bend the legs up and then try to solder the SOT transistors up-side-down on to the PCB. I notices the leads almost broke off when bending them back that far. I planned to fill the little crack with solder as part of the installation process, it would only require a slightly larger glob  of solder on the pin (actually the glob of solder is about the size of a flyspeck and can only be seen under the Microscope).

For the initial tests, I would not install the parts for the TR Relay Circuit, as it was not really necessary (I just shorted two pairs of the pads together, as if the Relay was always on).

PA-47 - Mounted to a CPU HeatSink
Output Matching Transformer and Connector
on the Right Side
PA-47 - DC Power and RF Input Connector
on the Left Side
I connected the Propeller Project as the Exciter, a Low Pass Filter, the 50 Ohm Dummy Load, and the 12.6 Volt supply to the PA, the Fan came on (a good sign)!. The Propeller was outputting its 2.2 Volts into the PA input. . . . . But nothing was coming out of the PA, - more Dang!

I spent the next several hours trying to figure out the circuit or problem. Remember is a NEW untried circuit, and there is a chance it would NOT work at all. After spending more time trying to make it work. I decided, maybe "I Should Have" breadboarded the circuit first!

So, I started loading a fresh breadboard with components. With still more time (several hours) I still could not get anything to work as expected on either circuit.

The Bench
Dishearted - I gave up - left the shop "without" putting away my tools and cleaning up (I normally always clean up before leaving).


Several days have past (about a week), it is now Sunday and I am planing to go to Jack's Homebrew Club evening meeting on Tuesday, and the following night I am going to the pQRP monthly meeting. I decided to clean-up the PA (to remove excess solder flux) to make it "look good", and take it to the two meetings, to show my epic FAILURE of a nice idea for the PA.

While cleaning the circuit board, I noticed one of the legs of the (necessarily) "dead bug" mounted transistors did NOT appear to be soldered (with filets) to its pad, it appeared to have just a glob (flyspeck) of solder on the bent leg. This was the drive transistor for the output power FET, maybe it was not actually soldered down!!

For me, it is actually quite difficult to see a single SOT-23 pad without a Microscope.

Under the Microscope I could see it only had flux between the pin and pad!! I heated the soldering iron, fixed the joint, re-connected the Propeller Beacon Exciter, the output Low Pass Filter, the 50 Ohm Dummy Load, and the 12.6 Volt supply, . . .

WOW!, I observed 43 Volts PP at the load. That is; (43/2*.707)^2/50 = 4.7WATTS - YES!!!!!

Which is a gain of 25.93dB.

The PA-47 Curcuit with a Sziklai Pair Driver works!!

I installed the rest of the TR circuit and Relay on the under side of the PCB, but - DANG! The relay would not work! Now what??

After several hours, I discovered that I had attached the Relay Coil to the opposite pins on the Library PCB pads within DipTrace, the Relay is polarity sensitive. Like the drive transistors I had jury-rig the Relay by cross wiring the coil under it to make it work (the fix can not be see in the photo).

After cross-wiring the coil pins and re-installing the Relay - All worked as expected :-)
PA-47 - With TR Circuit and Relay Installed
Regardless of Success or my Failures, Tess is always watching, to make sure everything is being done right! She was not involved with the PA design or PCB layout, but some how she must have missed the poor solder joint!! (actually, . . . she is watching her ball that I placed on the bench :-). If I had her concentration (and other admiral attributes), I may have NOT missed the above critical project details :-))))

Tess - Doing Her Job,
Production, Test, and Final Inspection
I am , . . . ah, . . we are NOW both Happy Campers!

. . . . . Tess has her Ball, and I have my PA!

I will later post more PA-47 Project details. More testing, and more work is needed to install angle brackets to support the PCB.


Increasing the DC supply of up to 14.2 Volts (via a heavy duty Lead Acid Battery on a Charger), I got 60 Volts PP at the 50 Ohm Load, (60/2*.707)^2/50 = 9.0 WATTS- Nice!!

Note to self:
A Fan control via the TR circuit or HeatSink Temp would be useful, it would save some power and would be quieter.


Here is the PA-47 running on Jack's Bench at his Homebrew Club meeting, the photo was taken by Jack and who suggested this photo would make a great (complex) puzzle picture :-)
PA-47 Running on Jack's Bench


1 comment:

  1. The little thing is looking good, what is the power transistor ?