10W Class AB with parallel transistors - Follow-up to Initial SOT223 Package Results

Eventually I found time to measure the input impedance of the amplifier. With 3 turns on the input transformer the input impedance measured 340 ohms on 80m. This is the right most curve of the three shown below - the labeling of the markers might cause confusion. With 2 turns it dropped to 159 ohms and with just a single turn it was 37 ohms. 

Input impedance to 10W "JBOT" Amplifier

 

Gain for the 10W JBOT stage has risen to 20dB with the 1 turn winding. However, I'm not completely satisfied with the "shark fin" distortion that is present on the output. Perhaps I am too picky since I have never previously looked at the output of an amplifier before any low pass filtering. 

Next steps:

Add a low pass filter and measure the level of the harmonics.

If the harmonics are acceptable then I will have met my goal of a robust 10W amplifier.

10W Class AB with parallel transistors - Initial SOT223 Package Results Encouraging

 Another trip to the races. 

The first SOT223 transistor I tried vindicated the switch to this package by being very robust. However, the gain was low and there was lots of distortion. 

The second SOT223 transistor I tried was very robust with over 10dB of gain. I left it running at 10W for several minutes with no ill effects. And I pushed it to 20W without blowing anything. An encouraging development. 

Now, if the gain was 10dB then the 1W driver amplifier board should not have a lot of distortion. However, it was distorted making me suspect the input to the 10W amplifier was not close to 50 ohms. While the results would only be ballpark and more of a pass/fail measurement I went to measure the input impedance to the 10W amplifier board with my nanoVNA. 

Unfortunately the shed PC was dead with what appears to be a power supply fault. Most annoying. Now I know where the magic smoke I smelled a few days ago when i powered up the shed equipment came from.

It is a distraction but I think I will buy a refurbished desktop PC to replace the now defunct XP machine. It was my programming workhorse but I have learned over the last couple of years that I can get all my software running on Windows 10. So rather than persevere with it now seems a good time to switch.

I will progress this as time permits and given the milder autumn weather now I'm looking forward to tackling this soon.

 

10W Class AB with parallel transistors - Not yet, but 5W is OK

It appears a cheap, stable and effective 5W HF amp can be realised with parallel SOT89's and thermal bridges. 

 

A variety of transistors have been tested. Some could deliver 10W continuously for extended periods thanks to the thermal bridges and heatsink. However, none appeared capable of delivering 10W with low distortion.

 

Gain and power handling varied between transistor types. Some test boards could deliver well over 10W into the dummy load with no mishap. Harmonic distortion was present so a good low pass filter is essential. No issues with thermal runaway were noted.

 

I learned the hard way that a 1206 resistor had an insufficient power rating as an emitter ballast resistor when testing for extended periods. After replacing several sets of resistors I switched to a larger SMD resistor.

Having destroyed many sets of SOT-89 transistors I have concluded the SOT-89 package is not going to deliver 10W of RF continuously.  Using 10 parallel transistors the per package dissipation is around 2W (1W of Rf requires at least 2W of electrical energy). At times I saw 5 Amps of total collector current on the "guess" meter fitted to the power supply which was just too much for many of the devices. 

While several transistor types could deliver 5W with low distortion that wasn't my goal. So it's time to move on to a SOT223 package.

73's

 

10W Class AB with parallel transistors - Qualified Success

I began some detailed measurements. In particular, I was interested at where the onset of distortion began. 

I started collecting data to build a chart on output power v input power. When I started measuring at higher output levels I found the buffer amplifier was in fact distorting above 1W of input power. Not surprising given it was designed to deliver 1W but something I hadn't considered till now.

But this means the first transistor I started measuring is probably not going to be the best transistor given the gain is lower than I wanted.

 

 

I call this a qualified success because I have a very robust and stable amplifier that delivers 10W, albeit distorted, and a maximum observed output of 14W with US$0.50 worth of transistors is outstanding.
At 5W it probably ticks all the boxes.

When time permits I'll test the second board I already have populated with a different transistor.

73's

10W Class AB - Almost a Success, then the Smoke Escapes

I tried the first alternative transistor and saw mixed results. The distortion on peaks was still present. I slipped with the attenuator control and pushed the amp to 20W. Briefly. Then all the emitter resistors let go. 

The distortion has me puzzled and is the same as the first amplifier with 10 parallel transistors. So I Googled and saw this waveform:

 

 

The term "sharkfin" describes it well. The distortion in my amplifiers can be similar when pushed hard which does point to harmonics being present. The amount of distortion at 10W appears to depend on transistor type from results to date.

 

It appears I'm close to achieving 10W of output with 10 parallel SOT89 transistors. I'll replace the resistors and give it another try. A post with more detailed test results in due course.

10W Class AB with parallel transistors - Take 2 preparations

Time for another attempt. A selection of SOT89 transistors was purchased allowing each board to be populated with 10 transistors of the same type.

One of two boards ready for through hole parts and smoke testing. It is sitting on the heatsink to check the hole alignment and the heatsink has been milled for clearance as necessary.

 

 

 

10W Class AB with parallel transistors - Almost

Update: I've learned that the cause of my waveform distortion (sharkfin) is most likely the presence of harmonics.

Time for a second attempt. The board was mounted to a CPU heatsink, milled for clearance underneath as necessary. Cautiously I increased the drive. I found I could easily achieve over 10W from 2 - 20MHZ. The distortion on peaks is evident so further work is needed. 

By testing with a single tone I am being harsh on the amplifier.  A two-tone test signal delivering 10 Wpep would have 2.5W in each tone. At 4W with a single tone there is no obvious distortion. I must drag my two tone generator out of retirement.

However, this JBOT approach with parallel SOT89's has merit. The distortion reminds me of what I saw with the 1W class A experiments when the biasing wasn't quite right. Even with 80mA of standing current in each transistor the distortion at 10W was present.

It appears a cheap, stable and effective 10W HF amp can be realised with parallel SOT89's. The application of a two tone test signal is needed before this particular transistor model is condemned.

73's

 

10W Class AB with Parallel transistors - Initial Success

Having drafted a board with parallel SOT89's I sent the files off to have it fabricated.Upon receipt of the boards I found many errors and it transpires I generated the gerbers from an early draft which had not been finished. 

Out came the tools to manually fix the board and I populated the first board as a proof of concept. I ran out of 1210 resistors which I have been using as thermal bridges so I finished the board with 1206 resistors instead.

In my last blog I stated:

Since 2 of these in parallel class A could easily deliver 1W continuously it is reasonable to expect that 10 of them, operated class AB in push-pull, could deliver 10W. 

On reflection it would be reasonable to assume 10 devices could deliver at least 5W. I quickly found I could achieve over 10W but with some distortion. I also quickly found the smoke can escape.

Here is the board mounted on standoff above a heatsink which has a 50Ω resistor mounted to it. All of the emitter ballast resistors had burnt out. 

Presumably one transistor got too hot, developed a collector to emitter short which burnt out one resistor. Now 9 transistors were carrying the load, another transistor failed and resistor burnt out. Now 8 transistors.....Apologies to anyone with "10 in the bed and the little one said, roll over, roll over" now running through their head.

I will have to modify a second board and try again.

73's

Richard

10W Class AB - Next Steps

So I checked, and re-checked, every transistor I could lay my hands on. Nothing was suitable for 10W. Nor does it appear anything can be purchased that might be suitable that has not been tried.

Several transistors in plastic encased TO220's worked fine at a few watts but couldn't cope beyond that.

From here I have stopped looking for a general purpose transistor for a single-ended  amplifier.

A board is being layed out with parallel SOT89's. Since 2 of these in parallel class A could easily deliver 1W continuously it is reasonable to expect that 10 of them, operated class AB in push-pull, could deliver 10W. 

Something to be put to the test in the near future.

73's

Richard

10W Class AB amplifier - Modest Success

After letting the smoke out of yet more transistors I finally managed to get a repeatable 6W out of the amplifier. I made two changes to the amplifier:

1. The output transformer was changed to a larger binocular core, and
2. The input transformer was changed to a 9:1 transformer instead of a 4:1 transformer.

The first change appears to have resolved the issue where a few amps was reducing the return loss. See the post here for what was happening.

After popping yet another transistor I suspected the voltage across the base with the 4:1 transformer was exceeding the Vbe rating of the transistor with fatal results. With the new transformer I have yet to pop the transistor.

However, I still can't achieve a continuous 10W because the transistor, which I believe is rated for 17W,  starts shutting down as the junction temperature rises. I have a large finned heatsink so the cause of this is unknown at present. 

Note that I am measuring power using a single RF frequency, or carrier. Thus the 6W is equal to average power. This gives me hope that with a two tone test signal 12W PEP is possible.

Why the modest label? My goal was to achieve 10W PEP with a general purpose transistor. I had to use a RF transistor and at present I haven't measured 10W PEP though I believe the amplifier can achieve this with a two-tone test signal.

The best of the general purpose transistors available today that I tested was a 2SC6144 which achieved 4W with a single frequency on 80m. However, this was with the smaller output transformer so the full potential of this device has not been tested. If you try one please let me know what you achieved.

There are some obsolete transistors mentioned here that also delivered 4W continuous which shows the junk box can be a useful source.

73's

Richard

10W Class AB Amplifier - Further Development Results

To test the conventional 9:1 transformer I blogged here I revised the board. The populated board is shown below:

With scars of many replacement transistors

I started with a 2SC1306, a once common CB output transistor. The smoke escaped all too quickly. I then tried a few general purpose transistors with unsatisfactory results.

One of the key things I noticed was that as the output was increased many transistors had a threshold. When that threshold was exceeded the output quickly dropped. If the bias was too high then thermal runaway also occurred.

The presence of this self-heating effect was noted in all of the transistors I tested, sometimes resulting in transistor failure. TO220 packages were better than TO126 packages presumably because they have a lower thermal resistance.

With only a few 2SC1306 left I switched to a 2SC2146. I was able to get close to 4W (40Vpp) but  eventually the smoke escaped.

I'm going to replace the transistor and see if a larger transformer is needed. 

73's

Richard

10W Class AB HF Amplifier - Initial Testing

I mentioned in this post that while the 2:1 transformers were successful, they still needed to be tested at higher power levels. This post covers that testing and some initial observations on transistors to be used in this amplifier stage.

10W(?) HF Test Amplifier using a CPU Heat Sink

Above is a picture of the amplifier using the two transformers I tested back to back in this post. At 4W continuous the transformer used in the output had no issues at 80m. I didn't get a chance to try other frequencies for an extended period because I was too busy letting out the smoke!

The initial transistor tested was a 2SC1096. From the datasheet we read:

·For audio frequency power amplifier and low speed switching applications
·Suitable for output stages of 3 to 5 watts car radio sets and car stereo

I expected this to work well at 80m and 40m given the Ft of 65MHz and it did so. Gain decreased with increasing frequency but across 160-15m I was able to achieve 4W continuous by adjusting the drive. 

2SC1096 Initial waveform at 1W output on 80m

 

The initial waveform at 1W output on 80m (above) suggested an increase in standing current would beneficial. I pushed the output up to around 40Vpp. 

2SC1096 4W Out

At this point I was not entirely happy with that downward sloping waveform after the positive peak of each cycle. But I pushed the bias too far and popped the transistor. However, my initial goal of testing the bias circuit and 50:12.5Ω transformers was achieved.

I replaced the transistor with a 2SC1226, which is broadly similar to the 2SC1096 i.e. it's not a RF transistor. Again I could get 4W easily.

2SC1226 at 4W Out

Of note is the initial downward sloping part of the waveform is around 0.06uS long, against the 0.08uS of the 2SC1096. Which sounds better though I suspect this is still not ideal.

The 50:12.5Ω transformer was replaced with a Ruthroff Type 9:1 on Small Binocular Ferrite. This gave me the potential to output more than 4W. I could still get 4W or so out but increasing the drive popped the transistor.

So progress has been good. I might have popped a couple of junk box non-RF transistors but I established the development board, biasing and transformers are all working.

Next time I will replace the transistor with an RF transistor so I have something to compare a number of readily available general purpose transistors against. 

73's

Richard

9:1 Conventional RF Transformers 50:5.5Ω

I started with one of these binocular cores simply because it was already on the bench. It's also enjoyable to get something that has no data to work!

Using the braid from a piece of miniature coax cable as a 1 turn winding, I teased two holes in the closed end so I could wind 3 turns through the inside of the coax braid.

1 turn of braid, 3 turns inside braid

Good for 80m to 10m. What struck me some time later is the improved frequency response of this conventional style transformer compared to the 9:1 transmission line transformers I tested on the same core. This was contrary to my expectations that transmission line transformers had wider bandwidths. If I have learned anything in recent months it's that measurement beats folklore.

 I know that compensating caps are used in conventional transformers so I added 18, 22 and 27pF caps in turn across the 50Ω side.

Same transformer with different caps shunting 50Ω winding

The improvement in return loss is clear. I then repeated this on the 5.55Ω side of the transformer. The capacitors ranged from 220pF to 470pF. But these values appeared too large and I was getting inconclusive results. 

It appears a convectional transformer can be wound on these binocular cores. However, I need to do some reading on how to select the compensating capacitor values should they be needed. 

73's

Richard

9:1 Transmission Line Transformers 50:5.55Ω - More Results

Ruthroff Type 9:1 on Two Ferrite Beads

As an alternative to using trifilar windings I tried two of these cores, each core with 6 bifilar turns. These are connected in such a way that a 9:1 transformer is achieved. My expectation was this was just not enough ferrite or wire to work. I was surprised then to see a useful return loss from 160m to 20m. 

Ruthroff 9:1 Transformer Two lots of 6 turns on a single core

Ruthroff Type 9:1 on Small Binocular Ferrite

Switching to a regular binocular core I wound 6 bifilar turns on each half and repeated the measurements.

Binocular core, 2 x 6t bifilar, each winding on separate half of core

Comparing these two cores with the 4 bead 4 trifilar turns blogged about last time shows the 2c beads can be just as good as this binocular core:

Upper: 6 bifilar turns on each of two beads
Middle: 6 bifilar turns on each half of binocular core
Lower: 4 trifilar turns on 4 beads

 

Surprised again I looked at the Smith Chart of S11 for the binocular based 9:1 transformer. Since parallel capacitors are often seen compensating RF transformers I captured a few tests. Unfortunately I lost track of the compensating values used but the impact is clear. As the capacitor value increases the S11 curve rotates clockwise on the Smith Chart:

2 x 6t bifilar on Binocular Core
Upper no compensation, Middle some compensation and Lower More Compensation

 The resulting plot of the magnitude of S11 shows the outcome:

2 x 6t bifilar on Binocular Core
Upper no compensation, Middle some compensation and Lower More Compensation

Compensation can improve the performance of this style of transformer. It struck that a series capacitor could be used to "slide" the uncompensated curve around the Smith Chart. A 1nF capacitor was about right at 14MHz to cancel the series inductance present:

So yes the trace slid around the Smith Chart, but the transformer became narrower in bandwidth:

So if you need compensation to achieve the desired performance perhaps a conventional style transformer with compensation can be used? I'll look into that next.

73's

Richard

9:1 Transformers 50:5.5 - Initial Results

With the success testing 4:1 transformers I examined some 9:1 transformers. A trifilar winding, being a bit thicker than a bifilar winding using the same gauge wire, proved difficult with the small cores previously used. I wondered how a single winding through 10 of these cores would perform. I passed a single trifilar winding through 10 turns, arranged them into a U or long binocular shape, and secured them with piece of heatshrink.

1 trifilar turn through 10 cores

Note the Smith chart response is curving away from the 50Ω origin upwards and to the right. This is the same with a bifilar winding.

Returning to the test results here is the sweep of S11: 

1 trifilar turn through 10 cores

 

Very useful from 160m to 30m applications and so easy to wind. Just one turn. But it looked a bit cumbersome and not as broad as I had hoped. So I tried two turns through 6 cores. This was better but still didn't improve the upper frequency. So I tried 4 turns through 4 cores. 

Comparing 1t 10cores v 2t 6cores v 4t 4 cores

Clearly 4 turns through 4 cores is a better transformer, and would handle the 1W I intend to push through it at 80m. However, not as broadband as I had hoped given the success I had with 50:12.5Ω transfomres using these core.

At present these transformers are destined to be used in the 10W Class AB amplifier for 80m I am planing to drive with the 1W Class A amp I have developed. It will be interesting to see if they handle the power.

Overall, these tiny cores are a useful part. I certainly consider them useful for 4:1 transformers but trifilar windings are more time consuming to wind and solder.

73's

Richard

4:1 Transfomers 50:12.5Ω - Success

I hadn't expected this to work out so easily. I already had a selection of bifilar wound transformers wound on these cores so it seemed like a good place to start. Terminated in 12.5Ω I looked at the 50Ω connection with the nanoVNA:

8t bifilar on a single core

Another single core with 12turns bifilar was tested and it was too many turns. Good for 160m to 40m though. 

Shifting to two cores side by side like a binocular core with 7 turns proved useful for all of the HF bands. Five turns could be a suitable starting point for 6m.

7t bifilar on two cores binocular style

To check the insertion loss I wired two of these transformers back to back. (50:12.5 + 12.5:50) 

The loss was negligible.

Loss across two transformers back to back

With 1W applied to this arrangement for 15 minutes the cores showed no sign of heating, perhaps 1degree above ambient.  

The goal was to find a suitable transformer for matching 50Ω to input of a 10W amplifier. If a 4:1 transformer is the solution then this has been achieved. But a 9:1 transformer may be required so another round of testing is needed.

73's

Richard

1 Watt RF Amplifier using General Purpose Transistors - Latest Iteration

Taking on-board the test results to date the latest iteration of the 1 watt amplifier has the following characteristics:

• 1 Watt output
• 2SC3356 > 2SD1664 > 2x 2SD1664R in Parallel
  
• First two stages use a bifilar transformer to present the collector with 200 ohms for increased gain.
• Use of resistors to provide a thermal bridge to ground for heatsinking. 

The results of my experiments have overturned my previous position. You do not need a working RF transistor, suitably rated, recovered from another radio to build 1 watt Class A HF amplifiers. 1W HF amplifiers can be reliably made using many of the general purpose transistors available that cost a few cents each.

If there is sufficient interest I'll put together a kit. My back of the envelope estimate is the cost would work out around US$5 for a PCB with all surface mount parts fitted.

Save those working RF transistors for more demanding applications!

73's

Richard

Using 1210 Resistors as Thermal Bridges in 1W Transmitter

I mentioned in this post the concept of using surface mount resistors as thermal bridges. I've had a chance to explore this now and after various tests I'm completely satisfied this is a suitable way to heat-sink SOT-89 surface mount transistors in 1W HF amplifiers.

Balancing cost versus ease of installation  I settled on 4 pieces of 1MΩ 1210 resistors for each transistor. A 1MΩ resistor needs no further modification to isolate the collector voltage from ground. On an extended test the two parallel transistors had a  case temperature of around 130 degrees while dissipating a total of 3.4W and delivering 1W of RF to the dummy load.

 

I hope you find this tip useful.

73's

Richard

ps Please excuse the soldering. I'm using up some solder I would not recommend and and it's difficult to get a good joint without using too much.  That's compounded by it being too thick. 

My preferred solder is presently this one. 

My preferred flux is presently this one which I apply with a nylon type small paintbrush trimmed short to make the bristles a bit stiffer.

G.P. Transistors in RF Amplifiers - Parallel Ouput Transistors- Still More Success!

Seems I can't help myself but try different transistors. In the course of testing my resistor as a thermal bridge concept I needed to populate a test board for measurements. After that I finished populating the board and tested numerous transistors with it. 

Initially I had some problems, both a soldering fault and poor biasing due to an error in my spreadsheet. But I learned a lot about debugging this amplifier in the process. I have now tested a large assortment of transistors, both SMD and through hole. Most worked very well at 40m and 80m, some well beyond that. And I never encountered any instabilities.

Even on my narrow board I never let the smoke out of any transistor despite relentlessly abusing them. The only casualty was a 2.2ohm decoupling resistor burning out.

1 watt is about the limit with a 13.8 volt supply. The output signal is 20Vpp. After decoupling and loss in the collector choke you have about 13.2V at the collector. You need a bit over 1V on the emitter to set the current with a 5Ω emitter resistor. So you are left with a margin of say 2V to deal with saturation considerations. Even with everything going well the best you might achieve is an extra 1dB of output. Probably not worth the grief chasing it.

Too much standing current and the 2V margin is reduced. Too little standing current and distortion is present. What I do is calculate the bias component values based on a standing current of say 220mA since the bare minimum needed is 200mA for 1W of output into 50Ω. The base resistor to ground is then rounded up to a convenient value. Once built, check the emitter voltage. If higher than desired put a suitable value resistor in parallel with the base resistor to ground to reduce the emitter voltage to the desired value.

I then examine the emitter voltage (Ve) with my CRO. It must be greater than zero at all times when delivering the required 1W for a Class A amplifier. The exact value needed to avoid distortion varies with the transistor used. The minimum Ve is a function of standing current so if you are bottoming out then a small adjustment to the base resistor to ground is needed.

Parallel output transistor at this power level at HF are easy to heatsink using resistors as a thermal bridge and cheap. I recommend this approach over expensive RF devices in this application.

73's

Richard

Possible Alternative to Thermal Bridge?

It was ironic that investigating the use of surface mount transistors for QRP amps (see previous posts using the QRP tag) required a relatively expensive thermal bridge to heatsink the SOT89 package. These thermal bridges are made using aluminium nitrite. 

I noticed by accident that some high power surface mount resistors use aluminium nitrite as a substrate. Unfortunately, these are no cheaper than proper thermal bridges.

However, what about regular high power surface mount resistors? Reading various papers I learned they use a ceramic substrate with a thermal conductivity about 1/6th that of Aluminium Nitride. I hastily scratched a break in the resistance film on a few such resistors and constructed a test jig. Sure enough, I had a thermal bridge.

The initial result is very encouraging using two 2512 resistors, open circuit, on each transistor in the parallel 1W amplifier. I cranked up the supply until each transistor was dissipating 2W. The transistor temperature was stable at 125degrees Celsius while delivering 1W continuously. Some more suitable resistors are on the way and a re-design to the pcb means I will be able to properly test this soon.

73's

Richard