QRP Transceiver 40W Power Amplifier

Updated 20160820
There is no major advance to be made in class AB amplifier design, but improvements in economy are possible. Problems with designing for the HF frequency range are:
  • RF FETs are expensive and often difficult to source
  • The industry is producing transistors for VHF and above…
  • Which means those for lower frequencies are expensive
  • Switching FETs are cheap but have many shortcomings
  • Linearity issues
  • Efficiency issues

FETs designed for switching power supplies have high parasitics and poor thermal characteristics. Nonetheless I have embarked on a project to find a usable part, and make a low-cost amplifier using them. PCBs for the project have arrived as shown here.
Aug2016 PCB
The amplifier section is on the left, filter to the right. If anyone wants a low-pass as a separate project, it can be split off by cutting along the dotted line! I am beginning to build with the amplifier as the first block, testing to see which of the power FETs selected perform best.

The most common modern device for HF amplifiers of about 50W is the
Mitsubishi RD70HHF1. They have some disadvantages:
  • High cost >£25 each in small quantities
  • Output impedance is too low for an efficient output transformer
  • Doubtful linearity at 12V supply

Having discounted the conventional wisdom power devices, I looked through the enormous range of switching FETs available. These are the challenges of designing with cheaper devices:
  • High gate capacitance (except early generation e.g. IRF510), makes a flat 1-30MHz response impossible
  • TO-220 packages have inductive wire bonds
  • Switching FETs hotspot badly and the bias point is unstable…
  • Fairchild Semi agree
  • OnSemi agree
  • Infineon agree
  • Microsemi agree
  • Temperature compensation and other measures are essential

I found these are most suitable in terms of medium gate capacitance, low-ish transconductance and good thermal conductivity:
STP14NF12 (TO-220)
FQP13N10 (TO-220)
STW13N60M2 (TO-247)

The first two are similar, with the ST part having slightly higher V
DSS rating, and the Fairchild part having a track record in citizen band radio output stages. The STW13N60M2 is the lowest gate capacitance available in TO-247, and it's interesting to test a high voltage part against the older generation 100V ones. I spent considerable time looking on manufacturer's websites to find these. Silicon technology is reaching the limits of what is possible, and very few low gate capacitance (<500pF) parts are available.

Even these low capacitance FETs have higher transconductance than RF FETs. So they are more sensitive to bias point when running in linear mode. How much temperature compensation is needed to stabilise them can be estimated from the FQP13N10 data sheet which has a standard graph of V
GS against Id and temperature. It comes out as 0.45V/150C = 3mV/C.

3mV/C is above the temperature coefficient of a single diode but less than two. Having good thermal contact between a plastic FET and diodes is difficult. Hence getting a rapid response of the compensation circuit is difficult. Compensating the bias has a limited effectiveness agains thermal instability. The no-signal bias must fall well into the safe operating area, but dropping the bias too far doesn't reduce dissipation under high signal very much. It has the disadvantage of compromising linearity. So I include bias stabilisation but became aware of its limitations.

Besides the bias, a FET will hotspot and burnout under continuous high signal. So any amplifier like this can be rated in SSB for higher power than in CW modes like PSK31. Having neutralisation (feedback) from drain to gate is actually a form of bias. Having this allows the no-signal bias to be lower, giving the FETs a chance to drop well inside their forward bias safe operating area during speech pauses. The disadvantage is reduced gain and lower efficiency. I decided to use a current limited power supply and add a safety feature of switching the amplifier out (back to Rx) by sensing consistent drops in supply voltage.

Best heatsink insulator thermal resistance is now down to 0.25C/W but a little pricey. Having the best heatsink possible is the only non-electronic way of ensuring survival of the FETs. Beyond 50W dissipation heat-sinking gets more difficult with heat spreaders and finely polished surfaces needed.

Another concept is using FETs at higher voltage than 12V, because the output impedance at the FET drains is set by the supply voltage. I looked at designing a boost power supply using
TI WebBench. The cost of a 33V, 4A boost converter makes up half the difference to using RD70HHF1 FETs in the first place, let alone the extra constructional complexity. I went round and round lots of compromises to arrive at 33V !

A solution was found in "250W" boost power supplies from China/eBay. These can put out 100W if heat-sinked, and are adjustable, with current limit for only £3.50! With such a cheap way to get 33V @4A, plans for 50W+ amplifiers were abandoned, and I decided to aim for 30-40W output. A rough prototype was constructed using STW13N60M2 FETs and the Chinese boost module. It performed well, and has these advantages:
  • Gives reliable gain and power from 10V-20V input voltage
  • 40W PEP SSB was reached on bands up to 15m, less output on 10m
  • Avoids a 1:4 step up on the output, simplifying the design

The target for this design is cost reduction, which compromises the power output to 40W PEP/30W CW. I'm not in the business of selling overdriven or exploding transmitters. A number of extras are included with push up the cost but make a complete system - RxTx relays; Low-pass-filter; SWR bridge; micro controller for automatic band switching and protection.

The circuit diagram is
here and here. A photo of the Chinese boost power module follows…

s-l500

The real snag with all the amplifiers on this page is their in efficiency. As a designer I have to accept this. Commercial equipment such as 3G base stations have long since moved to other amplifier topologies to improve efficiency. But schemes like Class-F only improve to 75% at best, with a lot of extra complexity.

The big improvement in future is a class-D (switching) topology. Unfortunately the technology to switch high power at >50MHz is not quite here yet. Far as I know, the best FETs (even GaN) can't sink enough power or switch fast enough to provide a 20MHz class-D with 90%+ efficiency. But as a curious designer I have to keep a close eye on the possibilities in this direction…


20Watt Amplifier (previous project)

Previously I designed successive amplifiers to produce 15-20W of RF power from less than 0.5W of drive. Targeted at Soft-rock project builders, though adaptable to boost other QRP rigs to the 15-20W level. I added several unique options. All details are in the public domain. Anyone that wishes to build this design, get commercial standard PCBs made, or sell kits, is welcome to do so.

The final batch of PCBs for these boards is now sold out. This is the prototype undergoing tests:

Amp_test
There is also a mini-movie here. It is being driven by an Elecraft KX3, set to 0.3W output. Voltage supply and quiescent current are seen on the power supply. In the foreground is an SWR/Power meter with a dummy load sat on top. The power meter indicates 15W output from a “whistle test”. The next photo shows the completed board mounted on a heat-sink plate:

Amp_test3

It captures several years of design thinking, and has features not found on other similar amplifiers:
  • The power MOSFETs have dual footprint, for Mitsubishi RD16HHF1 or power supply FETs
  • Input PI network for matching circuit or attenuator
  • Temperature compensation of bias to help stability
  • Facility for single or multi-band filter
  • Integrated RxTx relay with pull-low or pull-high switching voltage
  • Options for external filter banks

Datasheet parameters to look for in switching FETs are low input capacitance (<350pF), Vds max >40V. Package TO-220 or TO-247. Most MOSFETs optimise RDSon and have high input capacitance. Check websites of ST, Fairchild, NXP, Texas Instruments, On Semiconductor, International Rectifier, Vishay, and others. New types of MOSFETs often come onto the market.


There is a matching multi-band low pass filter
here. The Kits & Parts single band filter is no longer available. I have some replacements.

SingleLPF

There are several options for external filter banks on the amplifier PCB. An external higher power amplifier can also be used. Look at the circuit diagram and see what is possible. That’s why there are several coaxial termination pads on the board. Anyone that builds this amplifier, then wants to cover more bands with automatic switching, can add an external board. If there is demand I can design a suitable filter bank. My previous design had 4 filter banks, and worked fine from 80m-15m.

If you want better efficiency on the bands <10MHz it is possible to use a BN43-202 twin hole ferrite rather than the type 61 ferrite.

Also there’s an option for a TMP100 temperature sensor, controlled by I2C. MoBo V4.3 users may find it useful to read the output PA temperature instead of the MoBo PA which becomes the driver stage.

Build Data Release (Iss.C)

NOTE THAT THE BOM HAS BEEN UPDATED TO CORRECT ERRORS AND ONLY LIST PARTS THAT FARNELL/MOUSER HAVE IN STOCK.
Gerber-X files (issue C) are found
here. Have a look at them with a free Gerber viewer like PentaLogix SmartDFM. I included top and bottom silkscreen, but to save costs the bottom silkscreen is not essential. Board size is 100 x 60mm. Recommended PCB type is 1.6mm thick FR4, plated through holes, hot air solder level (HASL) finish, 1oz copper. Top and bottom solder masks are required. Any PCB manufacturer who has a clue will accept those files.

The
circuit diagram is here, the bill of materials (BOM) is here.

NOTE: THE FOOTPRINT OF U2 (78L05) IS REVERSED ON ISS.C BOARDS, and CORRECT ON ISS.C1 BOARDS.

Development & Performance Notes - Mitsubishi FETs
At the outset I spent a while surveying available power transistors, before settling on the RD16HHF1 from Mitsubishi semiconductors. These are used by Icom, Yaesu, Kenwood, Elecraft…

The concept was based on the transformer layout by G6ALU which is functionally equal to other transformer layouts, and uses easily available BN61-202 twin hole balun cores. I made a first attempt with the issue A board, superseded by the issue B which was sold as a kit. It had many features to interface with Softrock RxTX V6.3. Unfortunately the MoBo V4.3 and Softrock ensemble projects came later, and were not so friendly in terms of needing pull-down RxTx switching.

The power output was near 20W on the lower bands, dropping to 15W on 28MHz. Looking at the PCB, the positions for RD16HHF1 connections are indicated on the silkscreen layer if using a commercial standard PCB. The drain and source pins are reversed between the Mitsubishi FETs and switching types.

When using Mitsubishi FETs, performance is equal to other amplifiers, such as
PennyWhistle from HPSDR. The original page for the Pic-A-Star amplifier is still available here. After 3 years of service, my 20W amplifier using these devices is still working despite severe abuse like transmitting without an antenna, and operating from a spiky field day power source. The Mitsubishi FETs are very robust devices. The RD15HVF1 from Mitsubishi is also suitable, and gives slightly better gain at 50MHz.

Development & Performance Notes - STP16NF06L FETs
The switching FETs are much cheaper, but how is their performance compared to the “RF” parts?

A major physical problem with switching FETs is the tab is drain. So tab is “hot” in voltage and RF terms, but has to be heat-sinked. I recommend a relatively thick heat transfer washer without heat transfer compound.

Before fitting the bias network resistors, check the Vgs threshold of your devices. Resistors at top and bottom of the trimpots VR1, VR2 are there to make adjustment easier. So calculate the centre point of VR1, VR2 to be at the point where the device
just switches on. Unfortunately switching FETs are designed with a very sharp DC switch-on point, as required in power supplies. This can make the bias unstable. I found the STP16NF06L to be very sensitive to adjust. Despite temperature compensation they will run-away and overheat. Always watch the current drawn by the circuit with a multimeter when adjusting, and observe the variation over several minutes.

An early prototype board undergoing bias adjustment and input network analysis is pictured next:

Cheap PA under adjustment

With the TE Axicom relay, quiescent current is 80mA at 13.6V. It is safe to put 200mA quiescent through each MOSFET when heat-sinked. The input SWR and of course the gain is affected by the bias current. Before running power tests, I looked at the response of the Kits & Parts 20m band low pass filter in-situ as in the picture. The first plot shows SWR and S21 (thru) response with the 20m filter plugged in.

20m filter

Input

The second plot is the input with resistors added to bring up the impedance closer to 50 ohms. It is possible to experiment with the input PI network and achieve better matching, though the input SWR is reasonable and will not be a problem for most driving stages. Certainly the Softrock output stage can drive this without problems. It is worth noting the high SWR on the right of the plot is caused by the low pass filter response, as expected.

The big question is
how good is linearity of these FETs in two tone intermodulation? Running into a dummy load with PSK-125 and listening to the signal on a receiver, while varying the power gives a rough idea. The signal sounds the same from power outputs of a few watts up to 25W at 14.070MHz. There is also no detectable splatter with the receiver.

My test equipment does not run to a spectrum analyser. I only have the SDR-Kits VNWA network analyser, which is brilliant at its primary function, but the spectrum analyser has a minimum resolution of 250Hz. I also have no proper signal generator, let alone the two required for an intermodulation test. The best I can do is generate a PSK-125 signal and sniff the output on the VNWA used in spectrum analyser mode.

Overall gain with STP16NF06L is actually higher than RD16HHF1 on the lower bands. If you only want to operate <10MHz they would be better. But see the compensation notes further down this page.


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In a PSK-125 intermodulation test, this is the result from the RD16HHF1 amplifier driven to 16W (post filter) on 21.070MHz. Two tones with no characters transmitted during analyser sweep.
RD16HHF PA 16W
Next the result of the STP16NF06 at same power level. The “shoulders” relative to the main signal are lower than the RD16HHF1 result. The “shoulder” is about -22dB and -25dB respectively, representing intermodulation products from the amplifier. The input power at 250mW is well within the Softrock Ensemble PA linear region.

STP16N06

Driving either type to an output of 25W makes the “shoulders” rise up towards the main signals, to within -10dB. This expected result shows the test is a valid one. Surprisingly the cheaper MOSFET has a better intermodulation performance than the expensive “RF” part. The Mitsubishi RD16 die is probably a standard FET die wire bonded differently to get the tab grounded. There is no magic in its construction.

The ST FETs drop off in gain more than the Mitsubishi's as frequency is increased. At low frequencies they will drive to maximum output (>25W) with <0.2W. But at 10m, you will need 0.8W to get 20W out. It is possible to put a compensation network in which will flatten out the gain considerably. With a bit of simulation and testing, the values are:

  • A 150pF leaded capacitor at the R7 position. There is a grounded via provided in the middle of the R7 position to fit one leg of the capacitor into.
  • A 180nH air spaced inductor at the R1 position. 180nH can be made from 7-8 turns of 22SWG wire wound on a 6mm former.

A picture of the frequency compensation circuit follows.

Compensation
It is best to start out with 8 turns and adjust the power output on 29MHz. If you find spacing the turns right out increases power, then desolder the coil and remove a turn. Gate capacitance varies widely on these FETs, it is best to adjust your own board individually. Also check the gain on 21MHz is peaked up. With the compensation network, I find the maximum power output to be 25W on 29MHz, with 1.0W input. At 3.5MHz it gives 30W for 0.5W input.

In conclusion, I find the STP16NF06 to be a capable and cheap amplifier device for HF transmitting.

Development & Performance Notes - Fairchild FQP13N06L FETs
An extensive survey of TO220 FETs shows the lowest gate and output capacitance is the Fairchild Semi FQP13N06L. They are cheap, so I tested a pair. With a dummy load, using the Elecraft KX3 as a signal source in FM mode at 0.3W. Quiescent current 700mA combined. Power out was:

7MHz 25W
14MHz 18W
21MHz 14W
29MHz 10W

So the gain drops a lot with frequency which is expected. The gain can be compensated in the same way as for the ST MOSFET. After a lot of tweaking about, I will make the statement that there is nothing to choose between the ST or Failrchiled FETS. Bearing in mind the output side is dummy load, the input SWR looks like:

FairchildVSWR
The FQP13N10L is on paper the lowest gate charge TO220 device available. They are often found in the PA stages of 27MHz CBs.

Builder’s Notes
Some of the previous notes are still valid for winding the transformers and setting up. The original builder’s notes file (PDF) is available here. The STP16NF06 or switching FET version only differs in the DC bias (gate) voltage needs to be set lower than the RD16HHF1 version.

The Softrock Ensemble should be arranged to switch the amplifier to transmit by connecting the /PTT line (U4 pin 4) to the J9 pin 1 of the amplifier. So pulling down the base of U3 on the amplifier when the Tx is keyed. Coax input at J11 can be connected straight from the Ensemble RF output. Details of how to terminate coax tails is in builders notes.

Previous Softrock 20Watt Amplifier Issue B

My previous project was an amplifier with integrated filters,
circuit diagram here. Design of HF amplifiers using Mitsubishi RD16HHF1 MOSFETs is old news now. The ones I built in early 2009 are still working reliably. This was the last “issue.B” unit, all of which are now sold.

20Wlast