Softrock or QRP Transceiver 20Watt Power Amplifier

Updated 20141130
Previously I designed a power amplifier to produce 15 to 20W of RF power from less than 0.5W of drive. The unit was a single board as shown at the bottom of this page. 25 boards were made, and I still have 2. There were no complaints about the design, and I was happy with the technical performance.

It was not popular due to high component costs in the UK, with 20% VAT. This page presents a price reduced version, targeted at Softrock builders, though it can be adapted to boost other QRP rigs to the 15-20W level. I also added several unique options. To prevent the technology being wasted under complaints of “too expensive” the Gerber files and 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.

A new batch of PCBs has been ordered and will be available mid-December 2014. This is the first prototype undergoing tests:


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:


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 or attenuator
  • Temperature compensation of bias to help stability
  • Facility for third party plug-in filters, or design your own
  • PCB footprints designed for generic components as possible
  • Integrated RxTx relay with pull-low or pull-high switching voltage
  • Extensive options for external filter banks

Many amplifier designs use power supply MOSFETs like the old IRF510. There are more modern equivalents to the IRF510, such as STP16NF06 from ST Microelectronics, or FQP13N06L from Fairchild.

Datasheet parameters to look for in switching FETs are low gate charge (<12nC), low input capacitance (<350pF), Vds max >40V. Package must be TO-220. Most MOSFETs optimise Ron to the detriment of gate charge, so giving high input capacitance. Check websites of ST, Fairchild, NXP, Texas Instruments (NexFET), On Semiconductor, International Rectifier, Vishay, and others. New types of MOSFETs often come onto the market.

I included an input PI network to help matching to previous stages of your transmitter. Again experimentation is required, and much to be learnt from doing so. Temperature compensation is from a dual diode.

There is a matching low pass filter
here. The Kits & Parts single band filter is no longer available. If there is enough demand I will design a simple single band filter and get some manufactured. I designed component footprints to be generic as possible, which means using surface mount. Nothing smaller than 0805 is fitted.

The amplifier is keyed to transmit by either a +12V supply, or an open collector pull-down to ground. The SOT23 PNP transistor can source up to 2.0A from the positive supply to drive external loads. The RxTx switching relay specified is from TE Axicom. Note the relay must be suitable for switching low level signals, not one with silver plated contacts.

There are several options for external filter banks. 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)

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.

circuit diagram in PDF format is here. The bill of materials (BOM) is here.


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 may give slightly higher gain on 10m up to 6m.

Development & Performance Notes - STP16NF06L FETs
The switching FETs are a lot 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 using a relatively thick heat transfer washer without heat transfer compound. It may also help to use M3 nylon nut/bolts to secure the transistors. Be aware nylon bolt thread strips easily!

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 heatsinked. 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.


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. Input SWR with STP16NF06 is not as good as RD16HHF1 FETs.

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 “spread” 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 RD16HHF. 0.25W of input will drive them to 20W output, and that measurement was taken with a low pass filter. Expect an extra dB of gain over the RD16’s. At 28MHz gain is rolled off by about 1dB.

Next picture is the prototype under test, seen with leads hanging off at the right, with Softrock Mk.2 and Power/SWR meter at centre of shot.


I have not stress tested STP16NF06 FETs by subjecting to short and open circuits. I expect they are less rugged than the Mitsubishi FETs. Going back to the 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.
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.


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.

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 a bit disappointing. The STP16NF06L did better. Maximum power output (limited) on the bands was:
7MHz 55W out, 1.6W in
14MHz 50W out, 1.6W in
21MHz 50W out, 2.0W in
29MHz 40W out, 2.0W in

Bearing in mind the output side is dummy load, the input SWR looks like:
So basically the result is stick to the other FETs tested on this page, or find another alternative to test!

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 plug in filter from Kits & Parts may need to be tweaked. In particular the input capacitor may need to be changed for a lower value. The versions I made cut off sharply at the top of their bands, be aware the Micrometals iron dust toroids are notoriously bad for tolerance. They generally give higher inductance than their published Al values suggest.

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.

The picture below shows the last unit of the previous version. All units have now been sold.