QRP Transceiver Power Amplifier

Updated 20150314
I'm investigating a higher power design. There is no major advance to be had in class AB amplifier design, but maybe improvements in economy to be made. The cost of Mitsubishi RD70HHF1 is so expensive (>£25 each) that switching FETs are worth another look. The output impedance of FETs at 12-14V is too low for an efficient 1:4 output transformer, and their linearity is not great either.

A rough prototype does 60W at 7.1MHz from
STP14NF12 FETs with 30V supply. A pair of them would go to 100W on the lower bands, but for coverage up to 30MHz things are not so easy.

Challenges in designing with switching FETs are:
  1. High gate capacitance, makes a flat 1-30MHz response impossible. 1-20MHz (-3dB) is possible and switching input circuitry 25-30MHz.
  2. Low gate capacitance devices like IRF510 and FQP7N10 are now obsolete
  3. Switching FETs have a snap-on characteristic which gives bad side effects when used in the linear region. Fairchild Semi and OnSemi have documents on this. The bias point is unstable compared to the lower transconductance of "proper" RF FETs. Temperature compensation is essential.
  4. A 30V supply eases linearity problems, reduces gate capacitance plus gives higher gain. Having a boost switcher adds noise, especially when using an economical double sided PCB
  5. Fitting it all onto an 80x100mm PCB will be a tight squeeze. Will have to use some difficult to solder advanced SMD parts

We want to leverage the best parts available. The
STP14NF12 and FQP13N10 are the lowest gate capacitance TO220 package FETs easily available. Comparing these for bias stability vs. temperature gives no clear winner. They both break with 0.3A bias current. The "maximum safe operating area" quoted on data sheets is for a 1 second pulse, running continuous class AB will be far below this.

Clearly the quiescent drain current (I
DQ) will have to be kept under control. But how high does IDQ have to be for respectable linearity? How high can we get without eroding safety margins? Previously I ran the RD16HHF1 amps at IDQ = 300mA per transistor, but with 30V supply a lower current will be acceptable. How much lower will have to wait for a real prototype to be constructed…

How much temperature compensation is needed can be estimated from the FQP13N10 data sheet which has a nice graph (maybe too nice) of V
GS against Id and temperature. Drawing on 2 lines for 300mA ID gives this result:

The 2 coloured lines coming down intersect the Vgs axis at about 3.95V and 3.50V. So 0.45V/150C = 3mV/C. The STP14NF12 is poorly characterised, but I am willing to bet it's not much different. 3mV/C is above the temperature coefficient of a single diode but less than two. Having good thermal contact between a plastic TO220 and two diodes is difficult. Hence getting a rapid response of the compensation circuit is difficult. It would be handy if these FETs had compensation diodes built into the packages but of course they do not!

I looked at other packages than the aged TO-220. The problem is replacing the FETs if they break then requires a hot air rework station. Having several in parallel is a nice idea, to spread the hotspots, but their gate capacitance is a problem that rears its ugly head again.

There has been a small improvement in heat-sink insulator technology. Thermal resistance down to 0.25C/W is available, if a little pricey. In view of the £20+ saving over RF rated FETs, £1 for a heat-sink tab is reasonable.

To generate 30 volts from an automotive supply voltage needs a boost power supply. There are dozens of boost mode ICs on the market. Switcher efficiency has improved recently. T
he best one with internal switch only puts out 1.7A at 30V. So there is no choice but to use one with an external switch.

Webench from TI (something they bought from National Semi) allows easy design of a 30V, 3A regulator. The LM3481 is an easily available part, along with CSD18502 FET which is a TO220 package. The inductor is a reasonable sized Bourns SRP1245A-6R8, according to Webench. The blocks for the new design will include the amplifier itself, boost converter, and SWR bridge.

20Watt Amplifier

Previously I designed an 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. I have now progressed towards the design of refined production standard PCBs, and higher power versions.

This page presents a cost reduced amplifier targeted at Softrock builders, though it can be adapted to boost other QRP rigs to the 15-20W level. I 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.

The final batch of PCBs for these boards is now sold out. This is the 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 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

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 input capacitance (<350pF), Vds max >40V. Package must be TO-220. 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.


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 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 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 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


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 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.


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. 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.

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. It would be possible to further compensate with a more complicated network.

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 a bit disappointing. 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:

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.