Mid-Power HF Amplifiers

Updated 20190602
New thoughts on HF amplifiers
When out on mobile radio trips I dream about a big signal to shout back at the DX I can hear (SSB). My Elecraft KXPA100 is wasteful, like all class-AB designs. It also needs a healthy 13V+ supply to get a full 100W output, whereas a good car battery drops to ~12.5V when off charge. Some amateur class-AB amplifiers are still in the 1980s with obsolete IRF510 or Motorola RF parts.

This page looks at two approaches to avoid these problems. One is a better class-AB, the second a new concept.

New Concept HF Amplifier (2018-)
Below is a frequency spectrum of a simulation running at 25% modulated power, 7.15MHz. The spurii looks worse because of the simulation's FFT resolution. For a sanity check I simulated a dc powered output stage against a modulated one -6dB down. The spurs stayed about the same, indicating the simulation’s noise floor is inaccurate. Trying to filter the remaining spurii (in the simulation) produced anomalous results, and convergence issues. This illustrates how circuit simulation has its limitations.

Pasted Graphic 1

The PCBs and initial components for placement are available for this unique project.

Class-E PCB

Initial tests confirm high efficiency from the power stage, with 65W produced from 13.8V, 5.3A = 93%. Modulating this raw power while keeping safety margins on the FETs is a difficult challenge.

The heatsinking and transformer arrangements proved inadequate. I am designing a new board to overcome these problems. A reliable 70W of CW power with 13V power supply is the target.

If anyone would like to collaborate on this project I can share much more information privately than given here.

100W Class-AB (2018-)
The new MRF101 LDMOS from NXP provide many advantages over the Mitsubishi RDxx series found in most HF power amplifiers.
  • Lower cost
  • Higher gain
  • Easy drive
  • Easy heatsinking
One major disadvantage - they need at least 25V to operate properly. As concluded below, there is no beating transistors designed for high power amplifiers. As a lot of ground-work was already done here in terms of auto-band switching; SWR protection and monitoring; boost power supply; Low pass filters; output transformers, it was decided to have a last push and productionise the best technology and combine with the latest FETs. This project should provide a minimal parts count 100W (PEP) amplifier which has several advantages over products like the Elecraft KXPA-100.

A prototype PCB and 160x100mm enclosure are near completion as shown here.
Economy HF Amplifier Project (2016-2018)
A previous project was with low-cost (<£1) switching FETs to advance the art beyond the very cheap, barely functional Chinese products on the market. They promise 100W for low cost but cannot deliver half that, and are very crude, not even providing an Rx/Tx relay - buyer beware!

There's 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:
  • 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

The most common modern device for HF amplifiers of about 50W is the Mitsubishi RD70HHF1. They have some disadvantages:
  • High cost >£30 each in small quantities
  • Output impedance is low, forcing an inefficient 1:16 output transformer
  • Doubtful linearity with 12V supply, see many rig reviews in magazines!

I looked through the enormous range of switching FETs available. These are the challenges of designing with them:
  • High gate capacitance (except early generation e.g. IRF510), makes a flat 1-30MHz response difficult
  • TO-220 packages with inductive wire bonds
  • Switching FETs hotspot badly and the bias point is unstable… OnSemi, Infineon, Microsemi agree…
  • Temperature compensation and other measures are essential
  • The drain or tab is hot to voltage and RF

Many were investigated but these TO-220 packaged FETs are best suited -
FQP13N10, IXTP2R4N50P. The Fairchild has a track record in citizen band radio output stages. The IXYS FET is obsolete, as Littelfuse acquired and closed the former IXYS semiconductor company.

DMOS power FETs can hotspot and burnout under continuous high current. So any amplifier like this must 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. I found the FQP13N10 stable with a heatsink of 1C/W, and bias of 250mA per device.

I looked at designing a boost power supply in
TI WebBench. The cost of a 30V, 5A boost converter makes up half the price difference to using RD70HHF1 FETs in the first place. A solution was found in “600W" boost power supplies from China/eBay. These can give 150W if heat-sinked, and are voltage adjustable, with current limit for £6! With such a cheap way to get 30V @4A, plans for integrating a boost power supply were dropped. Having a boost module has several advantages -

  • Operate at the same output power with 10-20V input voltage
  • Improved efficiency output transformer
  • Improved linearity at higher supply voltage

The boost modules use
the old TL494 chip from TI. Its dead-time control provides shutdown during receive.

The amplifier diagram is
here and here. I found IXTP2R4N50P FETs the best overall performers. FETs from St Micro and Infineon were also tested, but the Infineon (IRF520) part gave low gain. The ST part had a serious hot spotting and burnout issue. Here is the amplifier under test, with the Chinese boost module mounted next to the main board.
Power amplifier using switching MOSFETs
This project is now finished and a mini-movie is on Youtube. The play-safe RD16HHF1 version BOM is here.

Software - as mentioned on the video above, the software requires keying to transmit when first powered up. This measures the temperature compensation diodes and calculates the bias offset. Pressing the CAL button during transmit calibrates to the peak transmit power. Fitting a zero-ohm link across R22 forces the board to use a binary code on the BAND connection, instead of automatic band switching.

FQP13N10 Version
The RD16HHF1 FETs can only produce 20W, and that's with a healthy power supply of 14V. With the FQP13N10s and the voltage boost module, these results were measured:

80m, 6.2A, 35W
40m, 7.8A, 38W
20m, 9.0A, 40W
15m, 8.0A, 30W
10m, 6.0A, 25W

The centre column being total input current. The output will go higher, but it's not a good idea to push our luck with these cheap FETs. At 80p each, they are about 1/4 the price of the RD16HHF1's.
FQP13N10 BOM HERE and circuit PAGE1, PAGE2

The FQP13N10 works up to 30MHz with 2-3dB less gain than at 3MHz. This is done with negative feedback to swamp Miller capacitance, and careful matching at the input. FQP13N10 gives 20dB+ gain at lower frequencies without compensation. With compensation, 13dB gain at 3MHz and 11dB at 30MHz was measured on prototypes.

It's important to consider the tradeoff of drain voltage headroom vs. drain-source capacitance. Increasing supply voltage pushes up the impedance and allows optimum choice of output transformer. Using a 1:1 output transformer pushes up peak drain voltage. At 40W into 50Ω, the drain voltage peaks at 64V (assuming zero saturation voltage). Any SWR above 2:1, especially inductive loads, will easily push the crest voltage >100V and blow a 100V FET instantly. This is why FET amplifiers can be sensitive to high SWR.

So why not just use a (12V) low voltage supply? The answer is found in this graph.
Pasted Graphic

The horrible change of capacitance (Coss) of a FET causes non-linearity. Higher voltage FETs shift the above graph to some extent, so using a higher voltage FET to reduce intermodulation distortion is a trade-off of SWR tolerance against the non-linearity.

IXTP2R4N50P Version
The IXYS FETs are lower capacitance devices, and work really well up to 30MHz. Unfortunately they are now obsolete.