QRP Transceiver 40W Power Amplifier

Updated 20161125
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 embarked on a project to find a usable part, and make a low-cost amplifier using it. Blank PCBs (150x100mm) for the project are 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 too low for an efficient 1:4 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 can hotspot and burnout under continuous high signal. 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. The disadvantage is reduced gain and lower efficiency. I decided to use a current limited power supply and intelligently reduce bias using software.

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 30V, 5A boost converter makes up half the difference to using RD70HHF1 FETs in the first place, let alone the extra constructional complexity.

A solution was found in "250W" boost power supplies from China/eBay. These can put out 150W if heat-sinked, and are voltage adjustable, with current limit for under £3.50! With such a cheap way to get 30V @4A, plans for 50W+ amplifiers were abandoned, and I decided to aim for 30-40W output. The Chinese boost module concept allows a part cost of under £50 for a 40W amplifier (excluding heatsink) and these advantages over of-the shelf HF amplifiers:
  • Operate at the same output power with 10-20V input voltage
  • Improved efficiency output transformer
  • Improved linearity due to higher supply voltage

It's still impossible for me to compete with Chinese amplifiers based on surplus RF transistors. It has to be remembered that the majority of devices sold on eBay do not reach their specified output power. I decided to add a number of extras which push up cost but make a complete system - RxTx relays; Low-pass-filter; SWR bridge; SWR monitoring; micro controller for automatic band switching and protection.

The circuit diagram is
here and here. I found FQP13N10 FETs the best overall performers. The large STP13N60M2 FETs had some bizarre bias quirks, and are 3x the price of the FQP13 with no benefit in performance. The STP14NF12 has a tendency to self-destruct which I never fully understood but will avoid. Here is the amplifier under test, with the Chinese boost module mounted next to the main board.
This project is now complete as a technology demonstrator for low-cost FETs with micro-controller enhancements. A mini-movie is on Youtube. I have not yet completed the FQP13N10 version with confidence to release. The RD16HHF1 version BOM is here.

Further improvements will take one more PCB revision and include:
  • Dispose of LEDs and use a 16x2 alphanumeric LCD
  • Use ready wound baluns for the input circuit
  • Use larger, cheaper cylindrical ferrites for output transformer
  • Change 74AC74 to single D-flip-flop gate
  • Automatic bias setting, with a proper current monitor
  • Add fault protection diodes to gate bias circuit
  • Revise SWR bridge to use pick-up toroids
  • Optimise all SMD pad sizes

Most importantly, design to fit in a practical case, 160 x 100mm. This means having the LEDs on the "short edge" and a PCB mounted BNC socket to fit through an end panel. The Hammond case allows the PCBA to slot in and the FETs to fix onto a heatsink. That case allows room for expansion, to add a battery board or other advancements as described next.

Further thoughts on HF amplifiers

The big snag with all amplifiers on this page is their inefficiency. An HF amplifier of >80% efficiency and more than 100W output would be a game changer for portable and mobile operation. The UK full licence manual states a class-AB amplifier has an efficiency of under 50%. The 50% only achieved at full output. In other words a lot of power wasted when running off batteries or a small generator. Even if only available up to 40m it would be a massive step forward.

There is a wealth of academic papers on the internet about improved efficiency amplifiers. Almost all are looking at technology for the mobile comms industry. As usual, hams are left alone to research their own HF circuits.

Looking for efficiency improvements, the simplest way is to modulate the supply voltage. This is called either
envelope tracking, or class-H. It's used in mobile comms to reduce wasted power in base stations and in some handsets. Modulating the supply voltage ensures the optimum efficiency at mid and high power levels. Technology to do this has been available for a long time. But this technique just increases the <30% efficiency at middle power levels to 50%. Overall improvements are not spectacular.

Envelope tracking is very different to envelope
restoration which uses the power supply to modulate a class-D or class-E main amplifier. This technique can transmit SSB if the phase of the input signal is preserved. It looks feasible but would need driving from a DSP system to compensate for the non-linearity of the amplifiers. I don't think a stand alone amplifier using this technique would work.

Widely used in mobile comms, Doherty amplifiers combine Class-C and class-AB but are optimised for constant power transmission. They are far from ideal for SSB.

The best efficiency is class-D or class-E where the power devices are either on or off. Amplifiers to make CW and AM signals have been around for a long time. Up to 1GHz class-E is ideal for CW signals. Efficiency of about 90% is feasible, and moderate extra complexity over class-AB. Use of such AM transmitters on medium wave and lower HF is old news now. Some
amateur AM on the lower HF bands has used class-E.

Amateur HF amplifiers must work in SSB mode which is very different to the modulation used in short-range mobile infrastructure. Doing AM involves modulating at baseband (audio) as a simple PWM system. But SSB is much more difficult. Approaches to SSB will involve a mixture of DSP and sampling theory. It may not be possible at all as a stand alone amplifier. Amplitude and phase signals are needed, which means it will have to be part of a transceiver rather than an accessory.

Lower HF frequencies are within the switching speed of class-D and all HF is within the switching speed of class-E. This suggests that current technology can produce a 90% (approx.) efficiency amplifier for SSB - somehow! Class-D places much higher demands on the switching devices, but has several advantages over class-E.
  • No relatively narrow band resonating output circuit
  • No resonating output circuit means less phase distortion
  • Can run from a quite poorly regulated supply
  • Modulate by PWM, somehow to get amplitude
  • No need for an intermediate supply switcher, which will lose some power in itself

Switching speed demands can be met at least up to mid-HF frequencies, such as 20m. The fact is bands above 20m will be less useful over the next 10 years because of ongoing low sunspot activity.

All this has a number of challenges, and is 10x harder than class-AB !!
  • Requires amplitude and phase information
  • Have to switch high current in a few nanoseconds
  • Covering several octaves across HF bands
  • Attenuating all spurious responses (linearity)
  • Keeping the system >80% efficient to deliver the promise

After looking at
this wide ranging document, I decided these 4 methods are the most applicable to HF SSB.

1. Envelope restoration as mentioned above (Kahn's method). Difficult to do, and really needs a ready made FPGA-SDR platform. Such a platform is
LimeSDR or Red Pitaya. Moving away from the idea of modulating the power supply, and using PWM to modulate the switching devices themselves. Class-D removes the envelope amplifier from the system, simplifying it and should improve efficiency further. A circuit sometimes called "current mode" class-D, which I call balanced class-D looks suitable.

2. A commutating mixer technique, like a high power quadrature sampling exciter (QSE) stage. The baseband I/Q signals are chopped at 4x the signal frequency. So theoretically a high power "QSE" can be made. The drive signals can come from class-D audio amplifiers or SEPIC power supplies with an output bandwidth of several kHz. Providing the commutation (switches) is a problem. In a QSE the switches must carry the output voltages. At low power this is done by analogue switches. But switching high power like this is far more difficult.

3. Out-phasing amplifier. Use the relative phase of 2 non-linear stages through a combiner. The basic idea was from Henri Chireix in 1935. Its possible to use two class-D amplifiers, resulting in very high efficiency. Two big problems with out-phasing are: separating the two drive components from the original signal; recombining without incurring losses. Out-phasing is a very complex technique to implement. This is a very elegant idea but
extremely complicated mathematically.

4. Bandpass delta-sigma modulator. The Shannon-Nyquist theorem states the sample rate must be more than twice the bandwidth of the digitised signal. An SSB signal has <3kHz bandwidth, so if the baseband signal is somehow shifted, the system is simplified. It doesn't have to have an actual sampling rate (or switching rate) over twice the signal frequency.

No products using these concepts are available, they all use class-AB Mitsubishi FETs running off 12V. The big ham radio manufacturers are content to take the low risk approach of black boxes, with little in the way of real innovation.