QRP Transceiver 40W Power Amplifier

Updated 20170716
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:
  • RF FETs are expensive and often difficult to buy
  • 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 are not ideal but cheap, so 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!

The most common modern device for HF amplifiers of about 50W is the
Mitsubishi RD70HHF1. They have some disadvantages:
  • Very 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

Having discounted the conventional wisdom devices, 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 have inductive wire bonds
  • Switching FETs hotspot badly and the bias point is unstable… 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)
IXTP2R4N50 (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 IXYS FET is only available through Mouser in the UK, and has a higher voltage rating. 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.

To estimate the temperature compensation, the FQP13N10 data sheet gives a graph of V
GS against Id and temperature. It comes out as 0.45V/150C = 3mV/C, which 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. 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.

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. I found the FQP13N10 to be stable with a heatsink of 1C/W, and bias of 250mA per device.

I looked at designing a boost power supply using
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 "250W" boost power supplies from China/eBay. These can give 150W if heat-sinked, and are voltage adjustable, with current limit for £4! 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 about £50 for a 40W amplifier (excluding heatsink) and these advantages over off-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

I wanted to find a way to enable/disable the boost module. But the Chinese module has the chip number filed off, and I cannot find another chip with a pinout anything like. However, it seems to generate no RF noise on receive. So, I decided it is better to leave the boost module switched on all the time.

It's impossible for me to compete with Chinese amplifiers with surplus RF transistors. It has to be remembered 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. I tested a pair of IRF620's from the old IR FET family which are used in many amateur amplifiers. They produce very little gain above 20MHz, and need a lot of drive, overall not good devices to use.

Here is the amplifier under test, with the Chinese boost module mounted next to the main board.
Amp2016-3
This project is now complete as a technology demonstrator for low-cost FETs with micro-controller enhancements. A mini-movie is on Youtube. The RD16HHF1 version BOM is here, and the option to play safe with those devices was wise to include.

Software - as mentioned on the video above, the software requires keying into 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 RD16 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
FQP13N10 CIRCUIT
PAGE1, PAGE2
I offer the MSP430G2553 .txt file if anyone wants, but the source code is closed.

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 the supply voltage pushes up the impedance and allows optimum choice of output transformer. Using a 1:1 output transformer pushes up the 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 above 100V. This will 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.

I'm abandoning further work on class-AB circuits. The "further thoughts…" amplifier project promises a big step forward. Some of the ideas from the class-AB design are still applicable:

• Use a 16x2 alphanumeric LCD instead of LEDs
• Use larger, cheaper cylindrical ferrites for output transformers
• Automatic bias setting, with current monitoring
• Faster micro-controller
• Revise SWR bridge to use pick-up toroids (flatter SWR vs. frequency response)


Further thoughts on HF amplifiers
Now some blue-sky thinking. When out on mobile radio trips I dream about having a big signal to shout back at the DX I can hear. My Elecraft KXPA100 is hugely wasteful, like all class-AB designs. It also needs a healthy 13V+ supply to give its full 100W output, whereas a good car battery drops to about 12.5V when off charge for several days. Some amateur amplifiers are still using 1980s parts like IRF520 FETs. Its time for something different, and I have some breakthroughs to test out.

There's a lot of work on high efficiency amplifier technology, but for "toy radio" mobile comms applications. Their power levels are much lower than at HF, and specialised components are easily available.

Alternatives for low cost HF power amplifiers abandon class-AB. The design becomes more complex, but can utilise the latest semiconductor technology. Exactly what parts I am not ready to announce publicly.

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 sim'd a dc powered output stage against a modulated one -6dB down. The spurs didn't increase by much, indicating the simulation is not entirely accurate. Filtering the remaining spurii produced anomalous results, and convergence issues. This illustrates how circuit simulation has it's limitations.

Generally, results of my chosen topology are excellent, and warrant money spent on a real prototype. In simulation small MOSFET models (£1 each) were dissipating <1W each for 50W output.

Pasted Graphic 1