Mid-Power HF Amplifiers

Updated 20200710
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 designs of HF amplifier for mobile/portable operation.

100W Class-AB
The new MRF101 LDMOS from NXP has 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 much higher voltage than 13V. As concluded previously, it’s DMOS switching FETs cannot produce reliable RF power >50W. With the MRF101 easily available at about 10x the cost of DMOS FETs but half the price of the RD100xxx devices, they look a superior solution.

Lots of ground-work was done for auto-band switching; SWR protection and monitoring; boost power supply; Low pass filters; output transformers. The imminent release of the IC-705 from Icom fits well with a a minimal parts count 100W (PEP) amplifier with similar functionality to the Elecraft KXPA-100. Prototype hardware photo:

I WILL NOT BE PROVIDING KITS OR SUPPORT TO ANYONE THAT TRIES TO BUILD THIS. Selling ham radio kits is a money-loser and very time consuming, time which is better spent on designing future projects. The project is being re-designed with several new parts to improve many aspects, and interface well with the Icom IC-705. Provision will be made for a 12.5V auxiliary power output, making the amplifier and IC-705 a killer HF portable radio.

The preliminary circuit diagram and Gerber files are available from these links.
Amplifier (.PDF circuit page 1)
Micro+filters (.PDF circuit page 2)
Gerbers.zip (RS-247X format)
Preliminary BoM (for info only .xls format)
Firmware V0.9 (MSP430 .txt format)

Hardware Description
Connections are:
Input power, 20A maximum, 15A average
RF in BNC, 1.5W maximum
RF to antenna BNC, SWR 2.5:1 maximum
PTT phono socket, short to ground on transmit (provided on many radios)

The amplifier needs a 27V power supply, which can be made by “600W” boost modules available on eBay. Search for “600W boost converter” on eBay or Amazon. Internally there are positions for terminal blocks to accept wires from the boost module. Polarity is shown on PCB silkscreen.

Power Stage
Here are some secrets of HF PA design not often written in textbooks. The power stage is based on NXP MRF101A/B LDMOS transistors. A mirrored pair is used to improve the PCB layout, as the gates and drains are adjacent. The FETs are placed to be thermally attached to a large heatsink. Balanced layout is essential with this type of design, it spreads the power dissipation across 2 semiconductors, and reduces 2nd harmonics. At the expense of a small increase in complexity.

The output impedance of a linear amplifier stage is:
Where Vs = supply voltage, Vsat = saturation voltage (typically <0.5V).

The supply voltage sets the matching impedance. With a 1:4 transformer and 110W out at 27V supply, the impedance is about 12.5Ω assuming saturation voltage of 0.5V. That ratio enables a 1:4 transmission line transformer, far lower loss and lower cost than the dreadful tube-in-bead design. Alternatively a 1:1 transmission line (RF choke balun) can be used with a 36V supply and a low-pass filter with 25Ω to 50Ω impedance match. 36V is an odd voltage available from power tool batteries, Bosch/Makita/Dewalt. The 2nd of these choices removes a transformer, and is an ideal concept for a battery-portable amplifier.

At the input there is a min-loss-pad of 2dB which reduces the input impedance to 25Ω. That reduces the chance of overload at the input and ensures a reasonable match to the driver transmitter. 25Ω also helps broadband matching of the capacitive FET input. A balancing choke T3 ensures the 2 FETs are driven 180 degrees out of phase. The transformer is a ready wound part to cut costs. The input bias to the FETs comes from an MCP47FEB 10-bit DAC. The DAC is controlled by the MSP430 to give correct bias without manual adjustment. The bias is dropped down slightly at high temperatures.

The FETs drains are fed by a balancing choke, T8 which has 2 purposes. It ensures that the dc and ac supplied to the FETs is equal and opposite - i.e. balanced. A small amount of negative feedback is tapped off by a single turn thru T8 and connected to the FET gates. The feedback reduces distortion and gain. Gain is plenty high enough so a few dB reduction is actually better.

Input and output of the power stage is switched with Axicom IMC miniature SPDT relays, which have the highest current handling in such a small package.

Current Monitor and dc Power Input
A Texas Instruments INA138 high-side current monitor gives a voltage output for the MSP430. It needs to measure the current for setting the bias and for overload protection. The current is monitored by voltage drop across a 20mΩ resistor, so there is some inaccuracy at low currents due to the input-offset voltage. Better current amplifiers are available. A large capacitor helps absorb the high current pulses of the power stage. A large ferrite bead U8 helps filter the power stage.

MSP430 Microcontroller and LCD
The concept is to provide advanced features and remove expensive parts. For instance the MSP430 capture unit acts as a frequency counter for band selection, avoiding expensive band selection switches on the front panel. Automatic bias adjustment avoids expensive manual potentiometers. The micro has just 28 pins but each is heavily utilised, such as power save which turns off band selection relays and the LCD backlight. The only external logic chip is a 74AC238 which saves some pins.

A 16x2 LCD is cheaper than a large number of LEDs, once the cost of an LED driver is factored in. These modules are made in China at low cost and high volume. The display enables the secondary feature of copy protection too. The trimpot for contrast adjustment is more expensive than a DAC for contrast adjustment, perhaps in future I should do it that way. The LCD is used in 4-wire mode to save pins. These LCDs need an ICL7660 charge pump inverter fitted for their LCD bias

The MSP430 does all monitoring and control, like Rx/Tx sequencing and fault protection. The main defence against faults is simply switching back to receive. The main risk with that is welding the output relay if it switches at a peak of power. So the MSP430 does a sequence of reducing gate bias to zero and cuts off the boost power module, then waits 4ms before switching the Rx/Tx relays.

It would be better to have a solid state (PIN diode) input switch, that changes over much faster than a mechanical relay could. It can cut the input power much faster. That is for any future iterations. The system will not go into transmit until the input magnitude is fairly low. This avoids switching the relays to transmit under high power.

The MSP430 capture unit is a frequency counter for automatic band selection. If operation on a different band is detected while the amplifier is in transmit, it returns to receive mode to protect the band switching relays. Changing bands while transmitting is locked out on most radios anyway. The MSP430 has a maximum logic speed of 16MHz, so a divide-by-2 gate is used to bring down the possible 29.7MHz input to below 15MHz where the capture/compare is happy with it. Protective diode clipper is ahead of the frequency divider chip.

The software reads the supply current, forward/reverse power, input signal magnitude, supply voltage, and temperature sequentially every 1ms. It also sends characters to the LCD at that rate and checks for encoder movement. Fault conditions have to be seen on 2 adjacent cycles to register as a fault. Every 50ms the auto band selection frequency and temperature are checked. Until the band selection count has been registered twice, band switch is not made. The MSP430 runs at 8MHz but enters sleep mode once its processing cycle is complete. During sleep the ADC unit is reading the ADC channels.

Input signal magnitude is measured to avoid overdriving the input. LDMOS FETs are sensitive to being overdriven. If the input power is excessive, the unit does the return to receive mode sequence described above. The magnitude is carefully split from the drive to the frequency counter.

A second PCB is in the amplifier to boost the 12-14V car supply to about 27V. There’s a facility included to shut off the boost power supply during receive, to prevent noise. The Chinese boost modules use an old TL494 switching controller. Applying 3.3V to pin 4 of that chip disables it.

Temperature Monitor
An NXP PCT2075GV temperature sensor is fitted close to the power FETs. The temperature is read via I2C bus, and displayed on the LCD. The MSP430 uses the temperature reading to slightly reduce bias when the FETs are hot. It also returns the amplifier to receive if the reading is above 90C.

Lowpass Filters
A 6 section low pass filter covering 160m to 10m is onboard. The bands are selected thru a 74AC238 3-8 line decoder and 2N7002 FETs as pull downs. The FETs have body diodes, avoiding discrete diodes for discharging relay magnetic fields. The filters are uprated versions of the designs used on previous amplifier projects.

SWR Bridge
A “tandem-match” SWR bridge. It needs 18 turns to present enough impedance on 160m. The bridge produces tens of volts, so a resistive divider follows to reduce voltage within the range of the microcontroller.

Power from 36V battery
Many hi-end power tools from Bosch/Maikita/DeWalt have 36V battery packs, and come with chargers. 36V is a more suitable voltage for the MRF101 FETs. The output impedance at 100W output is 26Ω. Output filters can be designed with 1:2 impedance ratios, greatly simplifying the output transformer. A 36V lithium pack provides ideal portable energy. So, I am redesigning the project to use 36V battery packs. But the initial prototype must be completed first!

Software Description
The software is written in Code Composer Studio (CCS) for an MSP430G2453 target. The code is in .txt format and can be programmed in with any MSP430 programmer supporting Spy-BiWire 4 pin programming. MSP-FET or many of the Launchpad Kits (with 4 wire cable) are fine. The software is not open source.

Once programmed and run the software shows two 4-digit numbers. Turning the encoder changes the numbers, pressing the encoder makes the cursor move to the next number. The process to unlock the software follows:
  1. Email me the 4-digit code on the top line
  2. I send an unlock code back
Once I can get the new version designed, the code will not be sent for free.

The LCDtop line shows the amplifier status: TRANSMIT/RECEIVE/BYPASS

TRANSMIT (transmit mode with no faults)
RECEIVE (receive mode or return from Tx to Rx due to: hi-SWR, hi-curr, ovr-temp, ovr-volt, hi-input)
BYPASS (the PTT line is disabled and transmit power is passed straight through)

Rotating the encoder knob scrolls the bottom line through these sets of information.
PEP SWR (peak power output and SWR, zero on receive)
VLT CUR (supply voltage and PA current)
BND TMP (auto-band selection and temperature)

Pressing the encoder knob at any time sends the unit into bypass mode. The LCD backlight goes off after a minute in receive mode if the encoder is not touched. Turning the encoder or going into transmit makes the LCD backlight come on. This is done to save 40mA of backlight and relay current in long receive standby periods. Current with backlight off is <2mA. A fault condition latches in for 20 seconds and switches to receive. Keying to transmit is not possible during this timeout.

Zero-Z (zero output impedance) HF Amplifier
The high efficiency HF amplifier is now called Zero-Z: snappy and describes what it does.

Most HF amplifiers are designed around 50Ω output impedance. That gives them a maximum efficiency of 50% with a 50Ω antenna. Clearly it’s not good to transmit 100W and waste 100W, and at lower output they are worse! So far, I did some practical tests but a lot more simulation. Here’s a FFT spectrum in LTSPICE with 3kHz AM modulation. Peak power 130W from an 18V power supply at 92% efficiency.

40% modulation Blackmann window

Though the result looks spiky, the spurious is <50dBc which is the required limit for ham radio equipment. This is only an AM modulated test, the next plot shows a 2-tone test (SSB) signal simulated in LTSPICE. The frequency is 14.2MHz.
The FFT resolution of this 2-tone test is a coarse 4kHz on 10kHz spacing, and took a lot of processing to simulate, but shows the IMD at -30dBc. Another sim with an audio tone spacing of 2kHz showed it about -40dBc. The 2-tone waveforms in the time domain also show close reproduction between input and output. In other words no worse that a commercial class-AB transmitter. This is the first test PCB, which didn’t work well but was a major learning exercise.

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 yet keeping safety margins on the FETs is the challenge.

Much of the technology is new or not given in books anywhere. I found approaches to circuit design that overcome failures of previous experimenters with envelope restoration. An SSB amplifier can be made without generating the transmit signal as the
Polar Explorer does, saving hugely on complexity.

Bands above 20MHz will remain unreliable for many years. The only viable bands for mobile SSB operation are then 80m,40m,20m,17m. So, I’m designing for this frequency range to make things easier. If anyone would like to collaborate on this project I can share more information privately than given here. Eventually I hope to release the hardware as open source, but the software will be closed/licensed as with the class-AB projects.

Realisable Efficiency
A practical amplifier needs a drive source, transformers, filters, ancillary circuits. Looking at power loss in a typical class-AB amplifier in terms of dB and percentage:
  • 0.5dB = 12.5% loss in output transformer
  • 0.5dB = 12.5% loss in output filters
  • 0.4dB = 10% loss in switching and connectors/cables/PCB tracks
  • The drain efficiency will be 60% at best, 40% lost

So for 100W, dc input power will be >250W and a 12V amplifier takes 20A of current. For lower power outputs efficiency drops substantially, down to perhaps 30%. SSB duty cycle is generally only 25%, so amplifier efficiency will be way below 50%.

Class-E and similar switching amplifiers are listed in books as >90% efficiency. There are extra losses in a practical system from the rest of the circuit. Getting combined efficiency above 80% maybe impossible given the constraints of all the passive circuit elements.

For the example of a 100W amplifier, it will pull 125W, or 7A from an 18V power supply. Much better than class-AB, but there will always be some waste. Wastage is dependant on the power loss of every power handling component in the system. A zero-impedance transmitter stage is not realisable, but something close is.