2xIRF520 amplifier evaluation on 40m

I posted couple of pictures of my 2xIRF520 push-pull HF amplifier prototype. This time I measured its performance on 40m. I will continue with other bands once I build the appropriate low pass filters.

My prototype is derived from the famous article by Mike Kossor, WA2EBY, "A Broadband HF Amplifier Using Low-Cost Power MOSFETs". My prototype differs in couple of details:

• Instead of two IRF510 MOSFETs I am using two IRF520. These have 2x higher input and output capacitances, 2x lower on resistance and somehow higher maximum power dissipation and lower thermal resistance than the IRF510, which everyone uses.
• I am using an input transformer ratio 12t:2x6t wound on FT50-2 bifilar. This drives the FET gate harder, but requires twice the input power.
• I increased the gate bias resistors to 330 Ohm with the hope to decrease the drive power requirements. The amplifier was stable with the 1:1 input transformer, but not with the 1:2 input transformer. I inserted 4.7 Ohm resistors between the input transformer secondaries and gate capacitors.
• I made the output transformer with the same ferrite core, but the primary is a single turn made of silver plated brass tubing. The secondary is made of 2 turns of 22AWG silver plated teflon coated wire.

The amplifier of WA2EBY was wound with 2t primary and 3t secondary, which leads to 4:9 impedance ratio. My otput transformer has 1:4 impedance ratio. In ideal case the output power of my amplifier shall be around 1.8x higher at the same DC voltage. It would certainly not be the case if I just changed the output transformer ratio by keeping the 2xIRF510. But with half the on resistance of the IRF520 it may work.

Performance of the 2xIRF520 push-pull amplifier on 40m

The efficiency is very good when driving the amplifier very hard. The output power is sufficient for a backpacker operation and compared to the HF packer amp, the DC-DC converter to 28V is not required to achieve the same output power. The HF packer amp DC-DC converter regulates the PA voltage while the battery voltage drops keeping the output power constant, but the DC-DC converter increases power losses somehow.

The RF gain has been measured by setting the input power with a step attenuator and measuring the RF voltage on a 50 Ohm dummy load after the 40m low pass filter with a 1:10 scope probe, reading the voltage from an analog Tektronix scope. One clearly sees the compression at high drive levels. Driving the amplifier hard brings great power efficiency, but the amplifier is not linear enough even for morse code. The CW envelope shaping disappears, leading to key clicks. I am driving the amp with an ATS-3b transceiver, where I integrated a switch controlling the roundness or slowness of the CW envelope. Setting the switch to a "slow" position keeps the output envelope sufficiently round with 3dB RF input attenuator and power efficiency of 69%.

There is a dent in the RF gain curve around 16dBm RF input power. When looking at the drain curves at 7MHz loading the amplifier with 50 Ohm dummy without the low pass filter, I see a square wave. That's what I would expect. But there is a high voltage spike at the beginning of the square. Likely it is the effect of the drain/source capacitance of the FETs and one cannot do much about it with AB, C or D class amplifiers. One could eliminate the unwanted effect of the drain/source capacitance with the Class-E amplifier. If one decreases the RF input power level, there is another interesting effect. The high drain/gate capacitance causes the drain peak to influence the gate threshold voltage. This may switch the FET off until the input RF voltage increases again. If driving the amplifier hard, the slope of the input voltage is so high that the effect is negligible. But when driving the amplifier at low power levels, the effect causes ugly ripples of the drain voltage. Those oscillations are removed by the output low pass filter, but they harm the transistor linearity, as visible from the RF gain curve, and they may harm amplifier power output and efficiency on higher bands. The IRF520 with its drain / source capacitance twice of the IRF510 will likely not be usable on 10m band.

I tried to run the amplifier from a linear 24V power supply. The voltage dropped to 23.6V while producing 115W RF out with 197W DC in. That is 58% efficiency and 41W power dissipation at each IRF520. With forced air cooling I was able to keep the temperature of the heat sink around 40oC with brick on the key, but because of the high temperature resistance of the FETs the chip temperature likely raised over the maximum 175oC allowed. With the forced air cooling, the hottest part on touch was the output transformer core. I would not recommend to run the amplifier at these power levels, but running the amplifier at 18-20V sounds safe, if I only had such a power supply.

I set the bias current to 100mA per MOSFET. When the temperature of the heat sink rises, the bias current increases to around 150mA per MOSFET. With the ventilator running, the FETs are kept cool and the bias current stays at 100mA. This shows the efficiency of the Pentium heat sink. Passive heat sinks are designed differently from the forced air heat sinks and the Pentium heat sink has to be cooled if a significant power is to be dissipated.

My first prototype had 3 turns secondary on the output transformer. That lead to very much the same effects as running the amplifier from 24V. The input / output power increased by 9/4=2.25x. I expect the efficiency to drop a bit because of the higher saturation voltage at higher output currents. It would likely be a reasonable solution, when running the amplifier from 9-12V DC, producing 40-70W output RF power.

I am curious how will the amplifier perform on higher bands. I studied the low pass filter designs of the Juma amp, HF packer amp and the HardRock amplifier. The silver mica capacitors are very expensive and the Juma and HardRock amplifiers use 100V NP0 SMD ceramic capacitors. I will likely try those.

73, Vojtech OK1IAK