The Power Amplifier Unit

Overview

The Power Amplifier (PA) provides up to 100W output power on all HF bands. The power amplifier is a fully balanced design. It has three stages - OPA2674, 2xRD15HVF1 and 2xRD70HHF1.

The DAC is used to control driver and final transistors idling current. The DAC is controlled by the control processor via the SPI bus. The current and temperature sensors are also on the board.

The design goal was to built a simple and reliable power amplifier with a good linearity. I selected the negative feedback components to minimize the higher order IMDs while keeping IMD3/IMD5 at the reasonable level (better then -30...-36dB).

Results

The PA unit was tested with the LPF unit at the output and BPF unit at the input. The IMD were measured using two signal generators and PC soundcard based spectrum analyser.

The amplifier can produce >100W on all bands with the IMD3 performance better than -30dB PEP across the whole HF except of the 160m band (the IMD3 performance is 1dB worse there). The low noise 800rpm/120mm (Noctua NF-S12-800) fan provides PA cooling. The amplifier survives the shorted or opened load.

The frequency response is very flat across HF (I have no 50MHz band, but I bet it is not a problem for this design). All harmonics were at least 65dB below carier.

Here are the IMD test results.

160m Band

Two tone test output spectrum at the 25WPEP. The IMD3 is -36dBPEP, IMD5 is -50dBPEP.

Two tone test output spectrum at the 50WPEP. The IMD3 is -35dBPEP, IMD5 is -46dBPEP.

Two tone test output spectrum at the 100WPEP. The IMD3 is -29dBPEP, IMD5 is -39dBPEP.

20m Band

Two tone test output spectrum at the 25WPEP. The IMD3 is -39dBPEP, IMD5 is -53dBPEP.

Two tone test output spectrum at the 50WPEP. The IMD3 is -38dBPEP, IMD5 is -52dBPEP.

Two tone test output spectrum at the 100WPEP. The IMD3 is -37dBPEP, IMD5 is -43dBPEP.

10m Band

Two tone test output spectrum at the 25WPEP. The IMD3 is -37dBPEP, IMD5 is -53dBPEP.

Two tone test output spectrum at the 50WPEP. The IMD3 is -37dBPEP, IMD5 is -48dBPEP.

Two tone test output spectrum at the 100WPEP. The IMD3 is -33dBPEP, IMD5 is -43dBPEP.

Some words about the efficiency and cooling. Linear operation requires intensive cooling cause idling current is approx. 3A and efficiency is not spectacular - the PA draws 18A from the 13V power supply (with 100W RF output after the LPF unit). So I will probably try to reduce idling current in CW/RTTY modes to improove efficiency.

The schematics can be downloaded here.

The PA unit board top

The PA unit board bottom

The PA unit mounted on the heatsink with the cooling fan (the LPF unit will be mounted un the back side of the heatsink)