The Power Amplifier Unit

Overview

The Power Amplifier (PA) is optimized for 50W output (most of the time I use the external PA which requires 25-30W drive). The power amplifier is a fully balanced design. It has three stages - OPA2674, 2xRD15HVF1 and 2xRD70HHF1.

The idling current of the driver and final transistors are controlled 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 at the reasonable level (better then -30...-36dB).

Results (Preliminary)

I measured better than -36dB IMD3 on all HF bands (except of 10m) with 50W PEP and near -40dB with the 30W PEP. The efficiency is 40% approx. (final only!) with the 50W CW output.

The amplifier can produce >90W on all bands with the IMD3 performance better than -30dB PEP across the whole HF except of the 10m band (the IMD3 performance is 3-4dB worse there). But that was not my goal (the termal conditions will require bigger heatsink/noisier fan or duty cycle reduction, also the linearity at such power levels is worse and operation is less reliable). At the 50W ouput power the low noise 800rpm/120mm (Noctua NF-S12-800) fan is sufficient for 100% duty cycle. The amplifier survives the shorted or opened load with such output power.

The even harmonics are at least 35dB below the carier.

The frequency response is very flat across HF (I have no 50MHz band, but I bet it is not a problem for this design).

The schematics can be downloaded here. I will put more information after the completion of the LPF Unit.

The PA unit board top

The PA unit board bottom