The Ultimate Guide to Homebrewing a VU2EVQ Design HF Linear Amplifier – 2

This picture explains the various parts of the VU2EVQ design HF linear amplifier circuit. The Pi network of thick resistors form the attenuator for reducing the signal strength delivered to the input transformer. When I had accidentally delivered 60W SSB power from my FT-710 radio for the initial testing, this was probably the one which prevented the MOSFETs from burnout. Another factor would have been the current limiting by the relatively small UPS battery which I had used at that time.

RF sensing circuit which includes a transistor and diode detects the received signal from the input radio device and enables switching of the linear amplifier to transmit mode. So a PTT line from the radio is not needed for T/R switching. Default position of the relay will be in receive mode so that your radio can receive signals from the antenna. Bias adjusting potentiometers shown here are single turn cermet potentiometers. I had replaced them later with multi-turn potentiometers for better fine control of bias current and voltage. They are used to adjust bias to the final MOSFETs. The inductors and high voltage capacitors in the low pass filter are meant for filtering out harmonics like 14 MHz which may be generated in the final RF stage. Leakage inductance compensation capacitor is used to cancel the reactive impedance caused by flux that does not link the primary and secondary windings. Its primary role is to achieve series resonance with the leakage inductance, thereby eliminating the inductive voltage drop at a specific operating frequency, allowing for maximum power transfer and improved efficiency. RF choke prevents entry of RF into the DC supply line.

Now the VU2EVQ design HF linear board is nearing completion. The large heatsinks can be seen well in this video clip.

You may note the blue multi-turn rectangular potentiometers with a screw instead of the previous single turn cermet potentiometers. RF input and output cables can be seen in this picture taken today. Feedback resistor and capacitor connecting the MOSFET drain to the gate can be seen on the opposite side.