High Efficiency 4V supply QRP Amplifier

This is an example of a high efficiency QRP transmitter designed to work at very low supply voltages (3v-5v). It can produce 2 watts a 4 volt supply @ 70% efficiency. It uses small, inexpensive switching mosfets. The primary requirement for these mosfets is low output capacitance, a VDS of >20V, a logic level VGS and a drain current rating of a couple amps. There are many devices that will work. Unlike a Class E amplifier, this design requires no special alignment, providing for multiband operation easily. Only the output filter consisting of a L Network and Pi network in series need to be changed for a given band. It is tolerant of all kind of load conditions including infinite Z and maintains efficiency when poorly matched. While this circuit utilizes a microcontroller, display and clk generator, the logic buffer can operate from any oscillator source so the amplifier can be adapted to simpler designs.

QRP Transmitter Schematic

5 thoughts on “High Efficiency 4V supply QRP Amplifier

  1. Interesting PA, I have been looking for a circuit that could give a few watts out on low voltage supplies so I was pleasantly surprised by the mail notification on this one.
    I guess you do PWM for the bias to the FETs?
    What current do think the bias current is if I wanted to test out the circuit as-is without the MCU?

    Thank you for sharing another novel circuit!

    • No current just a bias voltage from the MCU’s DAC module. With most logic level FETS you don’t need it. That particular FET has a threshold V low enough that it works without any bias. I used the DAC to produce about 1 volt shift up(shifts the gate zero voltage above ground) to slam them a little harder. Any voltage source like a LED forward V..through a divider or POT would work fine if desired. You would adjust to just at or below turn on.

  2. Cool design. Is it working in Class-D? In my experience HF amplifiers claiming high efficiency often work at least partially in Class-E mode. Did you measure currents / voltages on the PA transistors?

    Does the keying circuit shape the dits?

    I am in a process of building a FOX transmitter for my kids from a junk box, your design would fit the bill.

    • Its not running class E, it’s not exactly class D either..Its a traditional push pull class C amplifier but driven with a switching waveform, resulting in class D like behavior and high efficiency. My efficiency calculation is simple: power out/power into the final. There is very slight wave shaping..when I have used this buffer drive circuit in the past I have had little key click effect. If you use it at 20 meters or below you can use the buffer drive chip as a 1 watt amplifier by itself..that is on my blog as well.

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