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

7 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.

  3. Interesting circuit. My LTSpice simulation suggests the output impedance is lower, around 5 ohms.
    Decoupling the coils on T1 appears to improve efficiency and power output.
    With the correct matching, at 3.7 V, the output power is simulated to be around 3.3 W and the MOSFET’s dissipate close to what they can safely handle.
    Be interesting to build the circuit and see what happens in reality.

  4. So the output transformer should perform better I would think as even order harmonics are canceled and the core size can be smaller as the currents cancel each other reducing net current across the transformer. I briefly experimented with separate inductors(trying to minimize specialized parts) early on and found the performance poor. I interpret this topology roughly as a switched push pull class C amplifier . From this premise, I did back of the napkin calculations for the design, so I wanted to get 1-2 watts from a 5v supply. This gives us a single ended Z of (5^2)/2*Pwr -> 6.25 ohms. we can then doubled this because of the push pull topology…this yields about 13 ohm @ 5V and 2 watts out. With the actual design I get 2 watts out at just over 4 volts…so my filter network may be off a little or my use of a class C model may be inaccurate. Regardless, it seems to work well.

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