5W QRP tranmitter for 30m with class D amplifier

10MHz is one of my favorite bands. Working DX with QRP is possible and the band is almost independent of sun spots.

Perhaps the best QSO I have ever had was working Peter 1 on 10MHz with 5Watts.

In this project I wanted to learn more about class D amplifiers. In D-class two active components act as switches.

One of the switches conduct on each half cycle. In the output either current or voltage is zero during all half cycle, which means that

power dissipated is zero and in ideal case efficiency reaches 100%. In practice during transition there are losses and only

efficiencies 70-90 % are reached.

There are two different modes of class D. In voltage mode class D or VMCD output voltage to filter is square wave and current sinusoidal.

In current mode class D or CMCD output current to filter is square wave and voltage sinusoidal. In this project I used VMCD.

My XTAL oscillator and isolation stages are made with CMOS NAND gates. +5V regulator is needed to serve the IC.

This simple capacitor tuned circuit is reliable and gives +/- 2kHz tuning range for one crystal.

One could make more fancy circuit to pull frequency more.

The driver stage is class D amplifier. It is wide band and could be used from 80m to 10m.

With complementary transistors we get good symmetry and attenuation of even harmonics without transformers.

The driver stage is keyed. Keying filter is an active integrator having rise and fall times 7ms.

The output stage is VMCD amplifier ( switch). Supply voltage is +24V which gives output impedance 14 ohms.

The output filter converts also impedance level, from 14ohm to 50 ohm. Harmonics are more than 40 dB down.

Symmetry helps to attenuate even harmonics.

My +24V power supply is from old HP deskjet 656C printer. It supplies 500mA.

TR-switch is with relay. Delay can be adjusted with potentiometer.

My construction is ugly style in RF shielded steel box made by TEKO. No PCB material is used.

Feed-through capacitors are used for DC connections.

Most difficult in construction was to get low inductance to emitter by-pass capacitors of the PNP-transistors.

Very short wires and multiple parallel capacitors helped but it is still not perfect.

Measured output power is 5.4W. Current consumption from +24V is 320mA giving 70% collector efficiency.

First DX worked with this transmitter was Alan N3BJ with 579 report.

Pekka

24.1.2009

OH1TV, OH1WX on QRP