Cheap KV PA with V-MOS

V-MOS IRF5xx and IRF6xx are suitable transistors for a simple broadband linear PA with a power of tens of watts. With a suitable supply voltage, we get a power of around 40 to 50W from a fifty-crown transistor. So if you have QRP device and you would like to build a smaller PA for it, I would recommend you just such a PA. I built one myself and it works great.

Broadband V-MOS PA with 1:4 transformers on suitable material. A single-band PA can be customized using pi- or L-cells. IRF5xx and IRF6xx are mainly intended for switching purposes (inverters, switching power supplies), the level of harmonic products is higher and we cannot avoid the use of low-pass filters. A push-pull design would certainly be interesting as well, which would make it possible to achieve a power of around 100W and the level of unwanted products would be lower.

Achieved results of the author

The author, DK7ZB, states that with a supply voltage of 30V and an excitation power of 1.5W, the described PA with IRF530 achieves a power of 30 (10m) to 50W (80m). It is at the entrance transformer 4:1, so the assumed input impedance in the gate is 12.5 ohm. The power is mainly consumed by R1 (powerless induction type), which significantly affects the resulting impedance. The DK7ZB thus solved the problem of the large input capacitance of the V-MOS relatively well. Use a higher quality capacitor as C2, RF current passes through it. We set the quiescent current with the trimmer R2 from the stabilized voltage.

We will use Zener diodes in the drain for IRF5xx, which have Uds=100V and have the task of protecting the transistor. The output is again a 1:4 transformer. Direct current is separated by capacitor C4. The supply voltage is supplied through a choke and is filtered by capacitors.

When connecting, it is necessary to pay particular attention to the terminals of the transformers - if the connection is incorrect, your PA will oscillate. Wind them bifilarly with wires of different color of insulation. It is also convenient to orient them differently (upright and lying down) or shade them. Before installing the transistor, it is a good idea to try the voltage setting for g1 and the RX/TX switching. Solder the transistor and attach it ISOLATED to the cooler (the drain is connected to the metal surface of the case).

Reviving the amplifier

It is advisable to start the revival at a lower supply voltage (e.g. half) with the circuit breaker switched on. With zero voltage on g1, the draw must be only a few mA. We gently turn the trimmer R2 and watch the drawn current, which must increase smoothly without any fluctuations. If this is the case, connect a wattmeter and an artificial load to the output, and a transmitter with a power of approx. 0.3 - 1W to the input. At full supply voltage, we set the quiescent current to approximately 200 mA. After applying the excitation signal, the output power should already correspond to the amplification of the PA (approx. 12dB). Finally, it is advisable to check the entrance fee PSV. We can try to improve it by slightly changing the quiescent current and the length of the coaxial cable between TX and PA (in my case it was necessary, but I used the MS1307 transistor at a supply voltage of 13.8V and it probably has a different input impedance).

We will build output filters according to some proven design. When reviving, I only had problems with the input PSV, but not otherwise. I can warmly recommend it to anyone who is considering a small PA.

Parts list