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 zariadenia a radi by ste k nemu postavili menší PA odporúčal by som Vám práve takýto PA. Sám som jeden postavil a pracuje výborne.

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

Autor, DK7ZB, uvádza, že pri napájacom napätí 30V a budiacom výkone 1,5W dosahuje popisovaný PA s IRF530 výkon 30 (10m) až 50W (80m). Na vstupe je transformátor 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

Oživovanie je vhodné začať pri nižšom napájacom napätí (napr. polovičnom) so zaradenou prúdovou poistkou. Pri nulovom napätí na g1 musí byť odber len niekoľko mA. Jemne otáčame trimrom R2 a sledujeme odoberaný prúd, ktorý musí plynulo narastať bez akýchkoľvek zákmitov. Ak je tomu tak, na výstup pripojíme Wattmeter a umelú záťaž, na vstup vysielač s výkon cca.0,3 – 1W. Pri plnom napájacom napätí nastavíme kľudový prúd na zhruba 200 mA. Po privedení budiaceho signálu by mal už výstupný výkon zodpovedať zosilneniu PA (cca.12dB). Na záver je vhodné ešte skontrolovať vstupné 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