Cheap KV PA with V-MOS

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

Broadband V-MOS PA with transformers 1:4 on suitable material. We can adjust the single-band PA using pi- or L-cells. IRF5xx and IRF6xx are mainly intended for switching purposes (converters, switching power supplies), the level of harmonic products is higher and we do not avoid the use of low-pass filters. The push-pull construction would certainly be interesting as well, which would allow to reach a power of about 100W and the level of unwanted products would be lower.

Achieved results of the author

Author, DK7ZB, states, that at a supply voltage of 30V and an excitation power of 1.5W, the described PA with IRF530 reaches the power 30 (10m) up to 50W (80m). There is a transformer at the input 4:1, that is, the assumed input impedance at the gate is 12,5 ohm. Power is consumed mainly on R1 (power non-inductive type), which significantly affects the resulting impedance. DK7ZB thus solved the problem of large input capacity of V-MOS relatively well. Use a higher quality capacitor as C2, the RF current passes through it. The quiescent current is set by trimmer R2 from the stabilized voltage.

We use Zener diodes in draine for IRF5xx, which have Uds = 100V and have the task of protecting the transistor. The transformer is at the output again 1:4. DC current separates capacitor C4. The supply voltage is fed through the choke and is filtered by capacitors.

When connecting, it is necessary to pay particular attention to the transformer terminals – if the connection is incorrect, the PA will oscillate. Wire them bifilarly with wires of different insulation colors. It is advisable to orient them in other ways as well (insisted and inclined), or shade. Before mounting the transistor, it is good to test the voltage setting for g1 and RX / TX switching. Solder the transistor and attach it INSULATED to the heatsink (the drain is connected to the metal surface of the housing).

Reviving the amplifier

It is advisable to start resuscitation at a lower supply voltage (e.g.. by half) with current fuse included. At zero voltage on g1, the consumption must be only a few mA. We gently turn the trimmer R2 and monitor the current taken, which must grow smoothly without any hesitation. If so, we connect a wattmeter and an artificial load to the output, to input transmitter with power approx.0,3 – 1W. At full supply voltage, set the quiescent current to approx 200 mA. After the excitation signal is applied, the output power should already correspond to the PA gain (approx. 12dB). Finally, it is advisable to check the input PSV. We can try to improve it by a small change in quiescent current and coaxial cable length between TX and PA (in my case it was necessary, but I used transistor MS1307 at a supply voltage of 13.8V and probably has a different input impedance).

We build output filters according to some proven construction. During the recovery, I only had problems with the input PSV, but not otherwise. Everyone, who is considering a small PA I can highly recommend it.

Parts list