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20W HF+50 MHz POWER AMP Print E-mail
Written by IK0OTG   
Friday, 27 May 2011 13:12
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20W HF+50 MHz POWER AMP AMP_20W_001



QRP amplifier with compact size.

The PCB measures 3.8" to 2.8".





Range 1.7 - 52 MHz
Voltage 13.8 V
Supply Current (CW 20W) 3.6 A
Maximum power (1.6MHz - 50 MHz) > 20 W
Gain 43 dB




Fig. 1



Wiring diagramm




Fig. 2


In fig.2 there is a separate connector (COX3 SMB Rx) if you are using a separate transmitter and receiver, in which case the jumper, visible in the upper left of Figure 1 should be positioned towards the interior of PCB. If you are using a RTx, the jumper must be positioned as in Figure 1.
The terminal blocks M1 and M2 are provided for power, one for the entrance a
nd the other, for supply other boards.


 In this version I omitted C26 Capacitor , to extentd the band until 70MHz (reduced power) and to increase the stability.

The primary of T3 is formed by two copper pipes, made ​​from a sheet of 0.1" copper and rolled on the shank of a drill Ø 4 mm, inserted in the holes of the core and welded into the holes of the two small PCB (see Fig .3). The secondary consists of two turns of Teflon-insulated stranded wire AVG 18. (see Assembly T3)

Be careful to properly solder the beginning and end of the windings of T2 and T1.

The MOSFET Q2 can be replaced with a RD06HHF1, which is cheaper and performance remain approximately the same.

The diode D2 is to be mounted under the PCB and must touch the heatsink with some thermal paste.

When you transmit, the center pin, of connector J1 must get a +12 V. This will polarize the MOSFET and energize the changeover relay IN / OUT RL1 and RL2. When receiving, the voltage must go to 0V, so that the relays are released and the MOSFETS do not heat up unnecessarily.

The MOSFET Q2, Q3, Q4 are TO220, with the source connected to the case, so the insulating mica plate is not required, but the thermal paste is always needed!

For the cooler, is sufficient, a heat sink in anodized or painted aluminum, 0.5 "wider and 0.5" longer than the PCB, with flaps of 1.175". Other solutions with the same surface, are still good



Fig. 3


The PCB, single-sided FR4 ​​material, must be fixed to the heat sink with metal spacers 0.3" high. MOSFETS are soldered by the track side with legs bent upward Fig. 4




Fig. 4




After checking that everything is correctly welded and that there are no jumpers of tin between the tracks and between tracks and ground. Turn the trimmer VR1, VR2, VR3, anti-clockwise (CCW).

Connect input and output of the amplifier to a dummy load of 50 Ω, ¼ W input and > 20W output.

With amplifier and heat sink at room temperature

  1. Connect voltage to M1(+13.8 V) via an ammeter in series, without giving voltage to J1. The current should be between 60 and 70 mA, take note of the measurement
  2. Connect voltage to J1 (12V on the central pin). The current on the M1, it should not be changed.
  3. Turn trimmer VR1 slowly clockwise until on ammeter will read a value equal to the first value increased by approximately 100mA (for example, if the first was 65mA, now you have read 165 mA).
  4. Slowly turn the trimmer VR2 clockwise until on ammeter will read a value equal to the previous value increased by approximately 200mA (for example, if the previous was 165mA, now you have read 365 mA).
  5. Slowly turn the VR3 clockwise until on ammeter will read a value equal to the previous value increased by about 200mA ( for example if the previous was 365mA now you have read 565 mA).
  6. The adjustments are finished. Turn off everything, unplug the meter, disconnect the dummy load and connect the amplifier as necessary for your needs


Assembly T3

Prepare two sheets of copper with dimensions of 0.1" to 0.5" to 1.2" (Fig. 5)

  • With the touch of embroidery or nail scissors , cut the short end for a depth of about 0.1"
  • Roll up the plates on the shank of a drill Ø 4 mm
  • Place the two tubes obtained in the holes of the ferrite
  • Place on the two ends, the two small PCB, with the copper side outside.
  • Make sure that the two copper pipes snack equally by both parties
  • Open as a daisy part shredded
  • Solder to the PCB both ends of the tubes.
  • With copper wire AVG 22 make 3 jumpers and solder as shown
  • Solder the capacitor C26
  • Wrap two turns of the secondary starting at the opposite to that which is welded C26




Fig. 5



. Download

In the Downloads section of this site, you have the schematic and the image of the PCB.
For those who wish, are available the three PCB with silkscreen, solder resist and tin.

73 IK0OTG Pietro

Last Updated on Sunday, 05 February 2012 17:57