Усилитель мощности на 40 ватт.

Performance summary

  • 40W min output power
  • 88 to 108 MHz frequency range, broadband
  • 20dB gain
  • +28V DC operation
  • High efficiency
  • Low component count
  • Integrated 7 pole Chebyshev low pass harmonic filter (LPF)
  • Single MOS FET gain stage in class AB

Suggested Rough and Ready Construction Procedure

  1. Cut out a piece of double sided PCB material for the main board (approx. 100 x 85mm)
  2. Create the aperture for the FET, using a selection of drills and files.  Use the FET as a template, if required, but don't blow it up with static.  Make sure you'll end up with the drain on the right side.
  3. Drill six holes in the PCB, these are to hold the PCB to the diecast box
  4. Place the PCB in the box and use the holes in the PCB to drill through the box
  5. Temporarily screw the PCB to the box
  6. Work out where the heatsink is going to go, underneath the box  The device should end up towards the centre of the heatsink.  Either drill some more holes through the whole lot, and re-use some of the existing PCB/box holes and extend these down through the heatsink.  Temporarily screw the heatsink to the PCB/box assembly.  When you look into the top of the box you should now see a piece of heatsink revealed, the same size as the base of the FET.
  7. Rig yourself up some static protection (if you've got an old blown-up device or a bipolar device in the same package you won't have to bother with this) and drop the device into the aperture in the board.
  8. Use the FET to give you give the centre positions of its' mounting holes
  9. Take everything to bits again.  Make two holes in the heatsink for the FET
  10. Drill the holes in the two ends of the box for the RF connectors and the feedthrough capacitor
  11. Tin the PCB, top and bottom, with a big iron.  Use just enough solder to get a smooth finish but not too much to create raised areas of solder, especially on the bottom, as these will prevent the PCB sitting flat against the box floor.
  12. Create the two islands for the FET gate and drain, as detailed in the above paragraph
  13. Solder copper tape between top and bottom faces of the PCB underneath where the source tabs will be
  14. Create the PCB islands, tin them, stick them on the PCB using the photograph as a guide
  15. Create and fit the screen between the amplifier and the LPF areas
  16. Fit all the remaining PCB components, with the exception of the FET
  17. Fit the PCB to the box and the heatsink
  18. Fit the and connect and the RF connectors and the feed-through capacitor
  19. Taking anti-static precautions again, apply the thinnest continuous film possible of heat transfer paste to the base of the FET.  This can be conveniently done with a wooden cocktail stick
  20. Bend up the last 2mm of each of the FET's leads.  This will make it much easier to remove, should the need arise
  21. Screw the FET to the heatsink.  Too loose and the device will over-heat, too tight and you will distort the flange of the device and once again it will overheat.  If you've got a torque screwdriver, look up the recommended torque and use it. 
  22. If you've understood the instructions correctly, the tabs of the device will be fractionally above the PCB  Solder the FET in with the big iron, first the sources, then the drain, finally the gate.  You may have to disconnect L4 and L5 while you are fitting the FET, but don't disconnect R3 as this provides static protection for the device.

Schematic

Parts List

Reference

Description

FEC Part No.

Quantity

C1, C2, C4

5.5 - 50p miniature ceramic trimmer (green)

148-161

3

C3

100p ceramic disc 50V NP0 dielectric

896-457

1

C5, C6, C7

100n multilayer ceramic 50V X7R dielectric

146-227

3

C8

100u 35V electrolytic radial capacitor

667-419

1

C9

500p metal clad capacitor 500V

 

1

C10

1n ceramic lead through capacitor capacitor

149-150

1

C11

16 - 100p mica compression trimmer capacitor (Arco 424)

 

1

C12

25 - 150p mica compression trimmer capacitor (Arco 423 or Sprague GMA30300)

 

1

C13

300p metal clad capacitor 500V

 

1

C14, C17

25p metal clad capacitor 500V

 

2

C15, C16

50p metal clad capacitor 500V

 

2

L1

64nH inductor - 4 turns 18 SWG tinned Cu wire on 6.5mm dia. former, turns length 8mm

 

1

L2

25nH inductor - 2 turns 18 SWG tinned Cu wire on 6.5mm dia. former, turns length 4mm

 

1

L3

6 hole ferrite bead threaded with 2.5 turns 22 SWG tinned Cu Wire to form wideband choke

219-850

1

L4

210nH inductor - 8 turns 18 SWG enamelled Cu wire on 6.5mm dia. former, turns length 12mm

 

1

L5

21nH inductor - 3 turns 18 SWG tinned Cu wire on 4mm dia. former, turns length 10mm

 

1

L6

41nH inductor - 4 turns 22 SWG tinned Cu wire on 4mm dia. former, turns length 6mm

 

1

L7

2 ferrite beads threaded onto lead of C10

242-500

2

L8, L10

100nH inductor - 5 turns 18 SWG tinned Cu wire on 6.5mm dia. former, turns length 8mm

 

2

L9

115nH inductor -  6 turns 18 SWG tinned Cu wire on 6.5mm dia. former, turns length 12mm

 

1

R1

10K cermet potentiometer 0.5W

108-566

1

R2

1K8 metal film resistor 0.5W

333-864

1

R3

33R metal film resistor 0.5W

333-440

1

D1, D2

BZX79C5V6 400mW Zener Diode

931-779

2

TR1

MRF171A (Motorola)

 

1

SK1

BNC bulkhead socket

583-509

1

SK2

N type panel socket, square flange

310-025

1

 

 

 

 

 

Diecast Box 29830PSL 38 x 120 x 95mm

301-530

1

 

Heatsink 16 x 60 x 89mm 3.4°C/W (Redpoint Thermalloy 3.5Y1)

170-088

1

 

Double sided Cu clad PCB material 1.6mm thick

 

A/R

 

Copper Tape or Foil

152-659

A/R

 

M3 nut, bolt, crinkly washer set

 

16

 

Non-Silicone Heat Transfer Paste

317-950

A/R

Notes

  1. Farnell Part Numbers are for guide only - other equivalent parts can be substituted.
  2. Metal clad capacitors are either MCM series, Unelco J101 series, Underwood, or Arco MCJ-101 series available from, amongst other places, RF Parts.
  3. MRF171A available from BFI (UK), Richardson or RF Parts (US)
  4. Arco or Sprague trimmers are available from Communication Concepts (US)
  5. 18 SWG (standard wire gauge) is approximately 1.2mm diameter
  6. 22 SWG (standard wire gauge) is approximately 0.7mm diameter
  7. To make the inductors - wind the required number of turns round an appropriately sized former, initially use one wire diameter spacing between each turn.  Then pull the turns apart to get the length required in the parts list table.  Finally check the value using a network analyser and adjust accordingly.
  8. The exception to the above spacing rule is L4, which is close wound.
  9. Copper foil is available from craft shops (used in stained glass making)
  10. A/R = as required

Photograph of Prototype Amplifier

Note orientation of the FET. The lead with the slash is the drain, and is to the right

 

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