1.5 Watt 74HC240 QRP RF Amp using 74AC240 inverter for improved output

I am building a little QRP  rig for the 40 meter band , utilizing one of my DC receiver designs. I am using a CS2000 clk generator for the receiver and can use the clock output to drive a 1 watt or so RF amp for the TX. I am prototyping the transceiver now and will post that  design when it’s done. I designed a nice class E final at about two watts but it was tricky to test and tune…etc- so I thought I would come up with an amplifier that was easier to duplicate by other builders. This amplifier works well and is easy to build. It puts out in excess of one watt from a single  74AC240 (the 74HC240  will also work but not put out as quite as much power). You can also use a couple of 74AC04’s  also. The 240 is convenient however, because it has an enable pin for keying when using CW. The circuit uses two sets of four inverters driven by  a couple more inverters( to create differential inputs) and either a balanced L network(followed by a balanced to single ended combiner transformer) or  a LC tuned balun to match the output to a 50 ohm load. The schematic shows both methods and values for 7 Mhz. I have created a spreadsheet for calculating networks for any band desired. The inverters have very low impedance and so you need to match roughly 4 ohms at the inverters to 25 ohms for each side of the push pull sets. You need to heatsink the inverter IC’s! I used a  small heatsink for IC’s (seen on top of the 74AC240 TSSOP package) but you can use a piece of bent aluminum – or copper, etc. Because of the low Z, all of the matching inductors are small – so you can easily fabricate your own small  air wound coils or coils wrapped on scraps of ferrite from junk inductors- you don’t need to have specialized toroid cores.

The LC balun works well but was trickier to get adjusted (needed further matching by means of a L network). The simple “sortabalun” transformer  combiner in tandem with the balanced L network works  a little better and  is easier to design and  adjust. The only disadvantage is you need some sort of ferrite core material for the transformer – whereas the LC balun method can be all air inductors. If you have some ferrite material – use the transformer method. One could do impedance transformation with the transformer but the L matching provides filtering, can be easily adjusted for  parasitics in the inverters(capacitance) and  a 1:1 transformer is very easy to construct!

The only drawbacks of this design are, one; you can only use 5v up  8V max to power the amp and two; it is not very efficient (mine runs at about 40%- 45%). It can draw .4 amps @ 8V – which is quite a bit of heat! Having said that, it is fun to build and tolerant of mismatched SWR. Overall, not bad performance. If you used two of them (8×8 push pull) you would need to adjust down the L network inductors and increase the capacitor(this lowers the impedance at the inverters further). I expect you could get more than 2 watts.

Schematic Diagram(minor revision 07/29/2015) 

QRP RF Amp

View of main amp using LC Balun

 IMG_20150722_213556201

 Just over 1 Watt 2nd and 3rd harmonic down 37 dB with LC Balun

IMG_20150722_213704057 IMG_20150722_213722665

Over 1.5 watts with balanced L network and sortabalun!

Harmonics down more than 40 dB!

IMG_20150723_193630353 IMG_20150723_193652596

Link to network designer spreadsheet: https://www.adrive.com/public/5wtBBr/network%20designer.xlsx

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11 thoughts on “1.5 Watt 74HC240 QRP RF Amp using 74AC240 inverter for improved output

  1. Hello,
    Congratulations for the wonderful pages.
    I tried replicating your design (toroid version). It works nice: i get a steady output with very small signals as input. The problem is that I get low power: on a 50 ohm load it only draws 120mA at 5V and 200mA at 8V.
    I’m using all SMD components, except for the toroid which is a T50-6. Do you think that might be the problem?
    Thank you in advance
    Alain

  2. Hello,
    Congratulations for the wonderful pages.
    I tried replicating your design (toroid version). It works nice: i get a steady output with very small signals as input. The problem is that I get low power: on a 50 ohm load it only draws 120mA at 5V and 200mA at 8V.
    I’m using all SMD components, except for the toroid which is a T50-6. Do you think that might be the problem?
    Thank you in advance
    Alain

    • Hey there…I’m traveling right now so can’t give you a detailed answer referencing my design info…But try changes to the 50 ohm dummy load (lower and higher) and see what value extracts the most power across the load. This will tell you if and how your matching network may be need to be adjusted. If using higher frequency then 7 mhz….It still works but gets less efficient. Keep me posted…And in about week I can delve more deeply into the issue if you are still having trouble.

    • Not enough info…What are you using the powder iron toroid for? T1? If so it won’t work well…You need more permeability…You need to use a high permeability ferrite material for the sortabalun T1

  3. Here is some additional info, but please don’t rush… take your time.

    Current peaks around 7MHz, as expected.
    Altering the load doesn’t have much of an effect. The peak is around 50 ohm.
    A toroid was used for T1. The material is AL=4 (details here: http://toroids.info/T50-6.php)

    Thanks,
    Alain

    • You need to use a ferrite type material with much higher permeability than the type 6 powder iron.you need higher reactance to choke current flow such that best balun behavior is achieved…Also you can try adjusting 1200 of cap up or down a little. Also I assume you are using the AC series part not the HC series for the inverter

  4. Ray,
    I will look for a ferrite and also doe a AC series inverter (I’m currently using HC) then I’ll get back to you. Thanks!

  5. Here is a little update: I replaced the iron powder toroid with another made with a ferrite bead. Not relevant differences found except for the fact that I was then getting most of of power around 4MHz (it might have been the case even before replacing the toroid but I’m not 100% sure about that).

    Then, as you suggested, I tried playing with the 1200pF capacitor. With a 470pF capacitor I am getting a solid 10Vpp over 50ohm at exactly 7MHz. Much better than before (6Vpp)! Absorbed current is still pretty low: 120mA.

    Question: what kind of increase can I expect by replacing the HC series inverter with the an the AC?
    Thanks!

  6. Ray,

    I tried every possible variation with bifilar, trifilar, iron powder / ferrite transformers. I can’t manage to get more than 8.4 Vpp at 7 MHz. The problem is that harmonics are huge… about (-18dB), so I can’t even use it.
    I am running out of ideas… have you got any suggestions? Thanks!

    • okay …give me a couple of days I am going to build one and revisit my values and output behavior and get back with you. I have built this amp standalone and in three other xcvrs with good results so ..sure we can get it up and running well for you. The impedance matching is somewhat critical and would be different for the HC part as opposed to the AC part at least from a maximum output perspective. Again my design uses the AC part.

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