W8DIZ 5 Watt CW Amplifier Analysis

I finally got the proper binocular ferrite cores that I needed to build the W8DIZ 5 watt amp correctly. You can see my previous post on this amplifier here. In my last post, I noted that I was seeing some strangeness in the drive level circuitry. I found that I had a very bad connection through my ammeter to the DC power supply, and once it was corrected the drive circuitry worked as it should.

For this basic analysis of the amplifier, I took measurements of the RMS voltage of the amplifier output into a 50 Ω dummy load with a constant input amplitude of 0 dBm. I also measured the total current draw of the circuit, which allowed me to calculate the amplifier efficiency. Note that no low-pass filtering was used at the output of the amplifier. The output waveform was not sinusoidal, but my DSO is able to do a good job measuring RMS voltage.

Test Equipment

  • Tektronix TDS 1012 Digital Storage Oscilloscope (100 MHz bandwidth)
  • Tektronix SG 503 Leveled Sine Wave Generator
  • Tektronix DM 502A Digital Multimeter
  • Tektronix PS 503A Power Supply
  • M3 Electronix FPM-1 Frequency Counter/Power Meter

Test Conditions

The DC power supply to the amplifier was set to a loaded voltage of 13.5 VDC. The signal generator for the input signal was set to 0 dBm power output into 50 Ω, which was verified with the FPM-1 each time the frequency was changed. Two sets of measurements were taken, one with R6 set to minimum and the other with R6 set to maximum.

Measurements

Min Max
MHz VRMS (V) PO (W) IDC (mA) VRMS (V) PO (W) IDC (mA) Eff.
1.8 13.7 3.75 440 22.4 10.03 957 0.776
3.5 20.2 8.16 768 23.9 11.42 992 0.852
7.0 13.8 3.81 470 21.7 9.42 844 0.826
10.1 9.3 1.73 300 19.8 7.84 732 0.793
14.0 5.24 0.55 174 17.6 6.20 635 0.723
18.7 3.12 0.19 103 14.4 4.15 514 0.598
21.0 2.07 0.09 78 12.6 3.18 452 0.521
24.9 1.53 0.05 65 9.05 1.64 326 0.372
28.0 1.19 0.03 60 6.56 0.86 232 0.274

Conclusion

As Diz states in his original post, the efficiency of the amplifier is quite good. However, both the power output and the efficiency starts to droop a bit above 20 meters. It's my belief that this is a function of the gain-bandwidth product of the two PA transistors. According to the datasheet, the FT of a 2SC5739 is 180 MHz. Given the rule of thumb of having a FT at least 10 times the output frequency, it makes sense that the output starts to get a bit weak around 18 MHz. I do have some similar devices (2SC5954) with a slightly higher FT of 200 MHz that I will probably substitute in the circuit to see if I can improve the upper HF response a bit. There seems to be some kind of strangeness at 3.5 MHz, which doesn't allow me to get much power output range. I'll have to check with Diz about this. Regardless, this would still make a very fine QRP amplifier up to the 15 meter band. The amplifier is extremely stable and the PA transistors don't get very hot during long periods of use. I currently have the transistors floating freely, but a modest heat sink would probably be a good thing if running the amp at full power output. This kit will be a great addition to the RF Toolkits line.

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3 thoughts on “W8DIZ 5 Watt CW Amplifier Analysis”

  1. Did this get resolved .... did the 2SC5954 increase the power above 15Meters ??

    I was planning on getting the 10W version but if the power out drops badly above 20M that will be a problem. Perhaps Q2/Q3 = mrf497's and Q4/Q5 = 2n3866 ??

    Ok the MRF497's are kinda spendy .... just off of the top of my head ...

  2. I am reading these, since I have to manually approve each comment. I'm sorry I wasn't able to reply quickly enough for your liking. I didn't have an quick and easy answer at the top of my head, so I wasn't able to send you an immediate reply. It is in my queue to look at, but you'll have to forgive me since I'm knee deep in the business of taking care of an infant, trying to start a business single-handedly and dealing with a family member with some painful health issues. On second thought, I don't think I even know where my notes are on this subject so I'm considering the topic closed. This would make a great project for your own homebrewing experiments.

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