G.P. Transistors in RF Amplifiers - Parallel Ouput Transistors- Success!

Having populated the board I previously blogged I applied power. Backwards of course. However, no harm done. Phew.

After noting the amplifier was working with no obvious issues I checked the temperature of the output transistors. Too hot for comfort so I added the thermal jumpers I had brought from Mouser for testing in just this kind of application. Never having used these before it was pleasing to see how they allowed a "hot" transistor tab to be thermally grounded but isolated for DC and AC voltages.

Now for a smoke test. With a 13.8volt supply I could only achieve a clean output of 16.6v peak to peak at 3.6MHz. Increasing the supply voltage to 15.9volts allowed a clean 1 watt output sine wave to appear on the CRO.

Tuning around and writing down measurements meant I noticed that the output of my signal generator is not as flat as you might expect from a HP product. Regardless, at a frequency of 20MHz I set the signal generator output to deliver 10v pp into the dummy load. I then tuned down and the output was quite flat until 10MHz when the output looked like some frequency doubling was happening. Changing the capacitor coupling the emitters together solved this. I had used a 15nF by mistake. But as I reduced the frequency still further the output resumed the appearance of a sine wave and started increasing until it reached 19v pp at 6MHz before declining as frequency was reduced.

I haven't time at present to investigate if that is the signal generator output level wandering around or something weird occurring in the amplifier. So I cranked the supply voltage up to 15.9 volts, set the output for 20v pp at 3.6MHz and walked away.  Later I returned and no smoke of any kind. I did note the output had dropped a few volts due to the temperature rise of the transistors. But no damage done and a few puffs dropped the temperature enough to show the output rise.

By now you  might be wondering what transistors I used. The very first packet I picked up were 2SD1007's. When I returned to the house I checked my LTSpice model and it was in loose agreement with my measurements. eg I had measured 1.4V on the emitter whereas LTSpice was suggesting is should be 1.18V. 

Some finessing is still required, but the proof of concept test board is actually a working amplifier.

 

Conclusion:

Cheap (A$0.10 each) SOT89 transistors can be made to work as HF amplifiers in the vicinity of 1watt when used in parallel.

However, you need a thermal jumper which costs 10 times more than the transistor. And my heat sinking using the ground plane could be larger. In practice that will require a wider board so I will abandon my plan to use the piece of extrusion I have on hand to house the amplifier.

So more thought needed regarding the thermal jumper.

73's

Richard

G.P. Transistors in RF Amplifiers - Parallel Ouput Transistors - PCB

 The PCB to test this idea is on it's way. Shouldn't take long to test the concept once it arrives.

I placed the second output transistor on the other side of the board. This kept the traces shorter and appeared cleaner.

The board is long and skinny since I am working towards mounting this inside a standard size of aluminium extrusion with an internal dimension of 38mm.   This plan may have to change depending on what happens with my heatsink test. More later.

73's

Richard

G.P. Transistors in RF Amplifiers - Parallel Ouput Transistors Preliminary

The test results for the 2SD882 showed it had the flattest response but at 160mW it appeared to reach a thermal limit that saw the output drop to near zero. After cooling off it appeared to work normally again. 

A LTSpice model suggested this transistor should have been capable of 1W, and with a much better heatsink and increased standing current perhaps 1.5W. However, achieving this would require a smaller emitter resistor with the result that Zin falls to 38Ω at 3.6MHz and 25Ω at 7MHz. 

Using two 2SD882's in parallel has some drawbacks:

• The gain is lower, so they have to be driven with a higher input
• The input impedance is lower still (30Ω @ 3.6MHZ and 18Ω @ 7MHz)

Looking at a few transistors with LTSpice returned the following  ballpark numbers when operated in parallel:

                                       Zin at 

    Transistor          3.6MHZ    7MHz

    2SD882            30                    18

    MJD44H11         36                  24

   2SC5824              42                  33

SOT89 style package 

     2SD965              43                  41 

    2SD1007           42                   35

    2SC5964             45                  38   

   

Conclusion

General purpose through hole transistors don't appear to be a good solution when operated in parallel. They don't have the frequency response of a true RF transistor and the input impedance is lower than desired for a 50Ω module approach.

Some of the SOT89 style transistors appear promising and if the heatsinking can be resolved may be worth pursuing.

73's

Richard

General Purpose Transistors in RF Amplifiers - Last Ouput Transistors Test

After testing some more transistors I found the 2SC4552 delivered 0.9Watt in my class A test amplifier at 3.6MHz. I am confident that a little more standing current and this transistor will deliver 1 Watt and it appears to be quite robust at this power level.

However, there are drawbacks:

• the input impedance of this stage is around 20 ohms at 3.6MHz. Increased current will see this fall further
• The efficiency is low with the transistor dissipating 3W's.
  
• at 20MHz the output had dropped to less than 100mW's
•  a unit price of $1.50 is approaching the lowest cost RF mosfets I can buy. 

So it's not the solution I had hoped for.

Nor have I found a suitable general purpose transistor that can work in a 1 Watt Class A HF amplifier. It's time to move on to an alternative approach.

 

Next Steps:

Investigate if parallel transistors will meet the 1 Watt target. 

Investigate if a 2SK3078A or 2SK3756 can be made to work at HF.

Consider a single ended Class AB final stage in the amplifier.

73's

Richard