4:1 (200:50 ohm) Transformers - Another Suggested Core

I had some more cores arrive, one of which appears to be as useful as those mentioned in this blog here.  This one is a binocular core from LCSC, part C498903. It is larger than the binocular core in the earlier blog.

Instead of showing you a Smith Chat of the swept response I present a plot of S11 versus frequency. This should make it easier to discern how good the transformer is. Six turns is good for HF, while 7 or 8 turns improves things at the bottom end of HF. I haven't measured a 5 turn transformer but it might be better for 6m.

6t bifilar

7t Bifilar

8t Bifilar

 

73's

Richard

Using 1210 Resistors as Thermal Bridges in 1W Transmitter

I mentioned in this post the concept of using surface mount resistors as thermal bridges. I've had a chance to explore this now and after various tests I'm completely satisfied this is a suitable way to heat-sink SOT-89 surface mount transistors in 1W HF amplifiers.

Balancing cost versus ease of installation  I settled on 4 pieces of 1MΩ 1210 resistors for each transistor. A 1MΩ resistor needs no further modification to isolate the collector voltage from ground. On an extended test the two parallel transistors had a  case temperature of around 130 degrees while dissipating a total of 3.4W and delivering 1W of RF to the dummy load.

 

I hope you find this tip useful.

73's

Richard

ps Please excuse the soldering. I'm using up some solder I would not recommend and and it's difficult to get a good joint without using too much.  That's compounded by it being too thick. 

My preferred solder is presently this one. 

My preferred flux is presently this one which I apply with a nylon type small paintbrush trimmed short to make the bristles a bit stiffer.

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

Seems I can't help myself but try different transistors. In the course of testing my resistor as a thermal bridge concept I needed to populate a test board for measurements. After that I finished populating the board and tested numerous transistors with it. 

Initially I had some problems, both a soldering fault and poor biasing due to an error in my spreadsheet. But I learned a lot about debugging this amplifier in the process. I have now tested a large assortment of transistors, both SMD and through hole. Most worked very well at 40m and 80m, some well beyond that. And I never encountered any instabilities.

Even on my narrow board I never let the smoke out of any transistor despite relentlessly abusing them. The only casualty was a 2.2ohm decoupling resistor burning out.

1 watt is about the limit with a 13.8 volt supply. The output signal is 20Vpp. After decoupling and loss in the collector choke you have about 13.2V at the collector. You need a bit over 1V on the emitter to set the current with a 5Ω emitter resistor. So you are left with a margin of say 2V to deal with saturation considerations. Even with everything going well the best you might achieve is an extra 1dB of output. Probably not worth the grief chasing it.

Too much standing current and the 2V margin is reduced. Too little standing current and distortion is present. What I do is calculate the bias component values based on a standing current of say 220mA since the bare minimum needed is 200mA for 1W of output into 50Ω. The base resistor to ground is then rounded up to a convenient value. Once built, check the emitter voltage. If higher than desired put a suitable value resistor in parallel with the base resistor to ground to reduce the emitter voltage to the desired value.

I then examine the emitter voltage (Ve) with my CRO. It must be greater than zero at all times when delivering the required 1W for a Class A amplifier. The exact value needed to avoid distortion varies with the transistor used. The minimum Ve is a function of standing current so if you are bottoming out then a small adjustment to the base resistor to ground is needed.

Parallel output transistor at this power level at HF are easy to heatsink using resistors as a thermal bridge and cheap. I recommend this approach over expensive RF devices in this application.

73's

Richard

80m Transceiver - Weaver Method of SSB : Preliminary

The building blocks are coming together. 

• Low pass transmit filter (a)
  
• Antenna changeover relay (a)
  
• Receiver front end filter (a)
  
• receiver detection, AF filter, AGC and PA  (b)
  
• Microphone amplifier and SSB Modulator (c)
  
• low level transmit amplifier (d)
  
• Power Amplifier (e)
  
• Synthesizer with LCD display - built and to be tested (f)
  

Analysis based on the inductors I had to hand suggested my receiver band pass filter may require changing. To facilitate that I decided to incorporate the three building blocks marked (a) onto one PCB. I will make the filters on a daughter board then fix that to the base board.

Rough layout and progress can be seen below.

73's

Richard

Possible Alternative to Thermal Bridge?

It was ironic that investigating the use of surface mount transistors for QRP amps (see previous posts using the QRP tag) required a relatively expensive thermal bridge to heatsink the SOT89 package. These thermal bridges are made using aluminium nitrite. 

I noticed by accident that some high power surface mount resistors use aluminium nitrite as a substrate. Unfortunately, these are no cheaper than proper thermal bridges.

However, what about regular high power surface mount resistors? Reading various papers I learned they use a ceramic substrate with a thermal conductivity about 1/6th that of Aluminium Nitride. I hastily scratched a break in the resistance film on a few such resistors and constructed a test jig. Sure enough, I had a thermal bridge.

The initial result is very encouraging using two 2512 resistors, open circuit, on each transistor in the parallel 1W amplifier. I cranked up the supply until each transistor was dissipating 2W. The transistor temperature was stable at 125degrees Celsius while delivering 1W continuously. Some more suitable resistors are on the way and a re-design to the pcb means I will be able to properly test this soon.

73's

Richard

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

After the success I wrote about last time I began poking around. I noticed that the driver, a BCX56, was contributing to the distortion at high output levels. I populated a second board, this time with what was an unknown SOT89 transistor taken from a FM92 sub-circuit. After some investigation it appears this transistor is a BFQ19. Mouser still stock them at around A$1 (US$0.75). However I have no more of them so I will experiment with some alternatives now that the proof of concept is working so well.

I also tried a different pair of output transistors (2SD1664R) and a different binocular core which I will write about later when I have a moment.

I doubt the binocular core was the reason for the different outcome. I could easily achieve 1 watt from 3.6MHz to 21Mhz. At the upper end I had to increase the drive but no distortion crept in. This new transistor costs just A$0.05 (US$0.04) a piece. That price is still a fraction of the cost of the thermal bridge but everything worked so well that this has become my new standard output transistor in this application.

A wider board with hopefully better thermal proprieties is planned. 

73's

Richard