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

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

Sourcing Parts - How times have changed

No - I'm not getting a kick back from Mouser though a voucher would be welcome.

Well it's certainly a turbulent supply chain at present. I was going to make a throw away comment within my conclusion to 4:1 transformers about sourcing ferrites. However, I realised that since the supply disruption began my perception of where to source parts has changed greatly. If I thought 2 years ago that Mouser would be my preferred source I'd have checked myself into the mental hospital. But for good reasons Mouser presently does justify having a look.

The supply chain issues also impact my plans. In the past I have held PCB's and some parts for those wanting them. But freight to me, and then to you, meant I was moving towards using a PCB assembler and having them send direct to you pcb's populated with surface mount parts. But they can't fit parts that are out of stock, and I can't continually have to make substitutions to overcome this. It is awkward all round.

So let's look at some outlets in the order in which I once used them.

Aliexpress - Avoid

Many reasons: 

1. Fakes. Fakes. Fakes.
   
2. You can't find what you want without wading through pages of rubbish.  I don't know how a search for "ferrite" returns results like diodes, transistors and electrolytic capacitors. 
3. In general it is now expensive
4. No consistency from one order to the next.
5. Chinese sellers rarely meet specifications
6. It was posted so you must have got it scam.
   
7. Disputes rarely amount to a refund.
   

I recommend ignoring Aliexpress at this time. Occasionally I buy a plastic project case or hand tool but that's about it. At the very least they really need to fix their website search engine before  it becomes worthwhile again.

LCSC

Still a reputable source for jellybean parts like chip capacitors and resistors and low cost semiconductors. However, move away from generic and prices have become too expensive. eg the STM8S003 purchased for $0.30 two years ago at one stage was quoted as $3, though the price is easing now. But they rarely have them in stock. And the filters when searching for parts, while adequate, is not as useful as Mouser, Digikey etc.

Mouser

While prices at Mouser have risen, the increase is not as great as say LCSC. Yes, Mouser started from a higher base but today I find:

• many parts outside jellybean status are available at Mouser but not LCSC,
• Mouser pricing is competitive
• Mouser holds many more stock items. eg I couldn't find any ferrite toroids at LCSC but Mouser has  a large range. 
  

 

Arrow, Digikey etc.

I still look at other tier 1 suppliers but I haven't found a compelling reason to choose them over Mouser. Arrow would be my least preferred alternative because their filter function when searching is cumbersome. 

It might simply come down to freight costs where you live.

Freight

This is perhaps the game changer. If you can find it on Aliexpress the cost with freight is too high. The free post bargain we used to enjoy has gone. 

LCSC freight costs have also increased. Must be a great time to be in logistics.

Occasionally you find "freight free" items at Mouser and this really does push Mouser into the lead at this time. Get the order value high enough and freight free becomes a possibility. With prudent purchasing I have not paid much, if anything, for freight for my last few orders. 

Conclusion:

Lead times are a big issue for items out of stock for all outlets. I run a cart at both LCSC and Mouser. Mouser are surprisingly competitive now and have a much wider selection outside the jelly bean range. Freight costs tend to see parts in my LCSC cart getting dropped in favour of sourcing from Mouser. The overall result is that today the ratio of Mouser to LCSC deliveries here is about 2 to 1 in favour of Mouser. That's a large change from  perhaps 3 to 1 in favour of LCSC a few years ago.

Happy to hear where your preferred source is. I'm always looking at alternatives.

 

73's

Richard

General Purpose Transistors in RF Amplifiers - Ouput Transistor Testing

After having my eyes opened by testing various buffer transistors I began substituting output transistors. Before testing currently available transistors I thought it useful to compare 6 alternatives in my junk box. These included fake 2SC2166 and 2SC1173 transistors, some genuine recovered CB finals, and a 30 year old BD139. All surprised me by appearing to work quite well. However, I wasn't driving them to get 1 Watt at this point.

Some of the testing results surprised me so I decided to give LTSpice a go to see if I could learn why I was surprised. That was a few days in my life I wont get back. However, using LTSpice made me accept that my spreadsheets may be in danger of becoming obsolete. The biggest challenge is being restricted to transistors for which a suitable model exists. But if you can find a spice model, LTSpice allows you to quickly rule out a device once everything is set up.

I have tested a number of devices now and I could not find a device that worked to my satisfaction. Issues noted were:

• the output dropping as the temperature rose, sometimes by 20dB!,
• distortion that required large increases in standing currents, and
  
• a roll-off in gain that ruled out use beyond 7MHz

I found that driving 50 ohms from a 13.8V appears to be pushing the envelope. If I increased the supply voltage, or changed bias resistors to increase the standing current, I could get 1Watt out. This came at the expense of the transistor dissipating 3 watts.

Conclusion:

From my measurements the KSC3503 is worth trying. If you settle for less than 1 watt you will be happy. At 7MHz and below the TTC004B is also worth considering.

However, from results to date, a working output transistor recovered from the junk box is still better than trying to buy a general purpose transistor for RF output stage applications.

Next Steps:

Investigate if parallel transistors will meet the 1 Watt target. (Update - yes they did and very nicely. See posts tagged "Parallel" for details.)

73's

Richard

General Purpose Transistors in RF Amplifiers - Buffer Test Results

Please see the previous posts to understand I am trying to replace obsolete transistors in my standard Class A amplifier chain with something readily available today. Especially for the output stage.

Having settled on a driver transistor I thought that replacing the 2N2219 with a generic buffer transistor was going to be without issue. However, this assumption was wrong and the final stage testing has been delayed.

The 2N2219, used as the buffer, has a claimed transition frequency of at least 250MHz. I've spent the last 40 years playing with small signal NPN transistors and regularly noticed in the datasheets Ft of 300MHz or more. So I assumed that pretty much any transistor I dropped into the buffer stage was going to work.

It wasn't that simple. I started with something marked G1. This was ~1.2dB worse that the 2N2219. Hmmn. 

Grabbed something marked 1GM. This was ~1.5dB better than a 2N2219. Hang on. If this was a MMBTA05LT1, a Motorola NPN part marked 1GM, it should have been comparable with a claimed Ft of 330MHZ at 25mA.

An element of confusion was setting in. I decided to put the junk box recovered parts away and grab some nice new and fresh from packet NPN transistors. 

The first was something labelled a BC549 from a long forgotten Aliexpress vendor. I agree, this could still have been a junk box part. It was marked G1 and was better than the first "G1" I tried and the 2N2219 itself. Rolloff was improved by 1dB over the 2N2219.

After trying many such parts I tried a 2SC3356R, Ft of 7GHz. The rolloff at 20MHZ was now 2dB better than the 2N2219 version. A similar outcome was achieved when I recycled something marked R22 from a Philips FM900 VCO board. Some claim this marking is a 2SC3356. I'm not sure this was a 2SC3356 however it was a high Ft device.

Clearly, the higher the Ft the better. Which I hadn't expected given the feedback networks being used.

I crunched up a spreadsheet with a Hybrid Pi model of a Class A amplifier without collector base feedback to see what difference Ft made. With all assumptions unchanged, shifting Ft from 100Mhz to 330MHz results in a lot more gain at 20MHz. Collector base feedback reduces this gain, but the numbers supported selecting a buffer transistor with a higher Ft.

Later I fired up RFSim99 and loaded up the S parameters for a Renasas 2SC3356. Fiddling with the emitter and feedback resistors suggested a minor variation in values would be better to improve the match to 50 ohms. I will use those values going forward. 

There are many 2SC3356 transistors on LCSC.  Since Renasas has discontinued this part I figured it would be prudent to include some in my next order so I opted for this 2SC3356 since it had curves that resembled those in the Renasas datasheet.

Conclusion:

Recycle, or spend a few extra cents if you're buying a transistor, and get something for the buffer with an Ft greater than 1GHz. This may be counter to the conventional wisdom that HF circuits do not need such a high Ft. But the difference could be as much as 3dB per stage. So for low level amplifiers with feedback to prevent oscillation I argue it makes sense since it could save an entire stage.

Now I can test a few final transistors.

73's

Richard

General Purpose Transistors in RF Amplifiers - Driver Test Results

Having reworded the amplifier chain to use 2:1 transformers instead of 22uH collector chokes I have commenced testing. My approach was:

1. Build the amplifier with transistors that were known to work and measure
2. Transfer the output and buffer transistors to a know board and start substituting SOT-89 packaged driver transistors
   
3. With a chosen driver transistors transfer everything to another new board and after confirming a SOT23 buffer would work start substituting final transistors.

Being a known good design the amplifier worked with the original transistors. I adjusted the output for 10v pp at 1MHz. The roll-off at 20MHz was 2.9dB. Looking at the inter-stage levels I concluded that most of the roll-off was split equally between the buffer and the driver. The output stage was almost flat (-0.2dB). 

After testing 10 different SOT89 driver transistors I found the average of the roll-off at 20MHz for the entire amplifier was now -3.7dB, a deterioration of 0.8dB. I explain that by the difference in Ft: the original driver, a 2N4427, has a transition frequency of at least 500MHz, compared with the transistors tested having a typical transition frequency of 100MHz. 

While I only tested one device from each packet, the best transistor was the BCX56 which was just 0.4dB worse than the 2N4427. I'm comfortable using that transistor because it also appears to have the highest Ft at the collector current being used (~180MHz at 42mA from the datasheet chart).

The worst transistor had a claimed 210MHz typical Ft at 500mA. But clearly the Hottech 2SC4672 does not perform very well at a 42mA collector current. That's probably why there is no chart! 

And be wary of just buying any BCX56. I checked a few datasheets and in this application I would only use a BCX56 where there was a chart of Ft versus current, or a stated Ft in the vicinity of 50mA. Of course, it you buy from Aliexpress then you have entered a lottery.

Conclusion:

In a class A amplifier around 30mW output you can use a 10cent transistor at HF. This BCX56 is now my standard and should work very well in my 80m Weaver transceiver being developed. This removes the need for a 2N4427 for low-mid HF applications.

Now to test some higher power alternatives.

73's

Richard

 

IF Crystal Filters - NDK Type numbering decoded?

I wanted to know more about the crystal fitlers used in the Tait Orca handhelds ie 45E12AA and 45E15AD. 

I could not find the specifcations for these exact filters. However, after looking at some NDK datasheets I believe these crsytal filter are as follows:

 45 - denotes the nominal center frequency

E - couldn't work the meaning out. Perhazps E means 2 poles and EE means 4 poles?

12/15 - bandwidth inkHz

A  - finalises model number

A/D at the end denotes the center frequency.

 

So, 45E12AA apears to be a 45.1MHz 2 pole crystal filter (seaching for 45E12A did not return a model datasheet, perhaps a special Tait specification?)

and 45E15AD is a 45.1MHz 2 pole crystal filter (search for 45E15A to see the model datasheet)

 

Seems strange that the center frequency is not denoted by a consistent letter.  But as expected when looking for relatively old datasheets, not everything is on the web. 

Best to keep the crystal used to mix the IF down to 455kHz with the filters so you can derive the center frequency if no data can be found.

 

73's

Richard

New part - SMA3109 MMIC

Something different....

I stumble across new parts from time to time that set a price / performance benchmark. So I thought it would be worth alerting readers to these parts. Here is the inaugural post on this subject.

The new benchmark in my shack for general purpose mmics is the SMA3109. It happens to be the cheapest mmic that LCSC stock. I had wanted to use this commercially but the initial lack of S parameters meant I passed it over in favor of another device. That decision might get revisited.

Despite the low price the SMA3109 offers outstanding performance from VHF through to 4GHz. Compare it with other low cost mmics such as the BGM1013 and you will see why I rate it so highly.

When I evaluate a MMIC I consider:

1. stability
2. input and output return loss
3. noise figure at intended frequency
4. gain
5. ease of hand soldering
   

This device is ticking all my boxes but the NF of 4dB rules out demanding low noise applications.

I suspect it is useful outside these ranges but the S parameters kindly supplied by Onsemi technical only cover 100MHz to 4GHz.

Band	Gain	S11	S22
2m	23dB	-28dB	-13dB
70cm	24dB	-24dB	-16dB
23cm	24dB	-38dB	-16dB
13cm	23dB	-15dB	-14dB
9cm	21dB	-11dB	-17dB

The input match on 23cm is outstanding and I will definitely use this device in my 23cm projects where filters are connected to the input. For driving filters I would consider at least a 3-6dB pad to provide a better source termination. Being very stable across the entire frequency range matching if needed is straightforward.

Where linearity is important I would budget to get around 0dBm out of the device.

My hunch is that the feedback within the device makes it useful below 100MHz except for the output becoming a poor match to 50 ohms.

The device appears to be very useful from 2m through to 9cm and represents terrific value for signal outputs up to the Po(1dB) level of +4dBm. Definitely worth a look at US 11cents each!

73's

Chinese Solder - An update

After months of using different solders I have updated my findings. Only 2 of the Chinese sourced solders I tested are a realistic alternative to the comparatively expensive Multicore solder. Thoughts? Opinions? Comments welcome.

73's

New Part - VHF Low Noise Amplifier MXDLN02C

So I was browsing for a high speed op amp and I came across this wrongly catalogued part at LCSC.COM. I am now testing it for a commercial project but it could have uses in ham projects.

But there are some caveats that go with a US$0.05 part:

1. The datasheet could be better
2. This is not a strong signal handling part
3. No details exist on how best NF is obtained

The MXDLN02C is meant to be used as a pre-amp for FM broadcast receivers. The datasheet suggests the noise figure is under 1.5dB and gain is above 20dB from 50MHz to 150MHz with good stability. I wrote to Maxscend as part of the commercial project requesting the S parameters in a table format. The response suggests Maxscend will never be one of my preferred suppliers.

So, in Ham spirit because this is a really cheap part, I simply hooked up the input to the nanoVNA and measured S11. Now I don't claim the nanoVNA is the last word in measurement, or that my test board was perfect. But the results I got suggest Maxscend was way off the mark.

From the Datasheet (2.85V) :

From measurement (also 50-150MHz) (3.3V):

I'm not dwelling on the possible reasons why there is such a disparity. What is important that based on my measured S parameters matching networks were easily calculated in RFSiim99. So, a few minutes of tweaking and I had a return loss of -30dB on the input at 6m and about 20dB of gain.

A similar story on the output port.

Summary:

It's ridiculously cheap.

Useful for 6m and 2m in non-demanding front-ends, still has +13dB of gain at 70cm.

This is not a MMIC so matching to 50 ohms, while easy, will be required

Like a MMIC no biasing calculations needed

Stability concerns limit HF applications

I always advocate using what is in your junkbox. But if you had to buy a device for low level amplification at 6m or 2m then this is what I would suggest.

73's