BLF245 Linear Amplifier

[Zozo]

So I've been working on an amplifier design using the "good old" BLF245 which is probably not the best choice of device with what's available today. However it has phenomenal gain, and was a very good device in it's day along with it's baby brother the "BLF244"

The main goal here was to at-least have a nice companion to go with the exciter board I put together. It may only be "QRP", but some of us like to run those levels of power.

I will probably still need to tweak some area's of the design before I order the first sample PCB, but here's the current design so far, which is subject to change. The use of mask-less "Meander" coils are perhaps a good choice to allow me to add shorting bars if needed, which can then be changed for the final run of PCB's.

Some of you with a keen eye will notice the amplifier contains many parts which have been "cherry picked" from other designs.

As before this will be an "Open Source" project with all Project / Gerber files available for download.

Any design suggestions welcome.

Z30BL.JPG

[Zozo]

Bottom side of PCB

Z30BLB.JPG

[MiXiN]

Looks great mate, especially if the BLF245 can be sourced cheaply.

The most inexpensive I've found is circa £25, but that's not too bad for 30W.

I'm a massive fan of your recent projects, and I'm looking forward to seeing the finished product.

[Zozo]

Thank you MiXiN, that's very nice of you to say regarding the projects. Speaking of which, this was something I started a few weeks back that I've been keen to get finalized. I have to get one project fully complete before I can really get committed into another "I think it's an OCD Issue".

I'm not sure how much interest this design maybe to people, but It's a possible option for anyone who may have a odd BLF245 kicking around doing nothing.

[radium98]

great thank you .

[zulu53]

Nice work PCB Zozo. I look forward to seeing BLF245 being tested.

[sinus trouble]

Nice neat layout ZoZo

Ive never used the BLF245 but i have used the BLF244 many times and its a great little device!

You could add SWR protection but i really dont think its necessary! Stick it on a chunky heatsink and it will be virtually bullet proof!

[teckniqs]

I've had about 30w out of a BLF244 in the past so this should probably be good for up to about 50w with a little bit of tweaking (opening out coils etc)...

[Zozo]

Thanks everyone for the comment's. I think I'm nearly there with the design. I did consider adding S.W.R. protection, but I think as Sinus mentioned it should be ok will a nice chunky heatsink. Going from memory, I'm sure the BLF245 is capable of withstanding a 30:1 continues mismatch without any permanent degradation of the device.

Hopefully the fuse will blow if there's any serious mismatch in the feeder or antenna, as the source to drain current will likely increase. I will have to monitor the current in both perfect and less than perfect conditions, and then select the appropriate fuse value. The point at-which the fuse does fail, it will at-least kill all power to the exciter board too.

I'm sure techniqs is absolutely correct that the BLF245 can possibly achieve more power than what I quoted, but I thought it best to underestimate it's performance, rather than be a disappointing overestimation.

I will be ordering the sample PCBs at the weekend, so sometime around the week after I will have something more to share with you all. Plus that gives me a few more day's to spot any error's I may have overlooked.

[zulu53]

It is nice to hear that some Necks have great experience with BLF244, a younger brother of BLF245. I guess that theirs amplifiers were based on the "old wire wound coil" method.

Given that (i) some of us may have some low to medium power RF Mosfet transistors laying around, (ii) application notes from the manufacturers are very hard to find for FM band and (iii) PCB facilities are not easily available for everyone, I would appreciate if Sinus trouble,Teckniqs and others could please share theirs old fashion circuit designs.

[Zozo]

[UPDATE]

The samples finally arrived this morning, I opted for the more cheaper and slower delivery service. At first glance they look to be all ok, I'll get round to population it with components soon.

More to follow soon.

20210603_103255.jpg

[sinus trouble]

I would appreciate if Sinus trouble,Teckniqs and others could please share theirs old fashion circuit designs.

In my experience? The 175Mhz test circuit worked fine! Increasing the LC component values slightly gives a better response.

Ofcourse you will need to add a LPF to be safe.

I will see if i have some pics? It wont help much with design? But is a basic visual aid!

[sinus trouble]

Excellent work ZoZo!!

It will be great to see how it performs!

I was thinking of designing a 'Generic' PA PCB based on the TO-220 package FETs? The main goal would be to able to switch out parts covering the B2 and B1 spectrum?

The power is not really a concern, 10 to 20 watt would be more than enough!

[sinus trouble]

Just another little Mod you could do?

You could substitute the 7812 for a 7824 and use a 24v fan? Which in turn may reduce dissipation?

[teckniqs]

Just another little Mod you could do?

You could substitute the 7812 for a 7824 and use a 24v fan? Which in turn may reduce dissipation?

That's a good idea but with 24v supply into 24v regulator the voltage will (as you know) be less than 24v unless 7824 has around 24+2v going in.

....So if main supply voltage is at least 26v that should be fine, it's a 28v device anyway so will be OK.

[MiXiN]

Looking forward to watching the progress, ZoZo.

Those boards look great by the way!

[mpx]

These sound like they might be good for a battery/solar installation. With an efficiency estimate of 50%, current draw should be about 2.5A at 24V

[sinus trouble]

Just another little Mod you could do?

You could substitute the 7812 for a 7824 and use a 24v fan? Which in turn may reduce dissipation?

That's a good idea but with 24v supply into 24v regulator the voltage will (as you know) be less than 24v unless 7824 has around 24+2v going in.

....So if main supply voltage is at least 26v that should be fine, it's a 28v device anyway so will be OK.

Yeh True! The voltage margin is pretty tight and i can see why some would prefer a 24V supply.

[MiXiN]

Zozo,

Did you get around to building one, and if so, how did it perform?

[Zozo]

Thank you to everyone that has replied, and keeping this thread active with feedback and comments. I've had sometime away whilst dealing with other things that have been keeping me very busy.

I never finished populating the board with all of it's components, but I did make a start before I was pulled away from this project.

As you can see from the picture below, there's still a few vital components to be fitted.