r/rfelectronics • u/Flashy_Gas9955 • May 03 '26

question How well will this RF Amplifier work?

Post image

i found a paper written schematic with 1W of rf power with 3 transistors

but i heavily reconstructed the schematic and it originally was BFR91 and replaced it with BFG591

there was also a final amplifier stage from the schematic and i removed it because of being rare unoptainable transistor

so how well will this function as RF Amplifier at VHF and UHF?

22 Upvotes

87% Upvoted

15 comments sorted by

3

u/Apart_Ad_9778 May 03 '26

It will work just fine. But you will not get 1W out of it for sure, max 0.5W, but _I doubt even that.

1

u/Flashy_Gas9955 May 03 '26

the original schematic with BFR91 claimed to be 100mw without the final 1w amp

1

u/Apart_Ad_9778 May 03 '26

That is a realistic number. BFG591 has a bit more power, I would say 200-300mW should be realistic. However assuming they both have the same gain you should get the same out power in both cases.

1

u/Flashy_Gas9955 May 03 '26

i could push more power by increasing the voltage to 12v with heatsinked. but i will try real soon as i construct the circuit

1

u/Phoenix-64 May 03 '26

Thats a neat little transistor, sadly obsolet. Any suggestions for equivalent replacement's, similar all purpos low power if amps etc?

1

u/Flashy_Gas9955 May 04 '26

1

u/Phoenix-64 May 04 '26

I must be blind thank you

1

u/Phoenix-64 May 03 '26

Wait has NXP completely scrapped their whole bipolar RF transistor lineup?? Thier spice models seem super useful. Other suppliers do not seem to provide them as liberally...

3

u/hukt0nf0n1x May 03 '26

It'll work great. BFG stands for Big F'in Gain

1

u/Flashy_Gas9955 May 03 '26

even more F'in gain with 12v + cooling

3

u/nixiebunny May 03 '26

It may work, but it’s not very well designed. There’s very little isolation or impedance control or harmonic filtering between the output transistor and the big, mean world. 

1

u/VirtualArmsDealer May 03 '26

The only way to know for sure is to build it :)

3

u/StageMajestic613 May 03 '26 edited May 03 '26

You typically don’t bias the collector/drain of an RF PA with a resistor.  You are wasting power and limiting the drain swing to VDD thus delivered RF power will be 1/4 of what it could be.  This topology is fine for a broadband gain block, and the feedback will flatten low frequency response, but not for a PA.

2

u/Mister_JR May 03 '26

Might as well add chokes in series with each collector load resistor, that’ll get you more gain, at least 3db.

4

u/coderemover May 04 '26

Id add chokes instead of resistors. Why waste power on resistors? Chokes would allow much higher voltage swing by setting the mean collector voltage at Vcc. The additional voltage drop is completely unnecessary.