How People Make Circuits without Printed Circuit Boards - Video from https://qrp-popcorn.blogspot.com/

 
Addendum added to the text January 13, 2025 - scroll to see -

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Addendum January 13, 2025

LOL -- a reader emailed and wants more signal power -- and asked whether they could just swap in a pair of medium power BJTs like the BD139-140 to get more output power. Sadly no. I have a higher power version of this amp in a receiver called Regen 5 plus one other on this Blog. However, there is another way to get more power without greatly increasing the parts count:

Above — My new Popcorn PA version that goes to 700 mW clean signal power without adding a ton more parts. The finals = a TIP 122 &127 pair. 2 more diodes get added to the bias stack to forward bias the 2 pairs of emitter followers. To avoid a big voltage drop, I moved the 22 Ω resistor outside the TIP power followers on the DC positive rail since these BJTs draw much more current than the basic version of this audio PA. Further, a 470 µF AF bypass capacitor gets added to filter the output pair on the positive DC rail.

The TIP 122-127 are low voltage complementary power Darlington transistors in a standard TO-220 package giving a current gain of ~ 1000. They only cost ~ 60 cents per transistor even in Canada with our nearly worthless Canadian dollar.  A BD139-140 transistor usually cost 25-30 cents more.

Above — Schematic and pin out of the TIP 122-127.  

Above —DSO screen capture at about the maximum clean signal power ~ 700 mW.

Above — FFT screen capture with the AF amp driven to 700mW. The harmonic distortion is not as low as the original audio PA shown in the video. However, this is the compromise of using the TIP Darlington transistors + easy-peasy diode biasing scheme with 4 diodes. The better way to bias the power followers is via a transistor like I did in the audio PA Regen 5 back in 2015.  Regardless, the TIP pair are a compromise compared to 2 separate BJTs pairs with optimum circuit design.

Still, to, the harmonic + crossover distortion will still be on par; or better than most of the audio amps you see in many lower power consumer electronics -- or a typical home brew Ham /SWL radio receiver.

Thanks for the email. If you have a question,or request,  please email. 

My readers are the best!   Popcorn Todd