The "magic" BC109C ?

[neazoi]

Hi, I would like to share my experience with you as far as concern the BC109C transistor and I would like your comments on this.
I have already read this thread https://www.edaboard.com/threads/289839/

This transistor has worked for me many times in different small signal circuits, like small QRP transmitters/transceivers, RF mixers and preamplifiers, low voltage radio circuits, amplified crystal radios etc.

What is weird is that most of the times, when replacing another small signal transistor (I have a few ten's of the most common types that hobbyists use) with the bc109c, the circuit works much better than with the previous transistor.

I know this is kind of a very general question, not constrained in a particular circuit, but I would like to ask if you had similar experience with it.

 

[Audioguru]

when replacing another small signal transistor

Please list the original transistor part numbers.

the circuit works much better

Please describe what becomes better.

The BC109 is a very old low noise transistor. I used them in audio circuits 52 years ago. It is basically a BC108 selected for low noise. Many "common" transistors produce lots of noise.
The "C" in its part number is its current gain group which is fairly high at 420 to 800 which is a narrow range. Many "common" transistors do not have groups so their current gain can be almost anything or be very low. Maybe the circuits you used BC109C transistors in were designed for a transistor with high current gain.

 

[neazoi]

Please list the original transistor part numbers.

For example 2n2222a, bc547, 2n4401, 2n3904, mpsa18 and some others I do not remember now.

Please describe what becomes better.

As said this is general question, but I will give 1-2 examples. For example in an HF "power" oscillator, I was able to get 200mW or rf output when using the bc109c, whereas with all other transistors (even more powerful) I could only get less than 100mW
Another example was mini audio amplifier for a small speaker I once made, these transistors made a good difference in audio volume at the same bias.

 

[Audioguru]

Most of the transistors you replaced have a low and wide range of current gain, much lower than the BC109C. The MPSA18 has many spec's identical to the BC109C including low noise and a high narrow range of current gain. So in a circuit the BC109C and MPSA18 transistors conduct more current when biased the same as before and produce more voltage gain and more output power than the other transistors.

The circuit could have its biasing changed for transistors with a low current gain then it will also produce more voltage gain and more output power.