Transistor selection question

[boylesg]

Given the series BC547, BC549 and BC548....


The only difference form the data sheet is the voltage rating so given an input voltage a 12V what is the advantage, if any, of choosing BC549 over BC547 concerning the low or high noise?

 

[godfreyl]

The only difference form the data sheet is the voltage rating....

Look again. Noise figure is also specified in most datasheets. Some have graphs of NF vs Ic as well.

 

[boylesg]

Look again. Noise figure is also specified in most datasheets. Some have graphs of NF vs Ic as well.

Yeah I did notice the 'low noise' thing associated with BC549. But can you tell me why you would opt for a low noise transistor as opposed to one with a higher voltage rating assuming that the in input voltage is below the threshold of all 3 transistors - BC447, BC548m and BC549.

It is not particularly important for anything I am doing at present but I would like to fill in gaps in my knowledge.

 

[godfreyl]

When dealing with very weak signals, e.g. in a microphone preamp, using a low noise transistor helps to minimize the noise added to the signal. Noise in audio signals sounds like hiss - it's what you hear from an FM radio when it's tuned between stations (or to a very weak station).

 

[boylesg]

When dealing with very weak signals, e.g. in a microphone preamp, using a low noise transistor helps to minimize the noise added to the signal. Noise in audio signals sounds like hiss - it's what you hear from an FM radio when it's tuned between stations (or to a very weak station).

OK makes sense....next question......inside the transistor what causes the noise or lack of noise.......briefly?

 

[godfreyl]

No idea. Try Google?