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Transistor selection question

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boylesg

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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?
 

boylesg said:
The only difference form the data sheet is the voltage rating....
Click to expand...
Look again. Noise figure is also specified in most datasheets. Some have graphs of NF vs Ic as well.
 

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

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).
 

godfreyl said:
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).
Click to expand...

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