Re: gEDA-user: BC547 vs. 2N3904

[Wojciech Kazubski <wk@xxxxxxxxxxxxx>]

> Does driving the transistor into cutoff but not saturation also count
> as saturation switching?
>
> CL<
>
No, saturation is when collector voltage drops below base level and the 
collector junction becomes forward biased. Charge stored in this junction is 
the cause of swithing delays. Swiching transistors have special doping that 
reduces carrier lifetime and speeds up the transistor.
Transistors can be operated into cutoff on high frequency, this happens for 
example in B or C class RF amplifiers. Their parameters (hfe, fT) degrade a 
bit when the transistor approaches cutoff, so C class amplifiers cannot be 
operated on the frequences as high as in linear mode (class A) with higher DC 
current.
But I think that this may be a problem above VHF band.

Wojciech Kazubski


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