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15th February 2004, 12:30 PM  #1  
diyAudio Member

What graph to look at?
New thread, as this became increasingly Off Topic for the thread What makes a mosfet linear?
Quote:
Always look at the SOA, as you don't want to destroy the device. Provided, that this is not at the output stage (where you have to look at transient thermal resistance graph and into the catalog of your preferred heatsink supplier) and not a low level (phono, mic) stage (where you have to look at the noise figures graph), there are three essential graphs: Transit frequency as a function of collector current: Stay to the left of the peak. DC current gain as a function of collector current: Stay to the left of the point where the hFE is 3dB below maximum. Apply (if provided) the graph best matching your expected junction temperature and Vce Collecter emitter saturaton voltage as a function of collector current: Stay below the Ic, where VCEsat is about 200..300% the minimal VCEsat (If there is no minimum, i.e. VCEsat drops ever more for lower Ic, a compareable point is 50% of the Ic where there is a "kink" in the double log plot). These are only my cookbook recipes. And they only apply for DIY, where you haven't to worry so much about decive cost  as these rules will often imply using a more expensive BJT with a much higher "rated" Ic, if you are looking for a specific collector current. Regards, Peter Jacobi
__________________
 YMMV 

15th February 2004, 02:13 PM  #2 
diyAudio Moderator Emeritus

Hi,
Thanks, Pjacobi for the advice. Since I'm only an amateur I cannot digest all instantly. I will try to understand the whole answer. I really wonder how you know so much about so many application of transistors outside audio. Since my introduction with transistor only in audio field, it opens my eyes about transistors in other fields. 
18th February 2004, 02:42 AM  #3  
diyAudio Moderator Emeritus

This is the first examination. Mentor=Pjacobi ; student=Lumanauw ; Case=MPSA06
Quote:
Quote:


18th February 2004, 02:56 AM  #4  
diyAudio Moderator Emeritus

Quote:
Also I cannot find any junction temperature graph. Quote:
Pjacobi, this 3 graph semms to limit the current. Since audio is about AC current, I assume this is for Bias/ Standing current. Since we have know the max current, is there any possible way to determine 1 operating point that is the best? How about determining operating voltage (Vce)? And is this method also works for mosfets? Or there is other method for mosfets? 

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