Transistor Hard Saturation - pg224 - Malvino

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'A designer who wants a transistor to operate in the saturation region under  
all conditions often selects a base resistance that produces a current gain  
of 10. '

http://pix.toile-libre.org/?img14%93526531.png
http://pix.toile-libre.org/upload/original/1493526562.png

I don't understand what he's doing with the math (it's not complicated math  
but he's trying to illustrate an idea that I don't get).  

1. betaDC=Ic/Ib but we only have control over Ib so.. how has he computed  
Ic=2mA?
2. Then he does Ib = 2mA/50 okay but then again the normal way would be to  
work out Ib first and multiply with the given BetaDC to work out Ic..

If he wants IcSat directly then that's at Vce=0 so you do Vcc/Rc = IcSat
What's he doing in Link-2 (Link-1 is his reference ckt)

Re: Transistor Hard Saturation - pg224 - Malvino
On 4/30/2017 12:35 AM, veek wrote:
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He isn't deriving anything, he's just calculating some values.  It is  
not a mathematical proof.


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He isn't calculating it, he picked it as an example by setting the Vcc  
and Rc.  20 V / 10 kohms = 2 mA.

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Again, he has a max Ic already.  Now he wants to pick an Ib that will  
saturate the transistor.  Since the beta is 50, use a gain of 10 which  
will use a lot more base current than needed and put the transistor into  
saturation.


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I don't see any links.

He picks a value for Ic and sets Rc and Vcc to get that.  Now to set the  
value of Rb he knows the beta is 50, so using a value of 10 is assured  
to provide much more base current than needed and puts the transistor  
into saturation.   So Ib = 2 mA / 10 = 0.2 mA which with Vbb = 10 V  
makes Rb = 100 kohms.  Er, um, wait, that's not right!  Ib of 0.2 mA  
with Vbb of 10 volts would be 50 kohms (ignoring the 0.7 volt Vbe drop).

Don't know why he is showing 100 kohms on the schematic, but do you see  
the logic here?

--  

Rick C

Re: Transistor Hard Saturation - pg224 - Malvino
rickman wrote:

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Thanks yeah I got it - I did it the hard way as well - calculated Ib for  
beta10% and beta50%.  

(He could have just said it out - instead of showing me the math - he's done  
these calculations before so the math is totally unimportant in that  
context)

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