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Simple question about saturated transistor circuit (from OpAmps for everyone)

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frilance

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Hello everyone!

I was reading OpAmps for everyone (available here) and found something strange on page 37.

Does anyone know if the I_L they use is the current flowing through a possible load resistor (i.e. 50 ohms) connected from V-out / V_C to ground? For me what it does not make sense is that they draw IC being the current flowing through R_C and then they use a different notation in the expressions, like I_C is the sum of the current through R_C and the current through the load resistor I_L.

They also refer to Equation 2-27 when they talk about the off case, therefore V_CE is not 0 but at least 10 V.

Did I get it correctly? Doesn't it look a bit incoherent?

Thanks in advance.
 

SunnySkyguy

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I agree, totally. Besides in 2-27 if transitor is off Ib=0 and Ic=0 but I(Rc) is 1mA.

Also when transistor is saturated, β does not apply as it is in the nonlinear mode Ic/Ib is typically 10% of β but often standardized as Vce(sat) @ Ic/Ib = 10 for old transistors
and only 20 to 50 for newer low Vce(sat) types

So if Rc is 2K, Rb must be <= 18k if both go to 12V ( not 35.6k 98% accurate??)

Even the load is poorly defined. Is IL= 1mA when off and (-)10mA when on??
then collector draws <6mA from 2k and 10mA external for a total of 16mA

then if Ic/Ib =10 , Ib=1.6mA or 11.3V/1.6mA = 7k not 35.6k

I probably misunderstood the question but the use of Ic is certainly confusing.
 

Audioguru

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There is no page 37. There are many sections. Maybe you have an old computer operating system that counts the pages but my Windows 8 does not.
 

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