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

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Member level 1
Aug 16, 2012
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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.

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.

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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