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Question on NPN Logic Inverter From Textbook

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Wimberleytech:
Born in the lab

Wimberleytech:

--- Quote from: eev_carl on September 30, 2019, 04:19:27 pm ---
--- Quote from: Zero999 on September 30, 2019, 04:05:55 pm ---That's better. I was just about to post the same schematic, but with R2 = 8k2, but you beat me to it. I suggest you try it.


--- End quote ---

Is there a rationale for selecting 8.2k?  Also, is this R a current limiting resistor or DC coupling (or both)?

--- End quote ---

Power, speed...  The resistor is to limit current.  All resistors couple dc.

edavid:

--- Quote from: eev_carl on September 30, 2019, 03:54:37 pm ---Is the general consensus that the book is wrong?  High current / low current, the book still seems about an order of magnitude too high (180mV versus 18mV)?

--- End quote ---

I wouldn't say that the book is wrong.  They are just describing a different type of transistor, probably something more like a 2N2369.  You can tell this from the high VBE.

Also, your base current is still unrealistically high.  Try a forced beta of around 10 and see what you get.

Wimberleytech:

--- Quote from: edavid on September 30, 2019, 04:26:54 pm ---
--- Quote from: eev_carl on September 30, 2019, 03:54:37 pm ---Is the general consensus that the book is wrong?  High current / low current, the book still seems about an order of magnitude too high (180mV versus 18mV)?

--- End quote ---

I wouldn't say that the book is wrong.  They are just describing a different type of transistor, probably something more like a 2N2369.  You can tell this from the high VBE.

Also, your base current is still unrealistically high.  Try a forced beta of around 10 and see what you get.

--- End quote ---

At best, the book is incomplete then.  Given that it's goal is to teach, leaving the student with such a quandary without explanation is sub optimal.  But, books are never perfect. 

fourfathom:

--- Quote from: Wimberleytech on September 30, 2019, 04:35:29 pm ---At best, the book is incomplete then.  Given that it's goal is to teach, leaving the student with such a quandary without explanation is sub optimal.  But, books are never perfect.
--- End quote ---

That book is quite complete (I found it online as a pdf).  Take a look at Chapter 5 (Bipolar Junction Transistors) where the behavior in saturation is extensively discussed.  That "inverter" example is just that, en example of an inverter, and not meant to by itself be a treatise on transistor characteristics.

So from this the OP learns that not all transistors are created equal.  Not a bad lesson!

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