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Question on NPN Logic Inverter From Textbook

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Wimberleytech:

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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.


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I don't want to start a flame war, but I downloaded the book and within just a few minutes I found more incompleteness (arguably just wrong)--see if you can tell me what is wrong with Fig. 2.17.  My point remains, no book can be complete, and the instance noted in this thread could have been more complete.

Zero999:

--- 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.


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

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I chose it because it would make IB ≈ 1/10IC and it's the same value as RC so it would cut down on the BoM cost, if you wanted to build it, as ordering multiples of the same value resistor, is cheaper than fewer of many different values.

For the purposes of simulation, how about changing V1 for a current source and sweeping its value from say 1pA to 10μA?


eev_carl:
This accompanies the example too.  I knew about Beta and was using different LT Spice models before I posted to see if I could back into the answer.

My confusion was that this graph has Vbe ranging from 0-5 without the discussion about Ib.  It seemed like a concrete circuit (ready to be built) to me with specific values for the collector resistor, Vbe, and Vcc.

edavid:
That is a confusing graph, since obviously you can't have a VBE of 5V.

Instead of VBE, the X axis should be the input voltage to the base resistor (that was left out of the schematic?!?).

It does make you wonder about the quality of the book.

Zero999:
Please do some image processing, rather than posting huge files. Here it is squashed, with the shadow removed and improved contrast.

Yes, it doesn't make much sense. If VBE really was 5V, then the transistor would burnout from excessive base current. It's probably the voltage before the base resistor, as mentioned above.

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