Author Topic: Question on NPN Logic Inverter From Textbook  (Read 3548 times)

0 Members and 1 Guest are viewing this topic.

Offline eev_carlTopic starter

  • Regular Contributor
  • *
  • Posts: 224
  • Country: us
Question on NPN Logic Inverter From Textbook
« on: September 30, 2019, 02:30:07 pm »
Hi,

I created an LT Spice of this logic inverter from a textbook (Jaeger and Blalock, 3rd edition "Microelectronic Circuit Design").  I'm getting a Vce that's 10x less than what's printed in the book. I was wondering if someone could help troubleshoot my model.  In the sim, I get a Vce of 18mV and the book (see attachments) has it listed at 180mV.  Maybe I'm not measuring the right thing?

Thanks,
Carl
 

Offline eev_carlTopic starter

  • Regular Contributor
  • *
  • Posts: 224
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #1 on: September 30, 2019, 02:31:13 pm »
It's cut off in the photo, but the figure from the book below the text is 5.32(a).
 

Offline Wimberleytech

  • Super Contributor
  • ***
  • Posts: 1134
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #2 on: September 30, 2019, 02:42:05 pm »
Pick a 2n3904 model for you simulation and see what you get.

You are being kinda hard on that poor transistor forcing 1 volt Vbe.

Add some base resistance.
« Last Edit: September 30, 2019, 02:46:54 pm by Wimberleytech »
 
The following users thanked this post: eev_carl

Offline eev_carlTopic starter

  • Regular Contributor
  • *
  • Posts: 224
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #3 on: September 30, 2019, 02:42:50 pm »
2n3904

V(vce):    0.00810523    voltage
 

Offline rstofer

  • Super Contributor
  • ***
  • Posts: 9964
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #4 on: September 30, 2019, 02:45:05 pm »
VceSat is usually on the order of 0.2V (MAX at Ic=10 mA).  Your simulation shows about 1/10th that amount and it is probably an issue with the model.

Look at the 2d page of the 2N3904 dataseet and you will see that VceSat is also a function of collector current.  The lower the current, the lower the saturation voltage.

https://www.onsemi.com/pub/Collateral/2N3903-D.PDF

Page 6 shows VceSat of 0.1V for currents less than about 10 mA. 

Try it with a real circuit!
« Last Edit: September 30, 2019, 02:51:10 pm by rstofer »
 
The following users thanked this post: eev_carl

Offline Wimberleytech

  • Super Contributor
  • ***
  • Posts: 1134
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #5 on: September 30, 2019, 02:51:49 pm »
Quote
The lower the current, the lower the saturation voltage.


Yeah, not enough current.  The book is wrong.
 
The following users thanked this post: eev_carl

Offline edavid

  • Super Contributor
  • ***
  • Posts: 3464
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #6 on: September 30, 2019, 03:24:09 pm »
I think the other replies have missed the point.

You have VBE set to 1V.  The base current is 3A.  Of course the VCE is very low.

Add a reasonable base resistor, and you will see a more reasonable saturation voltage.
 
The following users thanked this post: newbrain, eev_carl

Offline eev_carlTopic starter

  • Regular Contributor
  • *
  • Posts: 224
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #7 on: September 30, 2019, 03:54:37 pm »
Does this simulation look better?  I put a 1k resistor in series that brought Ib down to 0.3mA.

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)?

Thanks,
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 20363
  • Country: gb
  • 0999
Re: Question on NPN Logic Inverter From Textbook
« Reply #8 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.

Yes, I agree 180mV is on the high side, at such a low current, when VBE = 0.8V. It's probably an error with the decimal place, which is easily done.
 
The following users thanked this post: eev_carl

Offline eev_carlTopic starter

  • Regular Contributor
  • *
  • Posts: 224
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #9 on: September 30, 2019, 04:19:27 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.


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

Offline Wimberleytech

  • Super Contributor
  • ***
  • Posts: 1134
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #10 on: September 30, 2019, 04:22:24 pm »
Born in the lab
 

Offline Wimberleytech

  • Super Contributor
  • ***
  • Posts: 1134
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #11 on: September 30, 2019, 04:23:50 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.


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

Power, speed...  The resistor is to limit current.  All resistors couple dc.
 
The following users thanked this post: eev_carl

Offline edavid

  • Super Contributor
  • ***
  • Posts: 3464
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #12 on: September 30, 2019, 04:26:54 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)?

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.
« Last Edit: September 30, 2019, 04:30:56 pm by edavid »
 
The following users thanked this post: eev_carl

Offline Wimberleytech

  • Super Contributor
  • ***
  • Posts: 1134
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #13 on: September 30, 2019, 04:35:29 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)?

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.

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. 
 
The following users thanked this post: eev_carl

Online fourfathom

  • Super Contributor
  • ***
  • Posts: 2005
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #14 on: September 30, 2019, 05:06:57 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.

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!
We'll search out every place a sick, twisted, solitary misfit might run to! -- I'll start with Radio Shack.
 
The following users thanked this post: eev_carl

Offline Wimberleytech

  • Super Contributor
  • ***
  • Posts: 1134
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #15 on: September 30, 2019, 05:25:31 pm »
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.


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.
 
The following users thanked this post: eev_carl

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 20363
  • Country: gb
  • 0999
Re: Question on NPN Logic Inverter From Textbook
« Reply #16 on: September 30, 2019, 06:48:31 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.


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


 
The following users thanked this post: eev_carl

Offline eev_carlTopic starter

  • Regular Contributor
  • *
  • Posts: 224
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #17 on: September 30, 2019, 07:48:40 pm »
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.
 

Offline edavid

  • Super Contributor
  • ***
  • Posts: 3464
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #18 on: September 30, 2019, 08:14:48 pm »
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.
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 20363
  • Country: gb
  • 0999
Re: Question on NPN Logic Inverter From Textbook
« Reply #19 on: September 30, 2019, 08:34:02 pm »
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.
 
The following users thanked this post: eev_carl

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 7681
  • Country: ca
Re: Question on NPN Logic Inverter From Textbook
« Reply #20 on: September 30, 2019, 08:42:15 pm »
You'll never force an E-B junction to 5V without the transistor exploding. That graph should end at 1V.
Be careful, EE textbooks are written by academics and there are many errors. Always look for the latest edition or the errata. In university, the textbook errors and mistakes never ended despite the author having great credentials.
 
The following users thanked this post: eev_carl

Offline eev_carlTopic starter

  • Regular Contributor
  • *
  • Posts: 224
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #21 on: September 30, 2019, 08:46:46 pm »
Yeah, nothing in errata for 3rd, 4th, 5th editions (mine is 3rd)
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 7681
  • Country: ca
Re: Question on NPN Logic Inverter From Textbook
« Reply #22 on: September 30, 2019, 09:04:55 pm »
I think the book mistake is to use the datasheet maximum spec of BVEBO, instead of VBE(on). Example:
2SC5200 power transistor: BVEBO 5V; VBE(on) 1.5V
2N3904/2N2222: BVEBO 6V; no spec for, but saturation graph is VBE(on) 1.0V
 

Online fourfathom

  • Super Contributor
  • ***
  • Posts: 2005
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #23 on: September 30, 2019, 09:52:09 pm »
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.

OK, you're right.  I'm not a process/fab guy so I would have to study that figure long and hard, and still probably wouldn't spot the problem.

I did look at the graph showing +5V VBE, and that is obviously wrong.  Skimming through the book I also noticed Fig 6.3, showing a single-NPN inverter with a direct connection to the base, and showing a transfer function input ranging from 0V to V+.  So this is not an isolated problem in the book.
We'll search out every place a sick, twisted, solitary misfit might run to! -- I'll start with Radio Shack.
 

Offline Wimberleytech

  • Super Contributor
  • ***
  • Posts: 1134
  • Country: us
Re: Question on NPN Logic Inverter From Textbook
« Reply #24 on: September 30, 2019, 10:12:58 pm »
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.

OK, you're right.  I'm not a process/fab guy so I would have to study that figure long and hard, and still probably wouldn't spot the problem.

I did look at the graph showing +5V VBE, and that is obviously wrong.  Skimming through the book I also noticed Fig 6.3, showing a single-NPN inverter with a direct connection to the base, and showing a transfer function input ranging from 0V to V+.  So this is not an isolated problem in the book.

Yeah, I skimmed the book...all 1300+ pages.  I take issue with lots of things.  Why teach H-biasing of MOSFETS?  It is, frankly, useless information.  I don't have a problem with typos and an equation faux pas here and there (I am an author and understand how easy it is to miss such things), but pedagogical issues are more problematic.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf