Tue, 24 Sep 2013, 14:10

My students,

There are two reasons for me sharing this email with you.  One deals 
with 306 stuff explicitly.  The other deals with another important item.

I got the following email from a student last night:

 
> I was looking at the inverter gate and it seemed a bit complex for what it
> does, wouldn't this work as well? (assuming whatever the output was going
> towards had some resistance). I don't really understand why you need both
> the n-type and the p-type for the inverter?
>
> <<name withheld to protect the student who was not in class>>


With his email he included the following figure (which I hope my ascii
keyboard faithfully reproduces):

    -
         |
         |
         +- output
         |
   in - N
         |
         |
         v

If it doesn't, what he drew was an N-type transistor connected to both
power and ground, and the output connected to the wire connecting power
to the transistor.

I thought I went over that twice in class, once quickly, and then again 
more carefully, because Ben thought I had done it too quickly the first 
time.  So, I asked him if he had been attending class.  His response:

 
> Unfortunately I missed Mon and Wed last week due to career fair.
> However I did read through the chapter a couple of times and feel pretty
> confident about most of the material now. Still a little hazy on some of
> the material from 3.3 - 3.5 Would you rather I asked a TA this question?


So, I responded:

No, I will answer your question.  No need to take time from the TAs.
They have their hands full at the moment with the second problem set
due tomorrow.

The reason I asked if you had been in class is I went over that in detail 
in class twice.

If you look at your figure, you will note that if the gate is 2.9 volts (or
a "1"), the power supply is connected to ground.  That is, the output is 
connected to both the battery and ground.  That will cause the battery to
burn out.  Or, in a house without fuses or circuit breakers, it will cause
a fire that will burn down the house.

Using P type and N type properly guarantees that the output is connected to
EITHER power supply OR ground and NOT both.  Using N and P is not an
unnecessary complexity.

Finally, I do have to say something about missing class on Monday and Wednesday
to go to career fair.  Career fair is important, but only in the context of
SUPPLEMENTING your education.  As you know, I refuse to treat you like you are
still in high school.  I do not take attendance.  Your grade will be based on
what you earn on the graded work.  It is up to you to decide whether to come 
to class.  But if you choose not to schedule career fair around your class 
schedule and skip class, you may find yourself in a situation where career
fair is irrelevant due to your grades.  Again, you get to decide.

Good luck with the rest of EE 306.

Yale Patt