2.678  Electronics for Mech Systems

Come on by!

If you have any questions this weekend, put them up on the Piazza so the instructors (and other students) can help answer them.

-Ned

If you haven't yet picked up your components for the next pset/prelab, come by office hours and grab 'em!

-Ned

Dear Students,

Hi, tomorrow (Oct 20th) is one of the Mondays where we do not have a
lecture scheduled for 2.678. (we picked four lectures to cancel at the start
of term to give you a break -- the other remaining ones are Nov 10th
and Nov 26th.   So there is no lecture tomorrow. 

Our next major topic in the class, as we move into power electronics,  is
field effect transistors (FETs), which we will discuss in lecture on Wednesday. 
We have posted two of Professor Rowell's lecture handouts on FETs on stellar
(under the "Readings" section):

https://stellar.mit.edu/S/course/2/fa14/2.678/courseMaterial/topics/topic10/readings/FETs/FETs.pdf
https://stellar.mit.edu/S/course/2/fa14/2.678/courseMaterial/topics/topic10/readings/MOSFET/MOSFET.pdf

The first document gives a fairly detailed explanation of how FETs
work, first describing the Junction Field Effect Transistor (JFET) and
then the metal-oxide semiconductor field effect transistor (MOSFET),
which is very widely used in digital logic circuits and in power
electronics.  (See http://en.wikipedia.org/wiki/Transistor_count
for some amazing numbers about transistor counts in processors)


On Wednesday we will give a simplified model of how FETs operate, and
we will use FETs in several upcoming labs.

There is a question on the 2nd page of the PreLab that uses MOSFETs,
and since we haven't discussed them yet, we will not deduct any points
for incorrect answers in grading this prelab question. 

However we do encourage you to read about MOSFETs (using the handouts
linked above, and also the relevant sections in Scherz) and to do the
best you can to think about the problem.

The key thing to know about an FET is that it is a voltage-controlled
device, as opposed to BJTs which are current-controlled devices.  Just
like the BJT, the FET has three regions of operation: off, linear
(known for an FET as the "Triode" region), and saturation.  To make
things a bit trickier, the terminals of the MOSFET have different
names than from the BJT, namely: the Gate, Drain and Source terminals.
When a MOSFET is used as as switch, it turns "on" when the the voltage
between the gate and source (V_GS) exceeds a threshold (V_th, in the
range of 1 volt).  If V_GS is less than V_th, then the MOSFET is "off"
and there is effectively an open circuit between the drain and source.
When a MOSFET is "on", it behaves like a small resistor between the
drain and source pins.  We'll show you this in lecture and in lab.

As always, please let us know if you have any questions, and email me
if you every want to discuss any aspect of 2.678.

thanks
Prof. Leonard