Hi David,

There are really two issues here. First of all, the choice of N versus P
channel. The two circuits you describe are two different kinds of circuit.
Both are called "common source" configurations (because the source is
connected to a "common" or supply node, either GND or V+), but one is of
the type known as a "high-side" configuration and the other a "low-side"
configuration. This just refers to the fact that in one case, the
transistor is on the high-potential side of the load (in the PMOS case) and
in the other, it is on the low-potential (closer to GND) side (NMOS case).

Both configurations work equally well, and which you use depends on whether
it is acceptable to switch V+ or GND. For example, if you are controlling
something which has multiple independent ground connections, or has
multiple things connected internally to ground and you are just controlling
one of those via the V+ line, then you must use a high-side circuit (the
PMOS version) because you don't have control over the GND line, and you
can't stick a switch(or FET) in it.

For even more concrete examples, imagine that you have a device which
contains a motor and some other electronics. The electronics accepts a +5V
supply and the motor takes a 12V supply, but they share a common ground (so
you only have 3 wires). You want to control the motor via PWM, but it is
unacceptable to switch the power to the electronics on and off, it must be
stable. So, you can't put a switch between the device and ground, you must
put it between +12V and the 12V input line.

The opposite situation could occur, where you have a common supply but
different grounds. In that case, you want to use an NMOS FET to switch the
ground line.

The second issue is the one of symbols. There are many different symbols
for mosfets. I have seen at least 3 different conventions, and yes, some of
them have an arrow direction opposite to that of the others. Probably the
best thing to do is to become familiar with one convention, and then when
you are unsure, look it up, or try to figure it out from the circuit (i.e.,
think about whether an NMOS fet would work in that situation, etc.)  At
least for the 3 conventions I have seen, there are other distinctive
differences (other than just a reversed arrow), so it isn't completely
ambiguous.

Sean

At 08:50 AM 12/9/00 -0600, you wrote:
>Page 939 of AoE shows use of a mosfet to control power to a
>circuit.  Checking my Forrest Mims Getting Started, it is using a
>P-channel mosfet.  The load is between the drain and ground.  The source
>is connected to +5V.  The text indicates the p-channel is
>used because of the positive supply.
>
>Then in Switching Power Supply Design by Abraham Pressman, 2nd ed., I
>found similar circuits (p. 356) except he recommends an
>N-channel for positive supply and vice versa.  He also places the load
>between the power supply and drain (for positive supplies).
>
>Are these consistent?  Does it matter?
>
>Just a beef, but the symbols on p. 355 of SPSD appear intended to confuse
>newbies!  Neither match my Mims book.  There are slightly
>different symbols (extra line and dot).  Result is that for an N-mosfet
>sometimes the arrow points in and sometimes out.  (Must be a
>reason?)
>
>DS
>
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