Once upon a time, op amps
didn’t swing very close to
the positive or negative
rails. Even a couple volts
away from the rails—that
was okay in the old days
of transistor-ized op amps.
Hey, that was a lot better than with
vacuum-tube op amps that wouldn’t
swing within 200 V of the rail. Some op
amps could do a little better, but customers
never asked us for better 35
years ago.
The LM324 can swing pretty close to
ground (–VS) if you have a pull-down
resistor—or close to +VS if you have a
pull-up resistor. But, of course, it has a
pretty big (ac) dead zone when driving
its output. Crude. Slow.
Many modern low-voltage op amps
have common-drain outputs. This is
often called a “rail-to-rail” output, even
though it won’t really go to the rail.
Martin Giles always razzes me (quite
properly) if I talk about these amplifiers
as “rail-to-rail.”
Widlar’s 1976 LM10 was the first to
have a common-collector “rail-to-rail”
output, swinging within a few millivolts
of each rail. Most CMOS op amps don’t
swing that close. If you have an op amp
running on “+5 V dc,” it surely can’t
swing to even + 4.9 V if its power supply
is only + 4.75 V (i.e., if the supply has a
5% tolerance).
So if you have an analog-to-digital
converter (ADC) with a 2.5-V reference,
and a 2.5-V full scale, an op amp running
on +4.75 V can easily swing up to
the + full scale at +2.50 V. But it
still can’t swing that close to
ground. Even for a 10-bit ADC,
you are going to lose a few least significant
bits where the amplifier
can’t swing low enough (Fig. 1).
The output may swing within 12 or
22 mV of ground, but not closer.
The trick is to add a high-impedance
diode from (a) pointing to (b),
replacing the hard wire. (1N4004, not
1N914). Even if the amplifier’s output
can’t swing within 100 mV of ground, it
doesn’t have to. To get the output to
within 0.1 mV of ground, the diode
leakage merely has to go down to 50 nA
at VF = 0.1 V, even at your highest operating
temperature. Even at 95°C, you
can do this with a transistor’s C-B
diode. Try it.
The ~2k pull-down resistor in shunt of
the output and the diode in series with the
op amp’s output make the trick work. It
can drive the high-impedance input of an
ADC within a small number of microvolts
of ground—much better than a “rail-torail”
that can only get within a small number
of millivolts of ground.
So the circuit of Figure 1 can swing
close to ground nicely, but not close to
the positive rail. Is that what you needed,
bunky? Why didn’t you say so? Try
Figure 2. Install one high-impedance
diode from (c) toward (d) and another
one anti-parallel.
The 74C14 has a lot of “gain,” but
not a lot of output offset—and very little
power drain in this switching mode.
I have seen it drive an ADC within a
few microvolts of both rails.
One of the advantages is
that it won’t over-drive the
input of the ADC past the rails.
Good feature.
These circuits are not very fast,
and not low-distortion, and they
won’t drive much of a dc load. Do you
want to drive a dc load? If we add
another trick, you can drive a load. Go
to www.national.com/rap and click on
“rail-to-rail driver.” Have fun!
SCHOOL DAYS
When I was a kid
engineer of 18 at MIT, I took course
6.021 on Piecewise Linear Circuits with
graduate instructor Leonard Kleinrock. I
learned a lot from him.
Now, Prof. Kleinrock is a respected professor
at UCLA and one of the inventors
of the networks leading to the formation
of the Internet. AND 49 YEARS OUT, I
AM STILL HAVING FUN DESIGNING
PIECEWISE LINEAR CIRCUITS! I hope
that Lenny is still having fun, too!
BEST NEW RECIPE • For an excellent
casserole, boil 1 lb white beans (~Great
Northern) in 8 cups H2O, 3 minutes; let
stand 2 hours, then simmer an hour.
Meanwhile, bake ~2.5 lb of pork roast at
350°F. Cut into 1/3-in. cubes; discard
most chunks of fat. Scrub potatoes and
cut into 1/4-in. cubes to make 2 cups.
Add pork and potato to the beans. Add a
4-oz can of warm or hot chopped chilis,
to taste, and one (or two) (or 1/3, if you
are trying to not scare the kids) 4-oz cans
of hot chilis, chopped fine (3/16 in.). Add
salt and pepper to taste; simmer 1/2 hour.
RAP invented this Best Recipe of the year:
I had a hunch, and it worked out well.