314
IEEE JOURNAL
OF SOLID-STATECIRCUITS,
VOL.
sC-9,
NO.
6,
DECEMBER
1974
The Monolithic
Op
Amp:
A Tutorial Study
JAMES
E.
SOLOMON,
MEMBER, IEEE
Invited
Paper
Absfracf—A
study is made of the integrated circuit operational
amplifier (IC op amp) to explain details of its behavior in a simplified
and understandable manner. Included
are analyses of thermal
feedback effects
on
gain, basic relationships for bandwidth and
slew rate, and a discussion of pole-splitting frequency compensation.
Sources of second-order
bandlimiting in the amplifier are also
identified and som(s approaches to speed and bandwidth improve-
ment are developed. Brief sections are included on new JFET-
bipolar circuitry and die area reduction techniques using transcon-
ductance reduction,
I. INTRODUCTION
T
HE
integrated
circuit
operational
amplifier
(IC
op
amp)
is
the
most
widely
used
of
all
linear
circuits
in
production
today.
Over
one
hundred
million
of
the
devices
will
be
sold
in
1974
alone,
and
production
costs
are
falling
low
enough
so that
op amps
find
applications
in virtually
every
analog
area.
Despite
this
wide
usage,
however,
many
of the
basic
performance
characteristics
c)f the
op amp
are
poorly
understood.
It
is
the
intent
of
this
study
to
develop
an
under-
standing
for
op
amp
behavior
in
as
direct
and
intuitive
a
manner
as
possible.
This
is
done
by
using
a
variety
of
simplified
circuit
models
which
can
be
analyzed
in
some
cases
by
inspection,
or in
others
by
writing
just
a
few
equations.
These
simplified
models
are
generally
developed
from
the
single
representative
op
amp
con-
figuration
shown
in Figs.
1 and
2.
The
rationale
for
starting
with
the
particular
circuit
of Fig.
1 is based
on the
following:
this
circuit
contains,
in
simplified
form,
all
of the
important
elements
of the
most
commonly
used
integrated
op
amps.
It
consists
essentially
of
two
voltage
gain
stages,
an
input
differ-
ential
amp
and
a common
emitter
second
stage,
followed
by
a
class-AB
output
emitter
follower
which
provides
low impedance
drive
to the
load.
The
two
interstages
are
frequency
compensated
by
a single
small
“pole-splitting”
capacitor
(see
below)
which
is usually
included
on
the
op
amp
chip.
In
most
respects
this
circuit
is
directly
equivalent
to
the
general
purpose
LM 101
[1],
VA
741
[2],
and
the
newer
dual
and
quad
op
amps
[3],, so
the
results
of
our
study
relate
directly
to
these
devices.
Even
for
more
exotic
designs,
such
as
wide-band
amps
using
feedforward
[4],
[5],
or
the
new
FET
input
cir-
cuits
[6],
the
basic
analysis
approaches
still
apply,
and
performance
details
can
be
accurately
preclicted.
It
has
also
been
founcl that
a good
understanding
of the
limita-
Manuscript received July 1, 1974; revised August 8, 1974.
The outhor is with the National Semicondl~ctor Corporation,
Santa Clara, Calif. 95051.