Rarely Asked Questions R A Q ’ s Q.

advertisement
R A Q ’ s
S
p e c i a l
A
d v e r t i s i n g
S
e c t i o n
Rarely Asked Questions
Single Supply Amplifiers—They Sound Simple…are they?
Q. Running
Running rail-to-rail
rail-to-railopop
amps
amps
on
a
supply
sounds
like a like
winning
onsingle
a single
supply
sounds
a
combination,
but
what
drawbacks
winning combination, but what
will
I encounter
using amplifiers
drawbacks
willwhen
I encounter
when
like
this?
using amplifiers like this?
A.
A.
Single-supply
Single-supply and
and rail-to-rail
rail-to-rail outputs
outputs
are
a
great
combination,
are a great combination, but
but there
there are
are
a
a few
few parameters
parameters that
that warrant
warrant a
a secsecond
ond look.
look. Your
Your question
question doesn’t
doesn’t specify
specify
whether
you
are
talking
about
whether you are talking about single-supsingle-supply
ply amplifiers
amplifiers (a
(a particular
particular class
class of
of ampliamplifiers)
or
running
a
traditional
op
fiers) or running a traditional op amp
amp on
on
single
single supply;
supply; we’ll
we’ll discuss
discuss both
both cases.
cases.
By
By definition,
definition, a
a true
true “single-supply”
“single-supply” op
op
amp
operates
on
one
amp operates on one supply,
supply, and
and the
the
input
input common-mode
common-mode voltage
voltage range
range of
of
the
amplifier
includes
the
negative
the amplifier includes the negative supply
suprail.
Note
thatthat
even
though
the the
input
of
ply rail.
Note
even
though
input
the
amplifier
can
go
to
the
negative
rail
of the amplifier may be able to go to the
and
beyond
doesn’t
mean
the output
negative
rail itand
beyond
it doesn’t
mean
can.
I’ll
say
more
about
that
when
talk
the output can. I’ll say more aboutwe
that
about
rail-to-rail
outputs.
when we talk about rail-to-rail outputs.
Any
Any amplifier
amplifier can
can be
be run
run on
on a
a single
single
supply.
Op
amps
don’t
have
supply. Op amps don’t have a
a ground
ground
pin
pin and
and are
are equally
equally happy
happy when
when run
run
on
on a
a bipolar
bipolar supply
supply as
as they
they are
are with
with a
a
single
single supply.
supply. Additional
Additional bias
bias circuitry
circuitry is
is
required
required when
when running
running an
an amplifier
amplifier in
in this
this
configuration,
configuration, however.
however. As
As a
a result,
result, the
the
amplifier’s
amplifier’s performance
performance may
may suffer
suffer slightly
slightly
in
the
following
areas:
lower
bandwidth,
in the following areas: lower bandwidth,
degraded
degraded power
power supply
supply rejection
rejection (PSR),
(PSR),
and
higher
noise.
and higher noise.
The
The term
term rail-to-rail
rail-to-rail output
output is
is a
a misnomer.
misnomer.
While
the
amplifier
output
can
While the amplifier output can get
get close
close
to
to the
the supply
supply rails,
rails, it
it never
never quite
quite gets
gets
there.
there. In
In bipolar
bipolar amplifiers,
amplifiers, the
the rail-torail-torail
output
stage
is
typically
rail output stage is typically a
a common
common
emitter;
emitter; therefore
therefore the
the closest
closest the
the output
output
can
get
to
the
rails
is
a
saturated
can get to the rails is a saturated transistransistor
tor drop
drop Vcesat.
Vcesat. The
The value
value of
of Vcesat
Vcesat is
is
Contributing Writer
John Ardizzoni is an
Application Engineer
FPO
at Analog Devices in
the High Speed Linear
group. John has been
with Analog Devices
since 2002, he received
his BSEE from Merrimack College in N. An-
dependant on the amount of load current
dependant
the
amount For
of load
current
delivered byon
the
amplifier.
low curdelivered
by
the
amplifier.
For
low
currents, the output can come to within tens
rents,
the output
to within
of millivolts
to thecan
rail.come
For higher
cur-tens
of
millivolts
to
the
rail.
For
higher
rents Vcesat can approach 0.5 V orcurmore.
rents
can approach
0.5 V oron-chip
more.
SomeVcesat
new amplifiers
now feature
Some
new
amplifiers
now
feature
on-chip
charge pumps to make up the Vcesat
charge
pumps to
upto
the
Vcesat
drop, allowing
themake
output
swing
all the
drop,
allowing
the
output
to
truly
swing
way to the rail.
all the way to the rail.
An amplifier output swinging close to
An
output
swinging
close
to
the amplifier
supply rail
may appear
fine
when
the
supply
rail
may
appear
fine
when
measured on an oscilloscope, but a
measured
on an oscilloscope,
a
network analyzer
may reveal a but
different
network
analyzer
may
reveal
a
different
result. As the output swings closer to
result.
Asthe
theoutput
outputtransistors
swings closer
to
the rails,
no longer
the
rails,
the
output
transistors
no
longer
operate in the linear region. As a result,
operate
linear region.
As a result,
distortioninisthe
introduced.
Distortion
can
distortion
is
introduced.
Distortion
occur several hundreds of millivoltscan
away
occur
several
millivolts
away
from the
rails.hundreds
Thereforeofwhen
possible,
from
the
rails.
Therefore
when
possible,
try design in a little extra headroom
try
design
in a this
littlewill
extra
headroom
from
the rails;
help
improve the
from
the
rails;
this
will
help
improve the
amplifier distortion performance.
amplifier distortion performance.
We’ve only skimmed the surface on these
We’ve
skimmedapplication
the surfaceand
on these
topics. only
For in-depth
topics.
For
in-depth
application
and
product information, visit our website or
product
information,
visit our website or
click on the
link below.
click on the link below.
dover, MA and has over
28 years experience in
the electronics industry.
Have a question
involving a perplexing
or unusual analog
problem? Submit your
question to:
raq@reedbusiness.com
For Analog Devices’
Technical Support,
Call 800-AnalogD
SPONSORED BY
To Learn More About
Single
Supply
XXX
XXXAmplifiers
XXX
Go to: http://rbi.ims.ca/XXX-100
http://rbi.ims.ca/5726-100
d e s i g n n e w s 1 0 . 0 6 . 0 8 [ w w w. d e s i g n n e w s . c o m ]
##
Download