tional
amplifiers concerns operation from a single supply
voltage. “Can the model OPAxyz be operated from a single
supply?” The answer is almost always yes. Operation of op
amps from single supply voltages is useful when negative
supply voltages are not available. Furthermore, certain applications
using high voltage and high current op amps can
derive important benefits from single supply operation.
Consider the basic op amp connection shown in Figure la. It
is powered from a dual supply (also called a balanced or
split supply). Note that there is no ground connection to the
op amp. In fact, it could be said that the op amp doesn’t
know where ground potential is. Ground potential is somewhere
between the positive and negative power supply
voltages, but the op amp has no electrical connection to tell
it exactly where.
VIN
VOUT = VIN
G = +1
+VS = 15V
–VS = 15V
VIN
VOUT = VIN
G = 1
+VS = 30V
(a)
(b)
FIGURE 1. A simple unity-gain buffer connection of an op
amp illustrates the similarity of split-supply operation
(a) to single-supply operation in (b).
The circuit shown is connected as a voltage follower, so the
output voltage is equal to the input voltage. Of course, there
are limits to the ability of the output to follow the input. As
the input voltage swings positively, the output at some point
near the positive power supply will be unable to follow the
input. Similarly the negative output swing will be limited to
somewhere close to –VS. A typical op amp might allow
output to swing within 2V of the power supply, making it
possible to output –13V to +13V with ±15V supplies.
Figure 1b shows the same unity-gain follower operated from
a single 30V power supply. The op amp still has a total of
30V across the power supply terminals, but in this case it
comes from a single positive supply. Operation is otherwise
unchanged. The output is capable of following the input as
long as the input comes no closer than 2V from either supply
terminal of the op amp. The usable range of the circuit
shown would be from +2V to +28V.
Any op amp would be capable of this type of single-supply
operation (with somewhat different swing limits). Why then
are some op amps specifically touted for single supply
applications?
Sometimes, the limit on output swing near ground (the
“negative” power supply to the op amp) poses a significant
limitation. Figure 1b shows an application where the input
signal is referenced to ground. In this case, input signals of
less than 2V will not be accurately handled by the op amp.
A “single-supply op amp” would handle this particular
application more successfully. There are, however, many
ways to use a standard op amp in single-supply applications
which may lead to better overall performance. The key to
these applications is in understanding the limitations of op
amps when handling voltages near their power supplies.
There are two possible causes for the inability of a standard
op amp to function near ground in Figure 1b. They are (1)
limited common-mode range and (2) output voltage swing
capability.
These performance characteristics are easily visualized with
the graphical representation shown in Figure 2. The range
over which a given op amp properly functions is shown in
relationship to the power supply voltage. The commonmode
range, for instance, is sometimes shown plotted with
respect to another parameter such as temperature. A ±15V
supply is assumed in the preparation of this plot, but it is
easy to imagine the negative supply as being ground.
In Figure 2a, notice that the op amp has a common-mode
range of –13V to +13.5V. For voltages on the input terminals
of the op amp of more negative than –13V or more
positive than +13.5V, the differential input stage ceases to
properly function.
Similarly, the output stages of the op amp will have limits on
output swing close to the supply voltage. This will be loaddependent
and perhaps temperature-dependent also. Figure
2b shows output swing ability of an op amp plotted with
respect to load current. It shows an output swing capability
of –13.8V to +12.8V for a l0kΩ load (approximately ±1mA)
at 25°C.
®
©1986 Burr-Brown Corporation AB-067 Printed in U.S.A. March, 1986
SINGLE-SUPPLY OPERATION OF OPERATIONAL AMPLIFIERS
SBOA059
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