ETC AB-067

output to swing within 2V of the power supply, making it
possible to output –13V to +13V with ±15V supplies.
One of the most common applications questions on operational 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.
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
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.
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
+VS = 15V
G = +1
–VS = 15V
+VS = 30V
G= 1
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
1986 Burr-Brown Corporation
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.
Printed in U.S.A. March, 1986
As demonstrated in Figure 1b, an op amp with typical
common mode and output characteristics functions well on
a single supply as long as the input and output voltages are
constrained to the necessary limits. Circuit configurations
must be used which operate within these limits.
Common-Mode Voltage (V)
Figure 3 shows a circuit, for instance, which references the
input and output to a “floating ground” created with a zener
diode. The zener diode is biased with a current set by RZ.
Since VIN and VOUT are both referenced to the same floating
ground, the zener voltage accuracy or stability is not critical.
VIN and VOUT can now be bipolar signals (with respect to
floating ground). With +V = 30V and VZ = 15V, operation
is similar to standard split supply operation. The load current
in this circuit, however, flows to the floating ground where
it will add to the zener diode current (negative load currents
subtract from zener current). The zener diode must be
selected to handle this additional current. If the zener current
is allowed to approach zero, the floating ground voltage will
fall rapidly as the zener turns off. Rl must be selected so that
the zener diode current remains positive under all op amp
load conditions.
Temperature (°C)
Output Voltage (V)
+VS = 12V
Output Current (mA)
G = –RF/R1 = –4.7
FIGURE 2. The Common-mode Range of an Op Amp is
Usually Dependent on Temperature. This behavior is shown plotted in (a). Output voltage
swing will be affected by output current. (b).
Often the op amp load is connected to ground, so
load current is always positive. Furthermore, as
the output voltage approaches zero, load current
approaches zero, increasing the available output
swing. A split power supply voltage (normally
±15V) is assumed in preparation of these plots.
VZ = 5.6V
RLOAD = RL || (RI + RF)
FIGURE 3. Bipolar Signals Can be Handled When Input
and Output are Referenced to a Floating Ground.
Changing load current causes a variation in
zener current which must be evaluated.
So the circuit of Figure 1b is limited to +13V output by
output swing capability and –13V by negative commonmode range. A single-supply op amp is specifically designed
to have a common-mode range which extends all the way to
the negative supply (ground). Also, its output stage is usually designed to swing close to ground.
Figure 4 shows operation in a noninverting gain configuration. In this circuit, the feedback components present an
additional load to the op amp equal to the sum of the two
resistors. This current must also be considered when planning for the variation in current flowing in the zener diode.
Again, the zener current should not be allowed to approach
zero or exceed a safe value.
It would be convenient if all op amps were designed to have
this capability, but significant compromises must be made to
achieve these goals. Increased common-mode range, for
instance, often comes at the sacrifice of performance characteristics such as offset voltage, offset drift, and noise. General purpose applications may tolerate op amp performance
with these compromises, but high accuracy or other special
purpose applications may require a different approach.
Fortunately, there are many ways to use high performance
and special purpose op amps in single-supply applications.
Notice that in this example, a single +12V supply is shown.
Often, single-supply applications use supply voltages which
are considerably less than the 30V total (±15V) at which the
performance of most op amps is specified. While modern
op amps generally perform well at less than their characterized voltage, this needs to be verified. Some op amps,
although they are specified to operate at lower voltage,
well defined. Normally other system components would
sufficiently load the regulator to allow for plenty of op amp
load current.
Gain = 1 + RF/RI
= 11
Particularly demanding applications may require that a buffer
op amp be used to establish a very low impedance floating
ground. Input to the buffer (Figure 7) could come from any
of the previously discussed techniques. The buffer can both
source and sink load current up to the output current limits
Total Effective Load
RL' = RL || (R1 + RF)
G = 5.7
VZ = 5.6V
IZ = (12 – 5.6)/1kΩ
= 6.4mA
FIGURE 4. As with Conventional Split-Supply Operation,
a Noninverting Gain Configuration Can Be
Acheived. The feed back components create an
additional load for the op amp which flows in
the zener diode. Basic performance characteristics of the circuit are the same as for split supply
FIGURE 5. Even Though the Impedance of the Voltage
Divider is in Series with R1 to Ground, the Gain
of this Noninverting Circuit is Determined Solely
by R1 and R2. Since the input and output are
referenced to the same floating ground, its impedance does not affect the voltage gain of the
suffer degraded power supply and common-mode rejection
as their minimum operating voltage is approached.
Extremes of common-mode voltage on some amplifiers may
produce unexpected behavior. Certain types of FET input op
amps, for instance, exhibit much greater input bias current
when the common-mode voltage relative to either of the
power supplies exceeds 15V to 20V. This could occur with
single-supply operation of 30V and common-mode voltage
unbalanced nearer one supply or the other. The actual
amplifier performance should be verified with the expected
worst-case common-mode voltage conditions.
Resistor voltage dividers are sometimes used to establish
floating ground (Figure 5). The impedance of the ground is
determined by the parallel combination of the divider resistors. Unless these resistors are made very low in value
(consuming significant power supply current), this will lead
to higher “ground” impedance. But with careful attention to
the effects of varying load current in the reference point, this
approach may prove useful. In fact, it may not be important
in some applications that a truly “solid” ground be established since input and output are referenced to the same
node. Good bypassing, however, will help avoid transient
disturbances of VG, or oscillation problems by providing a
lower high frequency impedance without low value divider
Appropriate voltage points often exist in related circuitry
which can be useful in establishing a floating ground. In
Figure 6, a +5V source used to power logic circuitry is used
as a floating ground. Beware that most regulators used
to supply these voltages are designed to source current to a
load only. If sufficient op amp load current flows into the 5V
line, its voltage will rise. Again, load currents should be
evaluated to assure that the floating ground voltage remains
G = –3.3
+5V to Logic
FIGURE 6. Many Systems Have a +5V Logic Supply or
Other Appropriate Voltage Source which Can
Be Used as a Floating Reference Potential for
Analog Circuitry. Be sure logic noise does not
enter the analog system by providing an adequate decoupling network or additional bypassing.
0 to 10V
2 1
0 to 60V
8 10Ω
FIGURE 8. Unbalanced Power Supplies are Often Used
with Power Op Amps to Achieve Higher Unipolar Output Voltage Yet Provide Output Swing
Down to 0V. The negative supply voltage in this
OPA512 circuit is made large enough to provide
the common-mode voltage and output swing
requirements of the application.
(See Text)
output transistor, thus requiring greater safe operating area.
See Understanding Power Amplifier Specifications, Application Bulletin AB-123, for information on evaluation of
safe operating area.
FIGURE 7. A Very-Low Floating Ground Impedance is
Provided by Using One Section of the OPA2111
Op Amp Connected as a Unity-Gain Buffer.
Input to the buffer is a voltage divider which
can be heavily bypassed. The arrows indicate
the direction of positive and negative load current flow.
Other signal processing circuits which are normally powered from a split supply can be operated from a single
supply as well. These include such devices as instrumentation amplifiers, current transmitters, analog multipliers, log
amps, etc. The principles in assuring proper operation are
the same as for op amps.
The INA105 difference amplifier provides an instructive
example. This device is comprised internally (Figure 9) of
a precision op amp and four precision matched resistors. In
a majority of applications pin 1 is connected to ground. This
is the output voltage reference pin. If pin 1 is referenced to
a floating ground using one of the previously described
techniques, operation is similar to split-supply operation.
Unlike the op amp applications previously described, however, the differential input terminals (pins 2 and 3) will be
capable of accommodating common-mode voltages equal
to and even greater than the supply voltages. Voltages
applied to the input resistors are divided down, maintaining
common-mode voltages to the op amp within operating
limits. In this case, the voltage at pin 1 in conjunction with
the required output swing determines the technique required for single-supply operation.
of the op amp used as the buffer. The closed-loop output
impedance of the op amp provides a very solid reference
ground. Frequency response and open-loop output impedance characteristics of the buffer op amp will determine the
high frequency floating ground impedance. Bypassing the
output of the buffer amp may help lower the high frequency
impedance, but don’t exceed a safe capacitive load of the
buffer amp or oscillations may result.
Figure 8 shows a technique often used with high voltage and
high current op amps. Here, an unbalanced power supply is
used to produce the desired output voltage swing. In applications such as a programmable power source, the output
voltage is required to go all the way to the ground. A small
negative supply is used to provide the necessary commonmode voltage and output stage requirements to allow full
output swing to ground. A much larger positive voltage
supply can now be used to maximize the available output
A higher current limit (lower value current limit resistor is
set for positive output current in this circuit since the primary
purpose is to source current to a load connected to ground.
Be sure to consider the safe operating area constraints
carefully in this type of operation. Unequal supplies mean
that larger voltages will be present across the conducting
True instrumentation amplifiers (Figure 10) usually have an
op amp at their input. Therefore, common-mode range of
the input op amp again becomes a concern. Input voltages
must be confined to within the specified common-mode
range of the device. The output section of the instrument
amp is like the difference amplifier and output voltages
swing requirements will dictate the techniques required.
VOUT = V2 – V1
VCM = (V2 + 15V)/2
FIGURE 9. The Input Voltage to this Simple Difference Amplifier is Divided Down by the Input Resistors Before Being
Applied to the Op Amp. Thus it is able to handle voltages which are equal to or greater than the power supply
V1 ≥ 3.5V
V1 ≤ 8.5V
V2 ≥ 3.5V
V2 ≤ 8.5V
FIGURE 10. Inputs to the Instrumentation Amplifier Are Applied Directly to the Active Circuitry of the Input Op Amps and
Therefore are Subject to the Common-Mode Range Limitations of these Op Amps.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user's own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.