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Maxim > Design
support > App notes
> Amplifier
and Comparator Circuits > APP 656

Apr 03, 2000

Keywords: single-supply, op amps, amplifiers,
design, trade-offs, operational amplifiers
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APPLICATION NOTE 656

Design Trade-Offs for Single-Supply Op Amps

Abstract: The
trend toward low-voltage, single-supply systems is fueled by
designers' attempts to balance the often contradictory goals
of lower product size and cost vs. longer battery life and
better system performance. This trend may be good for
consumers, but it complicates the task of choosing an
appropriate op amp for a given application.

Single-supply
operation is generally synonymous with low-voltage operation,
and moving from ±15V or ±5V to a single 5V or 3V supply rail
reduces the available signal range. Consequently, the
common-mode input range, output-voltage swing, CMRR,
noise, and other op-amp limitations become much more
important. As in all engineering, you must often sacrifice one
aspect of system performance to improve another. The following
discussion of trade-offs among single-supply op amps
also explains how these low-voltage amplifiers differ from
their higher voltage predecessors.

Input Stage Concerns
Input common-mode voltage range
is one of the first issues a designer should consider in
specifying a single-supply op amp. The first impulse
is to eliminate this concern by specifying a Rail-to-Rail
input capability. Certain penalties must be paid, however, for
true rail-to-rail operation.

Most of Maxim's
low-voltage op amps have input common-mode voltage ranges that
include the negative supply rail (Table 1), but only
some allow inputs that extend to the positive rail as well.
Others allow input voltages only within one or two volts of
the positive rail. Op amps that allow signals only to the
negative rail will be referred to as ground-sensing
amplifiers. Those that allow signals to either rail will be
referred to as rail-to-rail input amplifiers.

Table
1. Maxim low-voltage op amps

VOS and IB Concerns
In many
applications, the amplifier provides a gain of +2V/V or more
to a ground-referred signal. In these cases, a ground-sensing
amplifier is generally adequate for handling the signal's
common-mode range. If so, it could provide better performance
than one with a rail-to-rail input. Typical rail-to-rail input
stages use two differential input pairs instead of one
(Figure 1).

Figure
1. A rail-to-rail input stage (a) has two differential pairs,
while a standard ground-sensing input stage (b) has only
one.

As the input signal moves from one supply rail
to the other, the amplifier shifts from one input pair to the
other. At the crossover
point, this shift can cause changes in the input bias current
and offset voltage that affect both the magnitude and the
polarity of these parameters. These offset-voltage changes
typically worsen the distortion performance and precision
specifications of rail-to-rail amplifiers (in comparison with
ground-sensing types). To minimize offset-voltage shifts and
smooth the transition from one input pair to another, Maxim
trims the offset of its rail-to-rail amplifiers at both the
high and the low ends of the common-mode range.

To
reduce offset voltages caused by input bias currents, the
designer should match impedances at the op amp's inverting and
noninverting nodes. Because input bias currents are typically
larger than input offset currents, this impedance
matching is good practice for all types of op amps, not just
rail-to-rail input amplifiers.

To illustrate this
point, Figure 2 shows the change in input bias current
vs. common-mode range for the MAX4122–MAX4129 family of op
amps (which feature rail-to-rail capability at both the input
and output). As the common-mode input voltage ramps from 0V to
5V, the input bias current makes an absolute change of 85nA
(from -45nA to +40nA). In contrast, the specification for
input offset current is only ±1nA. Thus, changes at the
inverting and noninverting inputs (input offset current) track
each other closely, despite significant changes in the
magnitude and sign of the bias currents. By matching
impedances at these nodes, you can minimize the offset voltage
induced by changes in input bias current.

Figure
2. As the common-mode input voltage of a rail-to-rail input
amplifier sweeps from one supply rail to the other, the input
bias current can change, both in sign and in
magnitude.

Figure 3 shows how to match
impedances in the classic inverting and noninverting op-amp
configurations. The inverting configuration (Figure 4)
offers one way to eliminate changes in the input bias current
by keeping the amplifier's common-mode input voltage constant
at a reference voltage (VREF). The output is given
by VOUT = (-VIN x R2/R1) +
VREF(1 + R2/R1). If R2 = R1, this becomes
VOUT = -VIN + 2VREF. For
VREF = 2V and VIN between 0V and 3V,
VOUT ranges from 4V to 1V. The common-mode range is
fixed, so CMR errors are eliminated as well. Table 2
lists references suitable for use in low-voltage systems.

Figure
3. Matching the resistance at the inverting and noninverting
nodes minimizes offset errors caused by input bias currents
for both the noninverting (a) and inverting (b)
configurations.

Figure
4. By holding the common-mode input voltage constant, the
inverting-amplifier configuration eliminates
common-mode-rejection errors.

Table 2. Maxim
low-voltage references

Slew Rate
Slew rate can also suffer when a
rail-to-rail input amplifier is used in place of a
ground-sensing amplifier. The ground-sensing amplifier's
simpler input stage can take advantage of many
slew-rate-enhancing circuit techniques that are simply not
available to amplifiers with the two-pair, rail-to-rail input.
For example, MAX4212 family op amps (Table 1) have
ground-sensing inputs that help them achieve 600V/µs slew
rates and 300MHz bandwidths with supply currents of only 7mA
maximum. If they had rail-to-rail input stages and all other
specs remained unchanged, the slew rates would be several
times lower.

Output Stage Concerns
While low-voltage designs may
not require op amps with rail-to-rail input stages, they
typically require rail-to-rail output stages to maximize dynamic
range. Because op amps provide gain in most applications,
the output voltage is usually larger than the input voltage.
Thus, a rail-to-rail input stage is not always required, but a
rail-to-rail output stage usually is. These output stages
differ from those in dual-supply op amps and cause different
circuit behavior in the rail-to-rail output amplifiers.

Rail-to-rail output stages usually incorporate a
common-emitter configuration, and standard output stages
usually have an emitter-follower configuration (Figure
5). For common-emitter output stages, the voltage drop
from input to output is relatively low (a single
collector-to-emitter saturation voltage, or
VCE(SAT)), but the classic emitter-follower output
stage cannot get closer to the rail than VCE(SAT)
(due to the current source) plus VBE (due to the
output transistor).

Figure
5. A rail-to-rail output stage (a) has a common-emitter
configuration, while a standard output stage (b) has an
emitter-follower configuration.

Because a bipolar
transistor's VCE(SAT) depends on the current
through the transistor, the output swing of a bipolar op amp
depends on its load current. Thus, despite claims of
rail-to-rail performance, an amplifier's output stage never
truly reaches the supply rail. A MAX4122 with 100kΩ load, for
instance, swings to within 12mV of the positive rail and 20mV
of the negative rail. With a 250Ω load, however, it swings
only to within 240mV of the positive rail and 125mV of the
negative rail. For CMOS
output stages, the analogy to the bipolar transistor's
collector-emitter voltage is the MOSFET's drain-source
voltage, which is caused by the product of on-resistance and
channel current in the MOSFET.
Thus, the output voltage swing for a MOSFET output stage is
also a function of the load.

Gain vs. Load
Besides offering a low input-to-output
voltage drop, the common-emitter stage of a rail-to-rail
amplifier differs from the emitter-follower stage in other
important ways. Common-emitter stages provide voltage gain and
have relatively high-impedance outputs; emitter-follower
stages provide unity voltage gain and have low-impedance
outputs. For that reason, rail-to-rail op amps usually include
the output node as part of the compensation network, while
standard op amps typically take their compensation at a
preceding stage. For rail-to-rail op amps, the resulting
dependency of gain on load current can make them unstable when
driving capacitive loads.

These properties of
rail-to-rail outputs can be suppressed with careful op-amp
design, but the trade-off tends to be a higher supply current
than required by op amps with emitter-follower output stages.
The MAX4122–MAX4129 family of op amps offers good capability
for driving capacitive loads (Table 1). Featuring rail-to-rail
inputs and outputs that remain stable while driving 500pF,
these op amps are useful for driving both improperly
terminated cables and the capacitive inputs of
analog-to-digital converters. The features that allow them to
drive heavy capacitive loads also allow them to maintain good
large-signal voltage gain, even with heavy resistive loads.

Open-Loop Gain vs. Output Swing
As is true for all op
amps, the open-loop gain for a rail-to-rail output amplifier
is a function of the output voltage swing. Thus, to evaluate a
rail-to-rail output amplifier, you must specify the gain both
at a given output voltage and with a given load. Maxim
specifies gain this way, but not all vendors include such data
in their data sheets. For example, an op amp may have 106dB of
open-loop gain and the capability to drive a 250Ω load to
within 125mV of the rails, but it may not be able to exhibit
those capabilities at the same time. The MAX4122–MAX4129 data
sheet, for instance, properly specifies large-signal voltage
gain and output voltage swing in its Electrical
Characteristicstable (Figure 6). Large-signal
voltage gain vs. output voltage and load graphs for these
devices are shown in Figure 7.

Figure
6. A proper specification for large-signal voltage gain
includes both the load and the output voltage swing. Output
voltage swing is a function of the load being
driven.

Figure
7. These graphs show a dependence of gain on the load and the
output voltage swing for rail-to-rail output amplifiers.

Charge-Pump Op Amps
The MAX4162 op-amp family
illustrates a novel approach to the problems of the standard
rail-to-rail output stage. These op amps have a classic
emitter-follower output stage, but achieve rail-to-rail
outputs with an internal charge-pump converter that provides
internal supply voltages to bias the output stage. The
charge-pump converter also provides power to the amplifier's
other stages. Thus, the input stage has a standard
ground-sensing configuration, but allows inputs to swing from
ground to VCC. Specifications for this family are
listed in Table 1. Each device draws only 35µA (including the
charge-pump converter) while providing a 200kHz bandwidth.
Supply currents are low, but these amplifiers can drive
relatively heavy loads of 20kΩ and 500pF.

Because a charge
pump enables the construction of op amps with standard
input and output structures, such amplifiers can offer
performance superior to that of rail-to-rail op amps.
Charge-pump op amps have very good common-mode rejection, and
their single input-transistor pair is not prone to the
offset-voltage changes caused by switchover between input
pairs. In addition, the classic emitter-follower output stage
provides high open-loop gain, even with a relatively heavy
resistive load. It also allows the amplifier to remain stable
while driving large capacitive loads.

General Issues
Single-supply operation also aggravates
the problems of noise, biasing, and distortion.

Noise
Single-supply applications are generally low
voltage, and lower supply rails force the designer to make a
corresponding reduction in noise just to maintain the system's
signal-to-noise
ratio. Unfortunately, low-voltage operation usually goes
hand-in-hand with low-power operation, and as supply current
decreases, amplifier noise tends to increase. All else being
equal, a lower noise amplifier requires higher power
dissipation.

To evaluate op-amp noise, consider all
noise sources: input voltage noise, input current noise, and
thermal noise caused by the gain-setting resistors. Figure
8 illustrates these noise sources with a voltage-feedback
op amp. C1 is stray capacitance at the op amp's inverting
input, C2 limits noise gain and signal bandwidth at the higher
frequencies, and R1/R2 are the standard gain-setting
resistors. R3 balances the resistances seen by the inverting
and noninverting inputs.

Figure
8. Major noise sources in a voltage-feedback op amp are as
shown.

At low frequencies, the noise gain is given
by 1 + R2/R1 (Figure 9). The noise gain sees its first
zero at a frequency given by 1/2ΠR1C1, then increases at 6dB
per octave until it hits the pole caused by C2. At this pole
(1/2ΠR2C2), the noise gain is flat and equal to 1 + C1/C2.
Noise gain then intercepts the open-loop gain of the amplifier
and rolls off at 6dB per octave (the standard single-pole
rolloff of the amplifier's open-loop gain).

Figure
9. This graph shows noise gain and open-loop gain for the
amplifier in Figure 8.

Because the input voltage
noise, noninverting current noise, and noise due to R3 are
integrated over the entire closed-loop bandwidth and
multiplied by the circuit's noise gain, you can see (from the
plots of noise gain and open-loop gain) that circuit noise can
be minimized by choosing an op amp with a lower
unity-gain-crossover frequency. For the inverting input,
current noise and the thermal noise due to R1 and R2 are
integrated only over the signal bandwidth (1/2ΠR2C2). Since
capacitor C2 is not present for current-feedback op amps,
noise for those types is integrated over the entire
closed-loop signal bandwidth.

Distortion
An amplifier's loop gain minimizes the
distortion that would otherwise result from nonlinearities in
its input-to-output transfer
function. Because amplifier gain falls off at higher
frequencies, the amplifier's harmonic
distortion increases.

Thus, for a given frequency,
an op amp can achieve superior harmonic performance if it
operates in its more linear
region, with maximum loop gain. This usually means biasing the
output away from the supply rails, as in Figure 4 (which
introduces signal inversion and offset) or Figure 10
(which introduces offset but no signal inversion).

Figure
10. Providing both gain and offset to the input signal, this
circuit biases the output voltage away from the supply
rails.

The inverting method shown in Figure 4
eliminates common-mode nonlinearities by keeping the
common-mode input voltage constant. This feature is
particularly useful for rail-to-rail input amplifiers, whose
nonlinearities are produced by changes in the common-mode
input (as the input stage shifts from one input pair to the
other).

Focus again on the output stage. A light load
will improve the harmonic performance of rail-to-rail
amplifiers, because gain is a function of load current. An
amplifier's voltage excursion also affects distortion. All op
amps tend to benefit from loads that require a minimal voltage
excursion (internal nodes don't have to travel too far, so
they tend to remain in their linear regions). An amplifier's
slew rate, which is related to full-power bandwidth, also
affects harmonic distortion. In running the amplifier above
its full-power bandwidth, the associated slew-rate limitations
cause severe nonlinearities.

Generating a Second Supply
High-performance,
single-supply op amps are becoming more common, but to
maximize performance you must sometimes choose a dual-supply
amplifier. The selection of dual-supply types is greater
because dual-supply systems have been available longer, and
dual-supply op amps are not designed with the same
restrictions as their single-supply cousins.

Countless
methods are available for generating a negative supply from an
existing positive one. Switching regulators are the most
flexible, but charge-pump voltage converters offer the
simplest, smallest, and cheapest alternative. Because
charge-pump converters provide voltage conversion with
external capacitors rather than inductors, they excel at
providing integer multiples of the input voltage
(-VIN, +2VIN, etc.). Their output
voltages are typically unregulated, but if load currents are
relatively light, the output voltages remain fairly close to
an integer multiple of the input.

Because charge-pump
converters can have very low quiescent
supply currents, they can be highly efficient under light
loads. In Figure 11, a charge-pump converter is
configured to generate a negative voltage that is equal to the
input in magnitude but opposite in polarity. Pin-strap options
set the internal oscillator frequency at 13kHz, 100kHz, or
250kHz, allowing the designer to trade off quiescent current
consumption, charge-pump capacitor size, or output voltage
ripple.

Figure
11. Simple, small, and inexpensive charge-pump converters can
readily generate a negative supply rail from a positive
one.

Related parts

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Download,
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APP 656: Apr 03, 2000

AN656, AN 656, APP656,
Appnote656, Appnote 656

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