Small-signal bandwidth in a Big Bandwidth era
Understand the interplay between bandwidth and op-amp specifications.
Bill Laumeister, Maxim Integrated Products Inc -- EDN, February 2, 2012
The popular music of several decades ago typically required 12 to 25 musicians, so people refer to it as the Big Band era. Today, bandwidth increase is again a sign of the times. The explosion of Internet usage; network-capable cellular phones, including 3G, 4G, LTE (long-term evolution), and Wi-Fi units; music players; and digital videocameras has expanded consumers’ expectations for bandwidth. We are on the cusp of wholesale data transfer to all portable devices. Bandwidth has become king, and we are therefore in the “Big Bandwidth” era. So, why discuss small-signal bandwidth?Many operational amplifiers include a specification of
small-signal bandwidth in their data sheets. All op amps
have a “sweet spot” for better bandwidth, even if the data
sheet does not mention it. Manufacturers typically base this
specification on a signal amplitude of approximately 0.1V. At
first glance, this figure seems primarily for use in comparison
and for boasting rights with other op-amp companies. Some
applications, however, can take advantage of the small-signal
bandwidth, which can be many times greater than the large-signal
bandwidth for an op amp. For example, the MAX4104
op amp has a small-signal bandwidth of 625 MHz at a signal
amplitude of 0.1V or less and a large-signal bandwidth of 11
MHz at a signal amplitude of 2V p-p. Most applications use
the large-signal bandwidth. Small-signal bandwidth is high
because the op amp is operating in its midrange sweet spot
(Figure 1).
As the amplifier output
approaches either power rail, less
feedback is available, which in
turn diminishes the ability of the
feedback to linearize the amplifier
response. As feedback decreases
outside the sweet spot, the frequency response decreases, and distortion increases.Op amps that offer rail-to-rail operation use special circuit configurations to minimize distortion near the power rails. A careful reading of the data sheet for a typical rail-to-rail output, however, shows that the output current diminishes to 0A at the rails.
Modern op amps are fabricated with processes in which transistors have multigigahertz bandwidths. An op amp, however, comprises tens or hundreds of transistors, resistors, and capacitors, and the net effect of that circuit structure is to reduce the overall bandwidth—often by an order of magnitude or more.
Among the effects of this natural bandwidth reduction are
phase and amplitude errors due to stray interstage capacitance
and resistance. Bandwidth reduction limits slew rate and is
amplitude-sensitive, as you would expect. Thus, small signals
have larger bandwidths than large signals.Some applications, however, can use the small-signal bandwidth. In one such application—an impedance converter for a remote sensor—a small signal drives a relatively long cable. System requirements may include amplification of as much as 0.1V, as well as the ability to drive 50 or 75Ω coaxial cable. The first amplifier in the system usually sets the SNR. With the close relationship of bandwidth to SNR, using the small-signal bandwidth by limiting the signal amplitude may allow the use of a less expensive op amp that draws less power-supply current.
Op-amp frequency characteristics
Although bandwidth limiting detracts from an op amp’s
performance, you can leverage bandwidth limiting to get
the most from an inexpensive op amp. For instance, what if
you need to limit the signal bandwidth with a simple 1-MHz
lowpass filter? For noncritical applications, you might use an
inexpensive op amp, such as the MAX4245 (Figure 2). For
a 3-MHz lowpass filter, you could use the MAX4330 (Figure
3). For more critical applications, a Sallen-Key active filter
that precisely controls the cutoff frequency and slope is more
appropriate.You can combine the lowpass bandwidth with other functions
to reduce system cost. A precision rectifier comprising
“perfect” diodes, for example, can smooth the edges of a
signal by reducing the signal bandwidth. A perfect diode is
an op amp with a diode in the feedback loop, which produces
a diode response without the usual forward-voltage drop
(Figure 4).
A circuit that converts differential to single-ended signals
and that reduces high-frequency noise can also operate
with an inexpensive op amp. As another example, you can
construct a comparator with hysteresis—that is, a Schmitt
trigger—that ignores high-frequency noise in its threshold
voltage (Figure 5). The circuit ignores noise below the
threshold, and positive feedback latches the output state
until the circuit exceeds the opposite threshold. The op amp’s
response limits the output’s slew rate.Slow op amps tend to be inexpensive, and they can reduce system costs by combining functions that take advantage of the op amp’s native frequency response. Bias and reference circuits for power supplies can take advantage of the lowpass characteristics to decouple noise and produce clean power. Op amps can isolate circuits from other circuits and act as lowpass filters.
In Figure 6, for example, op-amp voltage followers enable
an ADC, a DAC, and other circuits to share a single voltage
reference. The followers’ high input impedance and low output
impedance isolate the various circuits from one another.
This isolation mitigates the effect of trace lengths on the
PCB and prevents crosstalk between the circuits. Because
the voltage-reference output should be one dc value, the low-bandwidth op amps enhance its quality
by acting as lowpass filters.
We are in the middle of an explosion
in communications, in which consumers
have come to expect high-speed
communication networks to be widely
available. In the United States, government
agencies have begun to consider
universal broadband availability, which
is similar to the government’s infrastructure
mandates in the 20th century. This
infrastructure—rural electrification,
universal telephone service, and the
interstate highway system—has greatly
enhanced our standard of living.When you think about wider and wider communication bandwidths, you should also think about all the systems that control that bandwidth. Circuits for equalization, channel selection, automatic gain and frequency control, and many others require slower, low-frequency control. Even in this Big Bandwidth era, low-bandwidth op amps have an established place.
Acknowledgment
This article originally appeared on EDN’s sister site, Planet Analog.
Author’s biography
Bill Laumeister is an engineer with the Precision Control Group at Maxim Integrated Products, where he works with companies using DACs, digital potentiometers, and voltage references. He has 38 years of experience and holds several patents in the video field. Laumeister is the inventor of the VEIL (video-encoded-invisible-light) communications method, which the US Congress is considering as a possible patch for the “analog hole” in the Digital Transition Content Security Act.
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