Small-signal bandwidth in a big-bandwidth era

April 13, 2011 //By Bill Laumeister
Small-signal bandwidth in a big-bandwidth era
Bill Laumeister, Principle Member of the Technical Staff, Precision Control Group, Maxim Integrated Products Inc., examines how to take advantage of small-signal bandwidth in a big-bandwidth era.

Introduction: The popular music of several decades ago required many instruments, hence the name “big band era.” Today, bandwidth increase is again a sign of the times. The explosion of internet usage, network-capable cellular phones (3G, 4G, LTE, and WiFi), music players, and digital video cameras 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 again in a “big bandwidth era.” So, why discuss small-signal bandwidth?

Many operational amplifiers (op amps) include a specification of small-signal bandwidth in their data sheets. (All op amps have a “sweet spot” for better bandwidth, even if it’s not mentioned in the datasheet.) This specification is typically based on a signal amplitude limited to about one-tenth of a volt, and at first glance 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 (0.1V or less) of 625MHz, and a large-signal bandwidth (2V peak-to-peak) of 11MHz. Most applications make use of the large-signal bandwidth. Small-signal bandwidth is high because the op amp is operating in its mid-range sweet spot ( Figure 1 ).

Figure 1: Signal conditions determine the bandwidth through an op amp.

 Typically, the sweet spot for input signals is near one-half the power-supply voltage. The amplifier is most linear in that region, and produces the best signal quality. Op amps have a large open-loop gain, and they employ negative feedback to control the amplifier by trading this open-loop gain for stability and linearity.

As the amplifier output approaches either power rail, less feedback is available, which in turn diminishes the ability of feedback to linearize the amplifier response. As feedback is reduced outside the sweet

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