Electronic Design

Amplifiers Preach Painless Performance

The most conspicuous trend in amplifiers is the list of new features that help systems engineers who aren't entrenched analog specialists avoid catastrophic design errors. Consider how it demolishes a system-development schedule when an amplifier that worked perfectly on the bench does nothing but oscillate on the prototype project board. Suppliers have designed more bulletproof amplifiers and created GUI-based design tools so those amplifiers are more apt to behave themselves in real products.

Today, built-in features include overvoltage, overcurrent, electrostatic discharge, and reverse-polarity protection. In fact, they're already the rule in automotive apps. Greater functionality can also take the form of programmable gain and offset.

Filtering for electromagnetic-interference (EMI) susceptibility is getting lots of attention from analog chip makers. But according to Steve Sockolov of Analog Devices, while companies are putting internal RF filtering on inputs, supply pads, and even outputs, the industry needs a figure of merit for the EMI susceptibility. That requires a standard test methodology used across the industry.

Without some kind of filtering, though, stray RF that's invisible without high-frequency instrumentation can be rectified by an input transistor or electrostatic-discharge (ESD) device. Ultimately, this ends up affecting offset.

On-board filtering is still needed for low-frequency EMI. But parasitics in those discrete components make their frequency response unpredictable in the multigigahertz range. The really high-frequency noise is better dealt with using on-chip components.

Suppliers are also developing new process technologies and circuit topologies to improve amplifier stability. New designs with lower input capacitance make amps easier to compensate. One new amp even has a patented output circuit that senses load characteristics and uses that feedback to control current in the output stage.

In applications such as ultrasound where higher resolution is the goal, new variable-gain amplifiers (VGAs) can automatically adapt gain to the signal return. This helps the system accommodate differing tissue densities by optimizing signal-to-noise ratio (SNR) in each channel for the medium.

Video op amps have some new applications. Flat-panel arrays feature as many as 1600 by 1200 and even 1920 by 1600 pixels. The need to service those arrays increases overall amplifier bandwidth requirements to something near 1 GHz.

While gain flatness differential phase and gain specs are still the critical specs for video amplifiers, load-driving capability is becoming very important. Even keyboard-video-mouse (KVM) switches require a strong driver with high bandwidth.

Another important video-amplifier application is in video servers for Ethernet cable installations in the enterprise, in classrooms, and even in the county courthouse. CAT5 is attractive for large installations due to its cost, which is 12 times less. But something must be done about signal degradation down the twisted pairs.

While compensation could be achieved using discretes, it takes nine plain-vanilla op amps to build an equalizer, which tends to negate the cost advantage of CAT5 over coax. The response has ranged from suppliers offering recommendation for an equalization network for their basic differential amplifiers to Intersil's EL9110, which combines everything in one chip.

Running ordinary video down one twisted pair in a CAT5 cable is one thing. Running high-resolution computer video such as 50-MHz, 1280-by-1310 SXGA becomes an entirely different matter. SXGA uses five signals, and Ethernet cable has only four twisted pairs.

Intersil tackled that in another amplifier product by encoding horizontal and vertical sync as three discrete signals of different voltage levels. Unlike ordinary differential amplifiers where an external VREF pin controls the common-mode level of the differential output, the VREF for each of the three internal amplifier channels receives a signal from a logic encoding block with the encoded sync information. The final output consists of three fully differential video signals, with sync encoded on the common mode of each of the three RGB differential signals.

Regardless of the video format, there's an unpredictable delay down each of the twisted pairs

(see the figure)

. As a result, there was also an opportunity to create a three-way analog delay-line amp. That's a fair amount of silicon, but considering the cost of coax versus CAT5, the overall savings are substantial.

Video-distribution applications are mostly designed in the U.S. In Asia, where flat panels are made, the ability to drive large capacitive loads figures prominently, and customers are asking for custom-crafted amplifiers.

One example is a gamma buffer for the column drivers in thin-film-transistor (TFT) panels requested by a Korean firm. The firm saw lot-to-lot variations in gamma (the weighting of the brightness curve) and wanted the ability to correct for those variations in the column drivers. A simple resistive divider was out of the question because of the power losses it would entail, so it made sense to shape the gamma response with the feedback network for an op amp.

A few challenges arose, though. For one thing, the amp had to be able to drive a fairly hefty capacitive load with an 18- to 20-V signal. For another, the designers had to deal with the way TFT drivers flip the polarity of each pixel on successive frames (unless the point about which the signal switches is exactly zero volts, pixel colors shift). Those types of challenges continue to make life interesting for analog amplifier suppliers.

Another evolving amplifier application involves driving the laser diodes in optical storage systems. This is another situation that needs a fair amount of drive, because the amp must drive the laser through 20 cm or so of flex cable.

Also, the signal to the laser must be shaped. It must start with a strong pulse that rapidly gets the disc's leading edge of the mark medium hot. (There's a corresponding pulse at the end of the mark.) This pulsing requires precise level control, especially as CD speed increases with each generation. Now that drive speeds are up to 16 times greater, the solution is a custom amplifier part that includes a waveform generator, some on-chip RAM to program it, and a high-speed serial interface to handle the high data rates.

The optical read-head amp has its own set of interesting design challenges. And, they will get even more intriguing as drive makers switch to blue lasers that write smaller marks and store four times as much data as today's densest drives.

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