Electronic Design

Analog/Mixed-Signal> Amplifiers

Amplifiers Close In On The Ideal

Name the amplifier (op amps, digital amplifiers, servo amplifiers, and power amplifiers) and you'll find dramatic performance gains thanks to circuit design improvements and advances in semiconductor processing technologies. Lower power consumption, smaller package sizes, faster slew rates, wider bandwidths, and lower noise levels will drive op-amp developments. But these won't necessarily be achieved within the same package. User requirements will dictate more application-specific op amps that fit their designs better, without paying for unneeded performance parameters. While no "ideal" op amp exists, relatively low-cost, high-precision op amps with all of the right attributes are getting closer to that mark.

More lower-cost op amps will be made on all-CMOS processes. But higher-performance (and thus potentially higher cost) units will require some combination of processes involving CMOS, bipolar, silicon germanium (SiGe), and gallium arsenide (GaAs). Novel op-amp front-end architectures will further improve noise and distortion performance specifications in high-speed IC op amps.

Op amps are experiencing greater levels of digital programming for more cost-effective performance. A harbinger of this trend comes by way of Analog Devices. The AD8555 features a zero-drift bridge-sensor signal op amp with precision, gain, offset-adjustment, and fault-protection circuitry in a 4- by 4-mm package, This sets it apart from other op amps (see "Programmable Amp Merges Precision And Flexibility For Sensor Correction," Electronic Design, Nov. 24, 2003, p. 62). Such features were achieved by combining the company's low-noise Auto-zero and DigiTrim circuit techniques.

The next revolution may come from digital "Class D" amplifiers that provide users with large power savings and very small form factors. They produce a pulse-width-modulated (PWM) output train in response to an input linear audio signal. These amplifiers reduce the need for bulky metal heatsinks due to their dramatic improvement in power efficiency. Moreover, they can even use digital signal processors (DSPs) to reduce the number of output filters typically required.

Newer, more effective designs like Class T amplifiers from Tripath are also emerging. Using a proprietary technique it calls Digital Power Processing (DPL), Tripath has managed to combine the signal-fidelity advantages of discrete-component Class A and AB amplifiers with the power-conversion efficiencies of Class T amplifiers.

TOP TEN

  • MORE APPLICATION-SPECIFIC OP AMPS WILL BECOME THE NORM Expect users to ask for amplifiers with performance parameters that more closely match their application needs. This will mean that they won't have to pay for performance parameters they don't need.
  • MORE AND MORE DIFFERENTIAL-INPUT OP AMPS WILL EMERGE, particularly in high-speed op amps, as they encroach on precision op amps. High-volume applications driving this trend include communications networking and video multiplexing, which rely heavily on differential signals to achieve the required performance specifications.
  • DIGITAL AMPLIFIERS, ALSO KNOWN AS CLASS D AMPLIFIERS, WILL BE THE NEXT MAJOR DEVELOPMENT They provide substantial advantages in low-power dissipation and very small form factors. They also eliminate the need for digital-to-analog converters (DACs). One company, D2Audio, hopes that the growing popularity of home-theater "surroundsound" stereo systems with five or more speakers will secure slots for "drop in" digital amplifier modules like its XS and XP DSP-based modules.
  • MORE-POWERFUL SERVO AMPLIFIERS WITH WIDER POWER RANGES WILL EVOLVE to meet future industrial networking automation needs. One such example is the Accelnet family of panel-mounting amplifiers from Copley Controls, designed for CANopen networking automation. Other companies like Tripath are trying a hybrid DSP-based approach. Their Class T amplifiers combine the benefits of high levels of audio fidelity with the advantages of high levels of power efficiency.
  • EXPECT THE VARIETY OF LOW-POWER HEADPHONE AMPLIFIERS FOR CELL PHONES AND PDAs TO MULTIPLY Here, low-power requirements are essential, and filtering requirements can be minimized (thus making for smaller form factors). Fairchild Semiconductor, Maxim Integrated Products, and National Semiconductor are some of the companies that are perusing this area. National Semiconductor set the pace with its LMV1012/1014, the first "amps in a mic."
  • RF POWER AMPLIFIERS REQUIRING FEWER OR NO EXTERNAL COMPONENTS WILL BE DEVELOPED by exploiting the latest packaging technologies available. A recent example is a 1- to 2.4-V, 5- to 6-GHz power-amplifier module made on a low-temperature co-fired ceramic (LTCC) substrate, jointly created by designers at Germany's Siemens, Infineon Technologies, and the Technical University of Brandenburg. This module requires no external components, yet it delivers 22 dBm of output power at 1-dB compression and 36% power-added efficiency.
  • GREATER LEVELS OF DIGITAL PROGRAMMING WILL BECOME MORE WIDESPREAD IN MONOLITHIC OP AMPS, as evidenced by Analog Devices' AD8555 zero-drift bridge-sensor signal op amp. The unit combines the company's low-noise Auto-zero and DigiTrim circuit techniques for its high performance.
  • EXPECT A RUN ON IMPROVED DIGITAL AMPLIFIER DESIGNS like the TA2022 Class T amplifier from Tripath, which allows both high signal fidelity and high power efficiency. This amplifier doesn't use pulse-width-modulation (PWM) methods like others, yet it combines the benefits of PWM designs and those of pure analog approaches. Its operation is based on a DSP-based Digital Power Processing technique.
  • MORE RAIL-TO-RAIL INPUT AND OUTPUT OP AMPS WILL COME ON THE MARKET to keep pace with the constraints caused by operating from lower-supply voltages. Examples of such op amps are the EL820X and EL840X rail-to-rail output units from Intersil's Elantec Division, available in both dual and quad versions with a 23-dB bandwidth of 500-MHz and 600-V/ms slew rates, the fastest in the industry.
  • CMOS WILL BE THE DOMINANT PROCESS FOR LOW-COST GENERAL-PURPOSE OP AMPS But on the high-performance side, other processes like bipolar, silicon germanium (SiGe), and gallium arsenide (GaAs) will be called upon.
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