Let's begin with some broad observations. Precision amplifiers, originally designed for test and measurement equipment, recently migrated to sensor monitoring in automotive and industrial applications. The latest performance-monitoring subsystems in cars and factories need the precision amps' low-input offset voltage and offset current with low temperature coefficients and noise characteristics.
High-speed amps, traditionally boasting at least 50 MHz of bandwidth and at least 100-V/µs slew rates, can be found in high-speed signal chains and analog-to-digital converter (ADC) drive circuits. And with the advent of high-definition television (HDTV), video amps have moved into high-speed territory.
In audio power amps, the venerable Class AB push-pull architecture is being superseded by its bridge-tied load (BTL) cousin. At the same time, super-efficient Class Ds also are challenging push-pull in applications where electromagnetic interference (EMI) from pulse-switching can be tolerated or mitigated.
PRECISION OP AMPS Automotive OEMs want performance at lower prices than what precision amps used to go for. That means chip makers have had to figure out ways to achieve the same precision they got with ±15 V with only ±5 V or even ±3 V (Fig. 1). This is driving lots of innovation in architectures, trimming techniques, and to some extent, integration in terms of additional circuitry on the die to handle filtering or calibration, auto-zeroing, and digital trimming.
Thanks to shrinking CMOS device geometries, chip makers sometimes can add extra power supplies right on the chip. In certain designs, this permits input voltage swings greater than rail-to-rail. In other cases, negative supplies make it possible to drive resistive loads like headphones without incurring quiescent dc losses by driving ±1.5-V levels.
Also, channel counts in medical systems, from CT and MRI to basic obstetric ultrasound machines, are quadrupling. In terms of volume, amplifiers must keep pace with ADCs. In terms of processes, the 0.25-µm generation seems to be the sweet spot. No one is diving deeply into very fine-geometry CMOS.
HIGH-SPEED OP AMPS In the high-speed arena, demand grows for low-power video amps in portable applications. HDTV pushes amplifier performance, but with simultaneous downward price pressures because TV is a consumer-based business. That downward pressure on prices is something new for high-speed amps, where customers once were inured to paying premium prices for premium performance.
There's little CMOS in high-speed amplifiers. National Semiconductor has announced its VIP50 process, and Texas Instruments stands by its SiGe BiCOM-III. Analog Devices finds no problem in meeting market-mandated price points with one or another of its many bipolar processes.
Then there's dis-integration. As system digital content expands, it often makes sense to move an amplifier that was once integrated into an application-specific standard product (ASSP) off the ASSP and onto the board. It's a case of intelligent partitioning. For example, cell phones may have an MP3 decoder on a baseband chip set in some ultra-fine geometry, while a headphone amplifier makes more sense in a separate 0.25-µm chip.
AUDIO POWER AMPS Class D audio amps are beginning to reflect a move from pulse-width modulation (PWM) to pulse-density modulation (PDM) with a delta-sigma modulator on the front end.
Where PWM modulates the width of the pulses that drive the power FETs in the amplifier's output bridge, PDM controls the density of fixed-width pulses. Properly controlled, PDM helps spread the noise spectrum of the Class D switcher, eliminating harmonic spikes in the frequency domain (Fig. 2). The task is to lower energy in frequency bands used for AM and FM radio and cell phones.
Like other spectrum-spreading techniques, PDM doesn't reduce the energy in spurious emissions. Parseval's Law still applies. However, the reduction in big spurs may make it easier to deal with what's left. It takes more than just PDM, though. ADI uses the amp's sigma-delta modulator to shape and direct the out-of-ban noise energy to frequency bands where it may not matter as much in EMI-sensitive applications.
Of course, how the audio subsystem is built is as critical as the digital modulation method. Shaping the out-of-band noise energy is good, but subsystem implementation techniques with power-supply filtering and speaker-wire shielding are also necessary. The guiding philosophy is that conducted EMI eventually will become radiated EMI if nothing is done to shield the conductors.
Other trends include the holistic integration of a Class D amp, codec, and some DSP onto one chip. With so much audio now being stored and distributed in digital form, you will see more examples of the "digital-input" audio amplifier in which the only analog-signal will be the post-filter drive to the speakers or headphones.
Please do not put flashing or moving adverts alongside articles to read. They are extremely distracting.
F H Raab -April 17, 2006 (Article Rating: )
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