No doubt, CMOS amplifiers will continue to improve and gain more territory, but some applications' requirements are just far too tough. CB or some other bipolar process will continue to serve those applications. Lately, National Semiconductor has boosted the speed and noise performance of CB transistors by incorporating full dielectric isolation techniques to significantly reduce the parasitic capacitance of these transistors. They also offer a bandwidth of 240 MHz with high output-drive capability and a quiescent current of only 4.5 mA. The dynamic range is over 90 dB with low distortion and noise. As the process improves further, expect even better performing op amps in the year ahead.
Other major CB backers include Analog Devices, Texas Instruments' Burr-Brown Division (www.ti.com), and Elantec Semiconductor (www.elantec.com). Analog Devices, for instance, has refined its XFCB-12 process to give npn and pnp transistors a big boost in speed with a substantial cut in power consumption. The company will use new differential-amplifier topologies to exploit the advantages of silicon-on-insulator CB to deliver unprecedented IF and RF dynamic ranges. Distortion floors will drop to 100 dBc at 100 MHz. Others also continue to improve their respective CB processes to crank out high-performance line driver, video amplifier, and other high-end op amps.
To deliver high-speed data over fiber, cable, and phone lines, highly power-efficient, high-speed bipolar amplifiers will tap the benefits of JFET transistors at the amp's input. The result will be an unmatched combination of high speed, low noise, and several orders of magnitude lower supply currents.
Burning power to obtain high slew rates in voltage feedback (VFB) amplifiers is no longer acceptable. Novel circuit techniques offer very high slew rates of around 3000 to 4000 V/µs at less than a 5-mA supply current. Suppliers are looking forward to doubling this performance at sub-10-mA current levels. In addition, differential input and output is in vogue for VFBs.
Current-feedback amplifiers have adequately served the needs of ADSL and cable modem lines that require high slew rates with wide bandwidths and high gain. However, at low currents (2 mA), system designers have been struggling with the dependence of bandwidth on gain for some time. Modifying the input buffer, engineers at TI's Burr-Brown division have successfully unshackled the gain from the bandwidth. Also, the output stage of these op amps has been improved to handle large voltage swings at high output currents.
Dual-channel versions also are in the works and should be available this year. While the current introductions employ dual 12-V supplies, the trend is clearly toward a single 5-V supply. Currently, these parts use a CB process, but they're migrating to SiGe biCMOS technology.
At ISSCC next month, researchers from Analog Devices will detail a precision op amp that uses autozeroing and chopping techniques to achieve a 3-mV offset with input noise of only 20 nV/√MHz. It includes additional circuitry to reduce switching transients. It's implemented in a 0.6-mm double-poly, double metal CMOS process to keep power consumption at 4 mw from a 5-V supply.
See associated timeline.