Electrical engineers can be a gluttonous lot. Invariably, we want to have our cake and eat it too.

Not only do we want our systems to run faster, but we also want them to run on little or no power. That means increasing the speed and performance of all the components in a system while staying within a restrictive power budget. When it comes to operational amplifiers (op amps), achieving low power and high performance has been a daunting task until quite recently.

Traditionally, designers have faced three alternatives when they sought the right op amp for their application. The first alternative has been BiFET op amps, which have good ac performance characteristics, such as slew rates in the range of 10 ýV/ms and bandwidths around 10 MHz, but the vast majority of BiFET op amps have been optimized for systems with split power supplies, like ý5V or ý15V. In fact, BiFET op amps do not function well at low voltages and tend to consume a great deal of power. Some Bipolar op amps, the second alternative, share the BiFET's favorable ac performance characteristics and in some cases will function at low voltages. Bipolar op amps are also effective in high-precision applications. However, one big drawback to Bipolar op amps has been their low input impedances, which limit their usefulness in some applications.

The third alternative, CMOS, can be optimized for single-supply systems and, generally speaking, CMOS op amps will consume significantly less power than almost all BiFET and many Bipolar op amps. CMOS components can perform well at voltages well below +5V. It is also fairly easy for chip designers to develop op amps with rail-to-rail dynamic range using CMOS. A rail-to-rail dynamic range is a critical capability for op amps when they are used with lower supply voltages. The drawback on CMOS op amps has been their limited ac performance, which has been in the range of a couple of MHz of bandwidth and slew rates of 3 to 4 ýV/ms, approximately _ to _ BiFET performance.

At a time when the market is placing more and more emphasis on portability and low power consumption, designers of high-performance split-supply systems have faced a dilemma. Systems that were making the transition to single-supply systems could not use the older CMOS op amps because they were too slow. Single-supply op amps with extremely high bandwidths/high speeds, usually developed with a complementary Bipolar process, are available for very high-end applications like radar systems and high-resolution scanners, but the high frequencies of these devices introduce unnecessary difficulties with board layout and stability problems into mid-range designs. Therefore, designers were forced to continue using their split-supply Bipolar or BiFET op amps. In some applications, the migration from a split-supply to a single-supply design could be made without great difficulty, but where ac performance comparable to that of BiFET op amps is required, this migration is not an easy task.

Filling the Gap

Fortunately, designers aren't faced with this dilemma anymore. Advances in CMOS and BiCMOS processes are dramatically improving the performance of single-supply CMOS op amps. In recent years, process engineers have experimented with variables such as smaller transistor-channel lengths and thinner oxide layers to develop faster CMOS processes. Thanks to shrinking geometries, which are now well below one micron, the speed of CMOS op amps is increasing to serve mid-range applications. In addition, CMOS and BiCMOS processes have been optimized for low-power 5-V and 3.3-V systems, which is a distinct advantage over BiFET op amps (which cannot operate at low voltages.)

A new generation of single-supply CMOS op amps holds the promise of improved performance with higher bandwidths, much faster slew rates and lower power consumption. This development opens up a whole new set of applications for single-supply op amps that was previously not available with older CMOS devices.

Splitting the Difference

Now, during a time when digital signal processors have taken on such widespread importance, the need for single-supply op amps to front-end ADCs is at an all time high. However, most of the older CMOS op amps did not have the required performance to drive the input stages of many ADCs. Because of this, designers may have been forced to use op amps that were overkill for their applications, but these were the only devices they had available to them. New advances in CMOS op amps are producing op amps with faster slew rates, higher output drive and faster settling times. As a result, CMOS op amps are now able to drive mid-range ADCs more effectively. By using an op amp better aligned with the performance level of the system, designers will be able to squeeze some of the cost from the system.

The limited output drive of older CMOS op amps precluded their use in certain systems, but the output drive of next generation op amps will be on a par with Bipolar and BiFET devices. In certain applications, output drive can be critical. In some telecommunications applications, the op amp must be able to drive a 600-W load for impedance matching on the telephone wire. Traditionally, BiFETs have been used for this function because they provide adequate output drive. Next-generation CMOS op amps with much higher output drive will allow designers to migrate to a lower-cost single-supply design because they are able to drive 600-W loads with a rail-to-rail output swing.

Overcoming the Hazards of Lower Voltages

The industry in general is moving toward single-supply systems with lower voltages, but this trend is not without its complications. For example, signal-to-noise ratio is a very important aspect of data acquisition systems because a lower signal-to-noise ratio will compromise the accuracy or resolution of such a system. Migrating a dual-supply data acquisition system to a single-supply system will tend to decrease the system's signal-to-noise ratio, if all other factors remain constant. Using an op amp with rail-to-rail swing can help increase the signal-to-noise ratio because the amplified signal is able to use the entire supply range. Other steps can also be taken in the design of data acquisition systems to minimize the total error floor and assure a high-resolution system at lower voltages.

Fortunately for system designers, the developers of op amps are cognizant of the concerns caused by transitioning to single-supply low-voltage designs. New CMOS op amps are being optimized with lower input offset voltages, current and voltage noise, and input bias currents to alleviate some of the difficulties in this transition.

For example, older CMOS amps may have been able to drive only up to 4 V on the output. From a 5-V system, newer PROamps will drive to the rails, increasing dynamic range by 25 percent.

Mid-Range Op Amps