In selecting an amplifier, you must determine the maximum and minimum voltage for the part’s operation, its quiescent current, the current the op amp must deliver to the load, and any other current it uses. You might, for example, set up the two power pins for bipolar operation on split supplies or for single-ended operation by hooking the negative power pin to ground (Figure 1). Although you can connect any amplifier in a bipolar or single-ended circuit, other factors often make the part suitable for single-ended operation. In addition, the input pins almost always include ground in their input range or provide for rail-to-rail inputs, in which the input pins can operate at either extreme of the power-supply voltage. Further complicating the design is the fact that op-amp data sheets typically express specifications for single-ended operation, despite the possibility that a test engineer could change the part’s operating conditions and restate the specs to reflect bipolar operation.
The output current is a key spec. Rail-to-rail-output parts provide usable drive current even when the output pin is less than 0.6V from either power-supply rail. Parts that use FET outputs can swing closer to the rails than parts with bipolar outputs. For example, the Intersil 30-mA EL5020 can swing to within 15 mV of either rail at 5 mA. To ensure accurate, low-distortion performance, you must also understand output-pin impedance, which varies with frequency. In addition, the output pin must drive some level of capacitive loads. Some parts, such as National Semiconductor’s LM8272, drive unlimited capacitive loads, whereas typical video amplifiers oscillate with just tens of picofarads of load capacitance.
Dave Kress, director of applications engineering for Analog Devices, sees five important elements in amplifier selection (Figure 2): bandwidth, power supply, the requirement for multiple parts in a package, application, and cost. On the other hand, Tim Green, linear-applications manager at Texas Instruments’ Burr-Brown division, narrows down the criteria to three: voltage, current, and bandwidth.
However, Paul Grohe, an applications engineer at National Semiconductor, thinks more about the inside of the amp. “Bias current and bandwidth—the two Bs—are what matters,” he says. “A fast part will use more current, and a low-noise part will use more current. And, if you have a high source impedance, the input-bias current is the most important spec.”
Bob Pease, staff scientist at National Semiconductor, in a jab at the company’s competitors, notes that the spec doesn’t matter if the supplier can’t deli
ver the parts on time. He also says that noise is an often-overlooked, yet critically vital parameter. “There are no easy answers; you have to use your judgment,” he says. “In every application, there are one or two key parameters, and you have to figure out what they are. You can’t have everything.”
Tim Regan, application manager for Linear Technology’s signal-conditioning unit, uses the acronym SNAP (supply voltage and current/need for ac or dc performance/amplifier count/packaging) to help engineers remember the important trade-offs. Patrick Long, business-marketing manager for op amps and comparators at Maxim, also mentions packaging as an important criterion. If the part targets cell phones, for example, you would want to use a flip-chip or solder-bump package. These ultrasmall packages provide high performance analog functions with a board area the size of a silicon die.
One way to understand the scope of selecting an op amp is to look at the structure of the data sheet. The first page is a valuable tool that reveals key features and the intended application. By ignoring marketing adjectives, such as “slow” and “fast,” and looking for the actual speed figure, you can quickly see whether the amplifier is in the right ballpark for your application. The first page may describe the process that the manufacturer used to make the op amp (see sidebar “Op-amp processes”).
A section on absolute-maximum ratings typically follows the first page in an op-amp data sheet. This section always covers the highest voltage and temperature that you can subject the part to. It should be obvious from the prominence of this section that these parameters are critical in your selection because they are absolute-maximum values. The part cannot exceed these limits for a nanosecond.
Data sheets also include tables on dc and ac performance and on operating voltage. The tables clearly state the operating voltage that the part was running on when the designers created the tables. The first page may claim that the part works at voltages as low as 2.7V, yet the tables may show that the part can run at 3V. Although it may be acceptable to run a 3V part at 2.7V, you cannot use the specification in the 3V-data-sheet table. Either you have to ask the manufacturer to characterize the part at the lower voltages, or you have to do it yourself. The values in the tables are contractual obligations that the manufacturer must meet.
Pages of charts follow the tables in the data sheet. Although these charts do not represent a legal obligation, they are important. For example, the tables may claim a huge PSRR (power-supply-rejection ratio), whereas the charts show that this specification decreases drastically with increasing frequency. If an amplifier is operating from a 1-MHz switcher that has a 1-MHz output ripple, you must evaluate the PSRR at 1 MHz from the appropriate chart and remember that designers created the chart at a certain operating voltage that may produce more beneficial results than your circuit will produce. Similarly, the tables base voltage noise on the flat-band noise at higher frequencies. For dc or low-frequency applications, you must consult the charts to determine the noise in your circuit’s frequencies of interest (Figure 3).