OP AMP STRUCTURES

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This section describes op amps in terms of their structures, and Section 1-4 discusses op amp specifications. It is hard to decide which to discuss first, since discussion of specifications, to be useful, entails reference to structures, and discussion of structures likewise requires reference to the performance feature that they are intended to optimize.
Since the majority of readers will have at least some familiarity with operational amplifiers and their specifications, we shall discuss structures first, and assume that readers will have at least a first-order idea of the definitions of the various specifications. Where this assumption proves ill-founded, the reader should look ahead to the next section to verify any definitions required.
Because single-supply devices permeate practically all modern system designs, the related design issues are integrated into the following op amp structural discussions.
Single-Supply Op Amp Issues
Over the last several years, single-supply operation has become an increasingly important requirement because of market demands. Automotive, set-top box, camera/cam-corder, PC, and laptop computer applications are demanding IC vendors to supply an array of linear devices that operate on a single-supply rail, with the same performance of dual supply parts. Power consumption is now a key parameter for line or battery operated systems, and in some instances, more important than cost. This makes low-voltage/low supply current operation critical; at the same time, however, accuracy and precision requirements have forced IC manufacturers to meet the challenge of "doing more with less" in their amplifier designs.
In a single-supply application, the most immediate effect on the performance of an amplifier is the reduced input and output signal range. As a result of these lower input and output signal excursions, amplifier circuits become more sensitive to internal and external error sources. Precision amplifier offset voltages on the order of 0.1mV are less than a 0.04 LSB error source in a 12-bit, 10V full-scale system. In a single-supply system, however, a "rail-to-rail" precision amplifier with an offset voltage of 1mV represents a 0.8LSB error in a 5V fullscale system (or 1.6LSB for 2.5V fullscale).
To keep battery current drain low, larger resistors are usually used around the op amp. Since the bias current flows through these larger resistors, they can generate offset errors equal to or greater than the amplifier's own offset voltage.
Gain accuracy in some low voltage single-supply devices is also reduced, so device selection needs careful consideration. Many amplifiers with ~120dB open-loop gains typically operate on dual supplies— for example OP07 types. However, many single-supply/rail-to-rail amplifiers for precision applications typically have open-loop gains between 25,000 and 30,000 under light loading (>10kΩ). Selected devices, like the OP113/213/413 family, do have high open-loop gains (>120dB), for use in demanding applications. Another example would be the AD855x chopper-stabilized op amp series.
Many trade-offs are possible in the design of a single-supply amplifier circuit— speed versus power, noise versus power, precision versus speed and power, etc. Even if the noise floor remains constant (highly unlikely), the signal-to-noise ratio will drop as the signal amplitude decreases.
Besides these limitations, many other design considerations that are otherwise minor issues in dual-supply amplifiers now become important. For example, signal-to-noise (SNR) performance degrades as a result of reduced signal swing. "Ground reference" is no longer a simple choice, as one reference voltage may work for some devices, but not others. Amplifier voltage noise increases as operating supply current drops, and bandwidth decreases. Achieving adequate bandwidth and required precision with a somewhat limited selection of amplifiers presents significant system design challenges in single-supply, low-power applications.
Most circuit designers take "ground" reference for granted. Many analog circuits scale their input and output ranges about a ground reference. In dual-supply applications, a reference that splits the supplies (0V) is very convenient, as there is equal supply headroom in each direction, and 0V is generally the voltage on the low impedance ground plane.
In single-supply/rail-to-rail circuits, however, the ground reference can be chosen anywhere within the supply range of the circuit, since there is no standard to follow. The choice of ground reference depends on the type of signals processed and the amplifier characteristics. For example, choosing the negative rail as the ground reference may optimize the dynamic range of an op amp whose output is designed to swing to 0V. On the other hand, the signal may require level shifting in order to be compatible with the input of other devices (such as ADCs) that are not designed to operate at 0V input.
Very early single-supply "zero-in, zero-out" amplifiers were designed on bipolar processes, which optimized the performance of the NPN transistors. The PNP transistors were either lateral or substrate PNPs with much less bandwidth than the NPNs. Fully complementary processes are now required for the new-breed of single-supply/rail-to-rail operational amplifiers. These new amplifier designs don't use lateral or substrate PNP transistors within the signal path, but incorporate parallel NPN and PNP input stages to accommodate input signal swings from ground to the positive supply rail. Furthermore, rail-to-rail output stages are designed with bipolar NPN and PNP common-emitter, or N-channel/P-channel common-source amplifiers whose collector-emitter saturation voltage or drain-source channel on-resistance determine output signal swing, as a function of the load current.
The characteristics of a single-supply amplifier input stage (common-mode rejection, input offset voltage and its temperature coefficient, and noise) are critical in precision, low-voltage applications. Rail-rail input operational amplifiers must resolve small signals, whether their inputs are at ground, or in some cases near the amplifier's positive supply. Amplifiers having a minimum of 60dB common-mode rejection over the entire input common-mode voltage range from 0V to the positive supply are good candidates. It is not necessary that amplifiers maintain common-mode rejection for signals beyond the supply voltages. But, what is required is that they do not self-destruct for momentary overvoltage conditions! Furthermore, amplifiers that have offset voltages less than 1mV and offset voltage drifts less than 2μV/°C are also very good candidates for precision applications. Since input signal dynamic range and SNR are equally if not more important than output dynamic range and SNR, precision single-supply/rail-to-rail operational amplifiers should have noise levels referred-to-input (RTI) less than 5μVp-p in the 0.1Hz to 10Hz band.
The need for rail-to-rail amplifier output stages is also driven by the need to maintain wide dynamic range in low-supply voltage applications. A single-supply/rail-to-rail amplifier should have output voltage swings that are within at least 100mV of either supply rail (under a nominal load). The output voltage swing is very dependent on output stage topology and load current.
Single-supply op amp design issues
Figure 1-20: Single-supply op amp design issues
Generally, the voltage swing of a good rail-to-rail output stage should maintain its rated swing for loads down to 10kΩ. The smaller the VOL and the larger the VOH, the better. System parameters, such as "zero-scale" or "full-scale" output voltage, should be determined by an amplifier's VOL (for zero-scale) and VOH (for full-scale).
Since the majority of single-supply data acquisition systems require at least 12- to 14-bit performance, amplifiers which exhibit an open-loop gain greater than 30,000 for all loading conditions are good choices in precision applications. Single-supply op amp design issues are summarized in Figure 1-20 above.
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