The OP27 and OP37


As noted above, the OP07 lineage also included other related devices. Two such op amps, also designed by George Erdi at PMI, were the OP27 and OP37. These devices were released in 1980 (References 30 and 31: George Erdi, Tom Schwartz, Scott Bernardi, and Walt Jung, "Op Amps Tackle Noise-and for Once, Noise Loses," Electronic Design, December 12, 1980 (The OP27 and OP37 IC op amps) and George Erdi, "Amplifier Techniques for Combining Low Noise, Precision, and High-Speed Performance," IEEE Journal of Solid-State Circuits, Vol. SC-16, December, 1981 pp. 653-661 (Design techniques of the OP27 and OP37 IC op amps)). Figure 23 below is a simplified schematic of the OP27 and OP37 op amps.

From the apparent similarity to the OP07 schematic of Fig. 22, it might be easy to conclude that this amplifier was an adaptation of the OP07. However, the similarity ends in the fact that they are both three-stage amplifiers, and in truth the two different designs have been optimized with different end applications in mind.
The OP27 and OP37 monolithic IC op amps
Figure 23: The OP27 and OP37 monolithic IC op amps

In the design process of OP27/37, an examination of various noise sources was done, and the three-stage architecture is biased with the goal of both lower input noise, and higher speed (see Reference 31, again). Thus the stage operating currents are higher vis-à-vis the OP07, and provision for a decompensated version was also done (the OP37, stable at a gain of five). This was achieved by making the compensation cap C1 smaller on the OP37 version, while the basic OP27 is stable at unity gain. Towards the lower input noise, the current-limit protection resistors in series with the inputs were also removed.

The OP27 did achieve the goals of lower noise and greater speed, with an input noise density of 3.0nV/√Hz at 1kHz, a 1/f corner of 2.7Hz, while the slew rate was 2.8V/μs and unity gain-bandwidth was 8MHz. While realizing these new AC performance levels, the OP27/37 also retained impressive DC specifications as well. With a zener-zapped trim to the first stage, the offset was 25μV(max), drift was 0.6μV/°C(max), and voltage gain was typically 126dB. The OP27 and OP37 went on to also become widely second-sourced, and became standard devices for use as low noise, high DC precision amplifiers.

Single-Supply and Micro-Packaged OP07 Compatibles

It would be understandable for many to conclude that the high DC precision represented by the better performing versions of the OP07 and OP27 class devices would be sufficient for most applications. More recently however, the ground rules have changed.

While the high precision is still often sought, amplifier versions with single-supply capability are now in demand, as are tiny and even more tinier packages. The traditional chip designs of the OP07/27 generation often can't work in new applications, either because the circuit demands single-supply operation, and/or the package size is incompatible with the large chip size of the older products.
The relative scale of some modern IC op amp packages
Figure 24: The relative scale of some modern IC op amp packages

The small relative scale of some of these modern IC packages is shown in Figure 24, above. In the upper row, the decreasing size going from the 14 pin SOIC at the right to the SC-70 package at the left is quite clear. In the bottom portion of the figure, the SC-70 and SOT-23 packages are shown in another perspective, relative to a US one cent piece.

Two very recent OP07 lineage devices from ADI address these two issues. One is the OP777 op amp series, which includes the OP777 (single), the OP727 (dual) and OP747 (quad) devices (Reference 32: “Precision Micropower Single-Supply Op Amps Have 100-μV max Offset,” Analog Dialogue, Vol. 34, No. 4 (The OP777, OP727, and OP747 precision single and dual supply, rail-rail output single/dual/quad IC op amps)). Designed by Derek Bowers and released in 2000, these new devices feature rail-to-rail CMOS output stages, a ground sensing bipolar PNP input stage, and a 270μA operating current. These designs operate over a supply range of 2.7-30V, in MSOP, SOIC and TSSOP packages.

Even more recent is the OP1177 series, also designed by Derek Bowers and released in 2001. This series includes the OP1177 (single), the OP2177 (dual) and OP4117 (quad) devices (Reference 33: Data sheet for "Precision Low Noise Low Input Bias Current Operational Amplifiers OP1177/OP2177," http://www.analog.com). This design series has a slightly higher operating current than the OP777 series, at 400μA per amplifier, and it operates from dual supplies of ±2.5 to ±15V. While not aimed at single-supply applications, this design does offer a wide range of small packages, with specifications applicable over a –40 to +125°C range.

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