IC OP AMPS


Birth of the Monolithic IC Op Amp

The first generally recognized monolithic IC op amp was from Fairchild Semiconductor Corporation (FSC), the μA702. The μA702 was designed by a young engineer, Robert J. (Bob) Widlar. As will be seen, Bob Widlar was a man who was shortly to make an indelible mark on the IC world. But, his 1963 μA702 didn’t exactly take the world by storm. It wasn't well received, due to quirky characteristics— odd supply voltages, low input/output swings, low gain, etc. Nevertheless, despite these shortcomings, the μA702 established some important IC design trends. As pioneered by Bob Widlar, these concepts were to carry over to future op amps (Reference 1: Bob Widlar, "Design Techniques for Monolithic Operational Amplifiers," IEEE Journal of Solid-State Circuits, Vol. SC-4, August 1969 (Design methods used in μA709, LM101A and LM108)). In fact, they are standard linear IC design concepts yet today. While the μA702 isn't covered in detail here, information on it can be found in Reference 2(Reference 2: "The μA702 Wideband Amplifier," Chapter 5 within James N. Giles, Editor, Fairchild Semiconductor Linear Integrated Circuits Handbook, Fairchild Semiconductor, 1967, pp. 33-55. (A detailed design and application discussion of the μA702 IC op amp)).

The μA709

Not long after the μA702 a major IC op amp landmark came about, specifically the introduction of another Bob Widlar op amp for Fairchild in 1965, the μA709, (Reference 3: R. J. Widlar, "A Unique Circuit Design for a High Performance Operational Amplifier Especially Suited to Monolithic Construction," Proceedings of the NEC, Vol. XXI, October 1965, pp.85-89 (The uA709, the first widely used IC op amp)). The 709 (Although the original Fairchild designation was “μA709," the design was broadly second-sourced. The widely-used generic name became simply “709”. Likewise, the μA702 is known as the “702”) improved markedly on the 702; it had higher gain (45,000 or ~94dB), greater input/output ranges (±10V), lower input current (200nA) and higher output current, and operated from symmetrical power supplies (±15V). The 709 quickly became a standard, and was produced for decades. Figure 15 below is a 709 schematic.
The μA709 monolithic IC op amp
Figure 15: The μA709 monolithic IC op amp
So universal was the 709 that it can be regarded as an IC op amp classic. Although the individual specifications were surpassed by many subsequent designs, the 709 remains a milestone, as the first widely used monolithic IC op amp.
Many design principles from the 702 were used again in the 709, such as the use of matched transistors, for the first and second stages, and the logarithmic biased (delta-VBE) current source, Q10-Q11. There were also new wrinkles added. Because the 709 used what was basically an NPN IC process, Widlar resorted to some clever tricks to create PNP functions. He used a modified NPN structure for two PNPs, the level shifter Q9 and the output PNP, Q13. The output stage operated class-B, with no Q13-Q14 bias. Local feedback around this stage via R15 minimized deadzone.
Frequency compensation for the 709 was achieved with two RC networks, between pins 1-8, and pins 6-5. The associated network values could be changed for optimum AC response, using 4 networks for gains of 0 to 60dB.
Although the 709 was a vast improvement over the 702, it still had quirks of its own, and these gave rise to application problems. For example, without some user-added series resistance, the output stage could blow out for sustained shorts. Many saw the frequency compensation scheme as difficult, plus it took up board space. Also, the 709 could latch up, whenever the input CM voltage rose high enough to saturate the input stage. And, excessive differential input voltages could blow out the input transistors. Although savvy users could work around these 709 application quirks, it sometimes took extra parts to do it. So, in one sense the above use-related issues served as a general lesson towards the necessity of bullet-proofing an IC op amp against various application stresses.
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