Operational amplifier models

Chapter 8.14 Operational amplifier models

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 410CHAPTER 8. OPERATIONAL AMPLIFIERS8.14Operational amplifier modelsWhile mention of operational amplifiers typically provokes visions of semiconductor devicesbuilt as integrated circuits on a miniature silicon chip, the first op-amps were actually vacuumtube circuits. The first commercial, general purpose operational amplifier was manufacturedby the George A. Philbrick Researches, Incorporated, in 1952. Designated the K2-W, it wasbuilt around two twin-triode tubes mounted in an assembly with an octal (8-pin) socket for easyinstallation and servicing in electronic equipment chassis of that era. The assembly lookedsomething like this:The Philbrick Researchesop-amp, model K2-WGAP/RMODELK2-W4 inchesapprox.The schematic diagram shows the two tubes, along with ten resistors and two capacitors, afairly simple circuit design even by 1952 standards:12AX7+300 V220 kΩ220 kΩ-300 VInverting (-)inputNoninverting (+)input1 MΩ2.2 MΩ510 kΩ120 kΩ4.7 MΩ221 kΩ9.1 kΩ12AX7680 kΩ7.5 pF500 pFNE-68OutputThe Philbrick Researches op-amp, model K2-WIn case you’re unfamiliar with the operation of vacuum tubes, they operate similarly to N-