In the 1930s, the earliest vacuum tube op-amps were composed of discrete components: vacuum tubes and resistors. They were fundamentally large, bulky, power-hungry devices and were used primarily for analog computation.

Figure 1 shows a K2-W general purpose computing op-amp from George A. Philbrick Researches . It was the first commercial op-amp, introduced to the market in 1952, more than a decade before the first transistorized version. In the 1960s, vacuum tube op-amps began to be replaced by miniaturized solid state op-amps (Figure 2), which consist of transistors and resistors.

Figure 1 - Philbrick K2-W Op-Amp
Figure 2 - 741 Op-Amp
Integrated Circuit
A relatively large number of transistors and resistors all on the same silicon chip. It started a new era in electronic circuit design.
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