An op-amp is an operational amplifier consisting of multiple transistors packaged in an integrated circuit chip. It senses the fluctuating voltage difference between two inputs, uses power from an external supply to amplify that difference, and uses negative feedback to ensure that the output is an accurate replica of the input. Its amplification can be adjusted by changing the values of two external resistors. Op-amps were developed originally using vacuum tubes, for use in analog computers, before the era of digital computing. Their implementation in integrated circuits dates from the late 1960s, when chips such as the LM741 were introduced (lower-noise versions of it still being widely used today). Multiple op-amps in a single package were introduced in the 1970s. An LM741 is shown in the image. Inside the 8-pin, DIP package is a single op-amp.