Mason's Invariant - Origin

Origin

In 1953, transistors were only five years old, and they were the only successful, three-terminal active device. They were beginning to be used for RF applications, and they were limited to VHF frequencies and below. Mason wanted to find a figure of merit to compare transistors, and this led him to discover that the unilateral power gain of a linear two-port device was an invariant figure of merit.

In his paper Power Gain in Feedback Amplifiers published in 1953, Mason stated in his introduction,

"A vacuum tube, very often represented as a simple transconductance driving a passive impedance, may lead to relatively simple amplifier designs in which the input impedance (and hence the power gain) is effectively infinite, the voltage gain is the quantity of interest, and the input circuit is isolated from the load. The transistor, however, usually cannot be characterized so easily."

He wanted to find a metric to characterize and measure the quality of transistors since up until then, no such measure existed. Little did Mason know, however, that he would discover an equation that is still used more than 50 years later and does much more than measure the quality of a transistor.

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