Lange Couplers and Their Use in RF/Microwave System Design

SemiGen, Inc., an ISO and ITAR registered provider of RF and microwave contract assembly, foundry services, semiconductor devices, and RF supplies, has released a new blog detailing the development of the Lange Coupler by Dr. Julius Lange of Texas Instruments in 1969, while his group was engineering microwave amplifiers on thin film ceramic substrates. The team employed a Bell Labs approach that used 3 dB quadrature couplers, but initially struggled to find the ideal combination of low loss and wide bandwidth.
Coupled line connectors could not achieve coupling factors of 3 or 6 dB, as they were too loose. Conversely, the transistors between the interdigitated base and emitter fingers were packed too tightly. Through necessity, invention arose: Lange recalled that geometric symmetry could guarantee the 90° split between outputs known as quadrature. By only moving a portion of the crossovers to the middle, he achieved the desired loss and bandwidth targets.