- Rockin’ Patent: US Patent No. US 3,475,623 A
- Filed: 10 October 1966
- In the name of: Robert A. Moog
- Title: Electronic High-pass And Low-pass Filters Employing The Base To Emitter Diode Resistance Of Bipolar Transistors
- What’s claimed: “An RC filter comprising a plurality of capacitors connected to provide the capacity elements of the filter and a plurality of bipolar transistors connected with said capacitors to utilize the essentially exponential resistance of the base to emitter diodes of said transistors as voltage controllable resistance elements of the RC filter.”
- Why this patent (application) rocks: This patent describes the use of bipolar transistors arranged in a ladder formation to provide a flexible filter circuit. The filter circuit was available as a module for early Moog modular synths from the 1960s and is still available to buy today. The ladder filter module serves to clean up and provide colour and clarity to a variety sound sources. The patent earned the inventor Bob Moog entry into the National Inventors Hall of Fame in 2013 (see here and here).