How Robert Moog Used Transistors to Shape Synthesizer Sounds
A 1969 invention by Robert Moog that uses the internal resistance of transistors to create the iconic filters that define the sound of analog synthesizers.
Original patent title: “Electronic high-pass and low-pass filters employing the base to emitter diode resistance of bipolar transistors”
A 1969 invention by Robert Moog that uses the internal resistance of transistors to create the iconic filters that define the sound of analog synthesizers. Granted to ROBERT A MOOG in 1969 with 7 forward citations, and it is now in the public domain.
Key facts
Coverage
What does this patent actually cover?
This patent describes a method for building electronic filters by utilizing the base-to-emitter diode resistance of bipolar transistors. By controlling the current flowing through these transistors, the engineer can change the filter's cutoff frequency, which determines which sound frequencies are blocked or passed. This mechanism allows for the smooth, voltage-controlled adjustment of audio signals, which is essential for creating the expressive, sweeping timbres found in electronic music.
The gap
What does this patent NOT cover?
- Does not cover digital signal processing or software-based filtering algorithms.
- Does not cover passive filter circuits using only resistors, capacitors, and inductors.
- Does not cover vacuum tube-based audio filtering circuits.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
What made this novel
Moog realized that the base-to-emitter junction of a transistor acts like a variable resistor that changes based on current, allowing for a precise, voltage-controlled filter that remains stable and musical.
The Patent Drawing
Schematic visualization of the patent's claim structure. Hand-drawn diagrams in progress for each landmark patent.
Where you've seen this
Real-world examples
Moog Minimoog synthesizer
Moog modular synthesizer systems
Eurorack modular filter modules based on the ladder design
Why it matters
The bigger picture
This circuit is the foundation of the 'Moog Ladder Filter,' arguably the most famous component in the history of electronic music. It enabled the transition from bulky, unreliable modular systems to the expressive, musical synthesizers that defined the sound of 1970s rock, funk, and electronic music.
Filed
October 10, 1966
Granted
October 28, 1969
Market context
Who's building on this
Companies in this space
Moog Music continues to manufacture instruments based on this core architecture. Many boutique synthesizer companies and DIY modular synth manufacturers also produce variations of this ladder filter circuit.
Market impact
This patent helped establish the voltage-controlled synthesizer as a viable commercial product. It set a standard for analog sound design that remains a benchmark for both hardware synthesizers and software emulations today.
Claim 1 — Plain English
What this patent covers
This patent describes a method for building electronic filters by utilizing the base-to-emitter diode resistance of bipolar transistors. By controlling the current flowing through these transistors, the engineer can change the filter's cutoff frequency, which determines which sound frequencies are blocked or passed. This mechanism allows for the smooth, voltage-controlled adjustment of audio signals, which is essential for creating the expressive, sweeping timbres found in electronic music.
The clever bit
Moog realized that the base-to-emitter junction of a transistor acts like a variable resistor that changes based on current, allowing for a precise, voltage-controlled filter that remains stable and musical.
What it does not cover
- Does not cover digital signal processing or software-based filtering algorithms.
- Does not cover passive filter circuits using only resistors, capacitors, and inductors.
- Does not cover vacuum tube-based audio filtering circuits.
Patent Journey
From filing to expiry
PatentBrief Score
Impact Score
Limited data
Citation count
18/40
Early citations
Claim breadth
0/20
Narrow claimsclaimsThe numbered statements at the end of a patent that legally define what the inventor owns.Read more →
Recency
0/20
Older than 20 years
Assignee scale
0/20
Independent or smaller assigneeassigneeThe entity that owns the patent — usually the inventor's employer or a company.Read more →
PatentBrief Impact Score — based on citation count, claim breadth, recency, and assignee scale. Not a legal assessment.
Heuristic Value Estimate
What this patent might be worth
$7K – $22K
Midpoint $13K · expired or expiring · industry ×1.4
Heuristic only — blends forward/backward citation counts, claim scope, time remaining, litigation history, and CPC-derived industry baseline. Real valuations need a professional appraisal.
Concepts involved
Citations
Patent lineage
Cite this patent
Moog, R. A. (1969). How Robert Moog Used Transistors to Shape Synthesizer Sounds (U.S. Patent No. 3,475,623). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/3475623/moog-synthesizer-ladder-filter
Auto-generated from the patent record. Double-check author order and the issue date against the official USPTO document before submitting.
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Common Questions
Frequently Asked Questions
What does How Robert Moog Used Transistors to Shape Synthesizer Sounds cover?
A 1969 invention by Robert Moog that uses the internal resistance of transistors to create the iconic filters that define the sound of analog synthesizers.
Who owns patent US 3475623?
ROBERT A MOOG owns this patent, granted in 1969.
When does this patent expire?
This patent has expired and is now in the public domain — anyone can use the invention freely.
What is patent US 3475623 cited by?
This patent has been cited by 7 later patents that build on its ideas.
What problem does this patent solve?
This circuit is the foundation of the 'Moog Ladder Filter,' arguably the most famous component in the history of electronic music. It enabled the transition from bulky, unreliable modular systems to the expressive, musical synthesizers that defined the sound of 1970s rock, funk, and electronic music.
What does this patent NOT cover?
Does not cover digital signal processing or software-based filtering algorithms.
Patent monitoring