# 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.

- **Patent:** US 3475623
- **Original title:** Electronic high-pass and low-pass filters employing the base to emitter diode resistance of bipolar transistors
- **Owner:** ROBERT A MOOG
- **Granted:** 1969
- **Status:** Public domain (expired)
- **Times cited:** 7
- **Field:** consumer_electronics, mechanical

## What it does

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.

## 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.

## 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.

## Real-world examples

1. Moog Minimoog synthesizer
2. Moog modular synthesizer systems
3. Eurorack modular filter modules based on the ladder design

## Why it matters

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.

## 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.

**Full plain-English explainer:** https://patentbrief.org/patent/us/3475623/moog-synthesizer-ladder-filter

**Original patent:** https://patents.google.com/patent/US3475623

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_Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._


## Related patents

Semantically similar inventions in the PatentBrief corpus:

- [The Invention of the Transistor](https://patentbrief.org/patent/us/2524035/point-contact-transistor) — Bell Labs' 1950 patent for the point-contact transistor, the fundamental electronic component that makes all modern computing possible.
- [How the Theremin Makes Music Without Touching Anything](https://patentbrief.org/patent/us/1661058/theremin-leon-theremin) — Leon Theremin's 1928 patent for an electronic musical instrument that generates sound based on the proximity of a performer's hands to metal antennas.
- [How Laurens Hammond Invented the Electric Organ](https://patentbrief.org/patent/us/1956350/hammond-organ) — Laurens Hammond's 1934 patent for an electrical musical instrument that used spinning tone wheels to generate sound, forming the basis of the iconic Hammond organ.
- [The Invention of the Junction Transistor](https://patentbrief.org/patent/us/2569347/junction-transistor) — William Shockley's 1951 patent for the junction transistor, the fundamental building block of all modern digital electronics.
- [The Invention of the Modern Field-Effect Transistor](https://patentbrief.org/patent/us/3102230/mosfet-field-effect-transistor) — This 1960 patent describes the fundamental structure of the MOSFET, the tiny electronic switch that powers every modern computer processor.