Electret Mic
An electret condenser microphone converts sound pressure into a small electrical signal. Sensitivity, frequency response, noise, supply requirements and package size belong to the exact capsule, not to the word electret.
In the capacitive construction described by TI, sound changes the spacing between electrodes. With approximately fixed charge, changing capacitance changes voltage. An internal JFET buffers this high-impedance signal; its gate voltage changes the drain-source current. A common two-terminal capsule exposes the bias/output connection and return, not a two-terminal JFET. Follow the capsule's specified bias circuit.
In plain terms
Think of the electret as a tiny pre-charged measuring spring. Sound changes the spacing of the sensing plates, while powered electronics help read the change. Stored polarization is not a battery for the preamp.
Why designers use it
- Provides a compact sound-input element for an analog audio circuit.
- Stores its polarization charge without requiring a separate external polarization supply.
- Can be combined with a bias network and preamplifier selected for the required signal level and noise.
Best for
- Analog voice input
- Portable recording
- Microphone preamplifier projects
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- When its sensitivity, noise, acoustic overload or frequency response cannot meet the recording task.
- When the circuit cannot supply the specified capsule bias. Stored electret charge does not power its internal JFET.
- When assembly or environmental conditions exceed the exact capsule's limits.
Common mistakes
- Copying a bias resistor without checking the supply voltage and capsule operating voltage/current.
- Calling a two-terminal capsule a two-terminal JFET, or assuming every capsule uses the same internal circuit.
- Assuming every electret capsule tolerates a standard reflow profile or has the same charge-retention lifetime.
Where you will find it
- TI's TIDU765 reference design uses an electret capsule with a single-supply preamplifier; the bill of materials identifies POM-3535P-3-R. The design calculates microphone bias from the capsule's operating voltage and current, then removes the output's DC component with AC coupling.
- The 2009 Acoustics Today history describes Sony's use of electret microphones in portable tape recorders. This is a documented historical application, not a claim about the internals of a current phone or laptop.
A short history
Gerhard Sessler and James West developed the modern electret condenser microphone at Bell Laboratories in the early 1960s. Their 1962 publication described a self-biased condenser microphone, using stored charge rather than an external polarization supply. The Acoustics Today history recounts the charged metallized Teflon membrane and subsequent use in portable tape recorders. The capsule's signal-reading electronics still require suitable bias.
Good to know
- The sensing element and the readout electronics have different jobs: stored charge polarizes the capacitor, while the JFET helps read its high-impedance signal.
- A two-terminal capsule can contain a three-terminal transistor. Internal connections are not the same thing as externally accessible pins.