Mic Circuit Diagram: Electret Microphone Bias Resistor and Coupling Stage
This is a free printable mic circuit diagram: download the diagram as SVG or open it and print to paper or PDF.
A mic circuit diagram shows how to bias an electret capsule, couple the AC audio signal, and feed a low-noise amplifier stage for clean sound capture.
An electret microphone capsule contains a built-in JFET that converts mechanical diaphragm movement into a tiny electrical signal, but it cannot work without a DC supply and a drain resistor. The classic two-terminal electret bias circuit connects the capsule's positive (+) pin to a DC supply rail — typically 3 V to 12 V — through a bias resistor, while the negative (–) pin ties directly to circuit ground. The bias resistor (commonly 2.2 kΩ to 10 kΩ) sets the operating current of the internal JFET and therefore the capsule's output sensitivity and noise floor. A lower value supplies more current and slightly more headroom; a value that is too high starves the FET and raises the noise floor.
The AC audio signal appears as a small voltage variation across the capsule's output pin relative to ground. Because this output rides on a DC offset equal to roughly half the supply voltage, a coupling capacitor (typically 1 µF to 10 µF, non-polarised or polarised with correct orientation) blocks the DC component and passes only the audio AC waveform to the downstream stage.
The downstream stage is almost always a low-noise preamplifier. A single NPN transistor (such as a 2N3904 or BC547) in common-emitter configuration with a collector resistor of 10 kΩ and emitter degeneration resistor of 1 kΩ provides 20 dB to 30 dB of voltage gain. An op-amp (such as the TL071 or LM358) in non-inverting configuration can replace the transistor stage for more predictable gain and lower distortion.
Important layout considerations: keep the bias resistor physically close to the capsule to minimise RF pick-up on the high-impedance node, decouple the supply rail with a 100 nF ceramic capacitor to ground right at the capsule, and use a ground plane to shield audio traces from interference. Supply voltage directly affects sensitivity; always match the capsule's datasheet recommended bias range.
This type of circuit is found in everything from condenser boundary microphones to PC sound-card input stages and Arduino voice-detection modules.
How to wire mic circuit diagram
- Select and obtain the electret capsule Choose a two-terminal electret capsule rated for your supply voltage (3 V to 12 V is typical). Note the capsule's sensitivity in dBV/Pa (commonly −42 dBV/Pa to −60 dBV/Pa) and its signal-to-noise ratio. Higher sensitivity capsules produce a stronger output but may saturate in loud environments.
- Calculate and install the bias resistor Using R = (Vcc − 1.0 V) ÷ 0.5 mA as a starting estimate for a 5 V supply yields R ≈ 8 kΩ; a standard 10 kΩ is a safe choice. Place this resistor between the positive supply rail and the capsule's output (+) pin. Keep the trace between resistor and capsule short.
- Add supply decoupling Solder a 100 nF ceramic capacitor from the supply rail (at the junction of bias resistor and power) directly to ground. This suppresses high-frequency noise injected from the supply and prevents the high-impedance node from acting as an antenna.
- Install the coupling capacitor Connect a 1 µF to 10 µF capacitor from the capsule output pin to the input of the preamp stage. For a film capacitor, polarity is irrelevant. For a polarised electrolytic, the positive leg faces the capsule side if the capsule output DC voltage is higher than the preamp input DC bias.
- Build the preamplifier stage For a single-transistor stage, wire the base to the coupling capacitor output via a 100 kΩ bias resistor to Vcc and a 10 kΩ resistor to ground. Connect a 10 kΩ collector resistor to Vcc and a 1 kΩ emitter degeneration resistor to ground. The amplified audio appears at the collector.
- Add output coupling and load Place another coupling capacitor (1 µF to 47 µF depending on the load impedance) from the collector output to prevent DC offset reaching subsequent stages or a speaker driver. A 10 kΩ load resistor at the output helps bias checking during measurement.
- Test with a signal tracer or oscilloscope Apply power and speak near the capsule. Probe the capsule output pin — you should see an AC waveform of 5 mV to 50 mV peak-to-peak. Probe the preamp output to confirm gain. If no signal appears, check that the capsule ground connects to circuit ground and that the bias resistor is supplying the correct DC voltage.
Specifications
| Typical supply voltage | 3 V – 12 V DC regulated |
|---|---|
| Electret capsule drain current | 0.2 mA – 1.0 mA (per capsule datasheet) |
| Typical bias resistor range | 2.2 kΩ – 10 kΩ |
| Capsule output impedance | 1 kΩ – 3 kΩ (set by internal JFET) |
| Typical preamp voltage gain (single transistor CE) | 20 dB – 30 dB |
| Audio frequency range | 20 Hz – 20 kHz (−3 dB, coupling cap dependent) |
| Low-frequency −3 dB point | f = 1 / (2π × R_load × C_coupling) |
| Capsule sensitivity (typical electret) | −42 dBV/Pa to −60 dBV/Pa |
Safety warnings
- This is a low-voltage electronic circuit. Ensure the power supply is regulated and fused. Even low voltages can damage sensitive CMOS devices if reverse polarity is applied; verify supply polarity before connection.
- When integrating this circuit into a mains-powered product, the audio circuit ground must be properly isolated from mains earth by a qualified electrical engineer. Improper isolation can create shock hazards. Consult IEC 60065 or equivalent product safety standards.
- Electret capsules contain a permanently charged polymer film and an internal JFET. Do not expose capsules to temperatures above 60 °C, strong solvents, or ultrasonic cleaning — all will permanently damage sensitivity.
- ESD (electrostatic discharge) can destroy the internal JFET of an electret capsule. Handle capsules with proper ESD precautions — use an anti-static wrist strap and store capsules in anti-static packaging.
Tools needed
- Soldering iron (25 W – 40 W) with fine tip
- Solder (60/40 or lead-free, 0.5 mm – 0.8 mm diameter)
- Multimeter (DC voltage and resistance measurement)
- Oscilloscope (preferred) or audio line-in on a computer for signal verification
- Breadboard or PCB with appropriate footprints
- Wire cutters and lead-forming tool
- Anti-static wrist strap
Common mistakes
- Omitting the supply decoupling capacitor: the high-impedance bias node couples supply noise directly into the audio path, producing audible hiss or hum proportional to supply quality.
- Reversing the capsule polarity: the capsule case (ground) must connect to circuit ground; reversing polarity biases the internal JFET incorrectly, producing a very weak or absent signal.
- Using too large a bias resistor (above 22 kΩ): reduces drain current below the JFET's minimum, causing high noise and compressed dynamic range.
- Incorrect coupling capacitor orientation: a reversed electrolytic coupling capacitor will leak DC, shift the bias point of the next stage, and may fail over time.
- Long unshielded cable between capsule and circuit: at 5 kΩ to 10 kΩ source impedance, even 30 cm of unshielded wire acts as an effective antenna for RFI and mains hum. Use shielded cable or keep runs to under 5 cm on PCB.
Troubleshooting
- No output signal at preamp output
- Cause: Capsule not receiving bias voltage, or internal JFET failed Fix: Measure DC voltage at the capsule output (+) pin relative to ground. It should be 1/3 to 2/3 of Vcc. If 0 V, check the bias resistor and supply. If Vcc, the internal FET may be open-circuit — replace the capsule.
- Loud hum or 50/60 Hz buzz on output
- Cause: Mains interference coupling into high-impedance bias node, inadequate ground plane, or long unshielded leads Fix: Add 100 nF decoupling capacitor at the bias node, shorten leads, add a ground plane, and enclose the circuit in a grounded metal enclosure if interference persists.
- Signal present but very low level
- Cause: Bias resistor value too high, supply voltage too low, or coupling capacitor open-circuit Fix: Verify bias resistor value and that supply voltage matches capsule specifications. Measure voltage across the coupling capacitor — both ends should show AC signal. If one end is zero, the capacitor is open-circuit; replace it.
Frequently asked questions
What value should the electret bias resistor be?
A value between 2.2 kΩ and 10 kΩ suits most electret capsules. Start at 4.7 kΩ with a 5 V supply. Check the capsule datasheet for its recommended drain current (typically 0.2 mA to 1 mA) and calculate: R = (Vcc − Vcapsule) ÷ I. Avoid values above 22 kΩ as they raise thermal noise significantly.
Why does the electret capsule have only two terminals?
Two-terminal electret capsules combine the drain and output on one pin; the JFET source and the back plate of the electret element share the ground (case) pin. This integration simplifies PCB design but means the bias resistor must supply both the DC operating point and present a load across which the audio voltage develops.
What happens if I use a polarised electrolytic capacitor as the coupling cap?
It will work provided you orient it correctly — positive leg toward the higher DC potential (capsule output side) and negative toward the lower-potential stage input. However, a film or ceramic capacitor is preferred for audio coupling because electrolytics exhibit voltage-dependent capacitance and can introduce distortion at low signal levels.
Can I power an electret capsule from a 3.3 V microcontroller pin?
Yes. Many modern electret capsules specify a supply range of 1.5 V to 10 V. Use a 2.2 kΩ bias resistor, decouple the 3.3 V rail with 100 nF to ground, and connect the output through a 1 µF coupling capacitor. The ADC input may need a DC bias mid-rail for single-supply operation.
Why is my microphone circuit picking up hum and buzz?
High-impedance electret circuits are susceptible to 50/60 Hz mains interference. Ensure the circuit ground is a solid copper plane, route audio traces away from switching supplies and mains wiring, keep cable runs to the capsule as short as possible, and consider shielding the capsule in a metal housing connected to circuit ground.
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