Preamplifiers for Microphones and Sensors
A preamplifier is the first amplifier a signal meets. A dynamic microphone produces about 1–2 mV on normal speech; a moving-magnet record cartridge about 5 mV; a thermocouple 41 µV per degree; a strain-gauge bridge a few millivolts at full load. Before any of these can be mixed, recorded or measured, it must be amplified — by a factor of 100 to 1,000 or more — and whatever noise the preamplifier adds is amplified right along with it by every stage that follows. The preamplifier therefore sets the noise floor of the whole system.
Designing a good preamplifier is mostly about noise: understanding where it comes from, how it depends on the impedance of the source, and how to choose a transistor or op-amp that adds as little as possible. It is also about the practical details — gain staging, grounding, shielding and powering the sensor — that decide whether a quiet design stays quiet once it is built.
Three kinds of preamplifier: a balanced low-noise microphone preamp, a high-impedance JFET buffer for a piezo pickup, and an instrumentation amplifier for a bridge sensor. The source decides which amplifier is quietest.
View LargerWhere Noise Comes From
Thermal (Johnson) noise is generated by every resistance, simply because its electrons are in random thermal motion. It is independent of current and spread evenly across all frequencies (“white”). Its voltage density is √(4kTR): about 4 nV/√Hz for a 1 kΩ resistor at room temperature, rising with the square root of the resistance.
Shot noise arises because current is made of individual electrons crossing a junction. It is proportional to the square root of the current, so a transistor running at lower current has less of it — which is one reason low-noise stages run at modest currents.
Flicker (1/f) noise rises at low frequencies, roughly doubling in power for every halving of frequency. It matters most in audio below about 100 Hz and in DC measurements, where it shows up as slow wander. Some devices and some op-amps (especially CMOS types) have much more of it than others, and datasheets give the “1/f corner” frequency below which it dominates.
An amplifier's own noise is described by two figures on its datasheet: a voltage noise density en (in nV/√Hz), which acts like a small noise generator in series with the input, and a current noise density in (in pA/√Hz or fA/√Hz), which flows through the source impedance and creates a noise voltage across it.
Noise Density and Adding Noise Sources
Noise densities are quoted per root hertz because the total noise grows with the square root of the bandwidth. To find the total RMS noise voltage, multiply the density by √(bandwidth). Independent noise sources add as the square root of the sum of their squares, never as a simple sum:
Total input noise density: etotal = √(es² + en² + (in × Rs)²)
Total RMS noise = etotal × √B
Because of the square-root addition, the largest source dominates. A noise source a third the size of the biggest one adds only about 5% to the total. This is why a preamplifier only needs to be clearly quieter than its source, not perfect.
Noise Figure and Equivalent Input Noise
Two figures describe how much noise a preamplifier adds. Noise figure (NF) compares the total noise with the noise of the source alone: NF = 20 log₁₀(etotal / es) dB. A perfect amplifier has NF = 0 dB; 3 dB means the amplifier adds as much noise as the source itself. Noise figure is the usual figure in RF work, where sources are 50 Ω.
Equivalent input noise (EIN) is the total noise referred to the input, expressed as a voltage or in dBu (dB relative to 0.775 V RMS). Microphone preamplifiers are specified this way, usually measured with a 150 Ω source over a 20 Hz–20 kHz bandwidth. A good microphone preamp achieves an EIN of about −128 dBu; the theoretical limit set by a 150 Ω source is about −131 dBu.
Source: a 200 Ω dynamic microphone. Bandwidth: 20 kHz.
Source thermal noise: es = 0.128 × √200 = 1.82 nV/√Hz; total = 1.82 nV × √20,000 = 0.257 µV = −129.6 dBu.
Option A — an NE5534 op-amp (en = 3.5 nV/√Hz, current noise negligible at 200 Ω):
etotal = √(1.82² + 3.5²) = 3.94 nV/√Hz → NF = 20 log(3.94/1.82) = 6.7 dB; EIN ≈ −122.9 dBu.
Option B — a discrete low-noise transistor input stage (en ≈ 1 nV/√Hz):
etotal = √(1.82² + 1.0²) = 2.08 nV/√Hz → NF = 1.2 dB; EIN ≈ −128.4 dBu.
For a low-impedance source like this, the amplifier's voltage noise is what matters, and a few dB of noise figure is the difference between an ordinary and an excellent preamplifier.
Matching the Amplifier to the Source
The best amplifier for one source can be the worst for another. Voltage noise dominates with low source impedances; current noise, multiplied by the source impedance, dominates with high ones. A bipolar op-amp with very low en usually has relatively high in; a JFET or CMOS op-amp has slightly higher en but almost no current noise.
NE5534: en = 3.5 nV/√Hz, in = 0.4 pA/√Hz.
Current noise across 1 MΩ: 0.4 pA × 1,000,000 = 400 nV/√Hz — more than a hundred times its voltage noise, and far more than the source's own thermal noise of 128 nV/√Hz.
A JFET-input op-amp such as the OPA164x or TL07x (in of a few fA/√Hz) adds only a few nV/√Hz from current noise at 1 MΩ, even though its en of 5–18 nV/√Hz looks worse on paper.
The rule of thumb: divide en by in to find the amplifier's “optimum source resistance.” Sources well below it want a bipolar input stage; sources well above it want a JFET or CMOS input.
Microphone and Instrument Preamplifiers
- Dynamic microphones (150–600 Ω, 1–2 mV): need 40–60 dB of low-noise gain. Professional inputs are balanced: a differential amplifier rejects hum picked up equally on both wires of the cable.
- Condenser microphones: contain their own impedance converter and need power — 48 V “phantom power” fed through two matched 6.8 kΩ resistors onto both signal wires, so it does not disturb the balanced audio. The preamplifier input must be protected by coupling capacitors and clamp diodes from the 48 V and its switching transients.
- Electret capsules (in headsets, phones and many small projects) contain a single JFET source follower and need a few volts through a 2–10 kΩ bias resistor.
- Guitar and piezo pickups: high source impedance (tens of kilohms to megohms). The input impedance must be 1 MΩ or more so the pickup is not loaded, and the input stage should be a JFET or CMOS device.
- Phono (record) cartridges: need 35–40 dB of gain with RIAA equalization, a frequency response that boosts the bass by about 20 dB relative to the treble to reverse the curve used when the record was cut.
Sensor Preamplifiers
Sensor preamplifiers usually work down to DC, so low-frequency errors matter as much as noise:
- Offset and drift: a 1 mV input offset is an error of 25°C on a type K thermocouple (41 µV/°C). Precision and zero-drift (chopper) op-amps reduce offset to microvolts and remove 1/f noise almost entirely.
- Bridge sensors (strain gauges, load cells, pressure sensors): the signal is the small difference between two voltages near half the supply. An instrumentation amplifier amplifies the difference while rejecting the common voltage, with its gain set by a single resistor.
- Photodiodes: produce a current, not a voltage. A transimpedance amplifier (an op-amp with a feedback resistor) converts it to a voltage; a low-current-noise JFET or CMOS op-amp and a large feedback resistor give the best signal-to-noise ratio.
A type K thermocouple produces about 41 µV/°C, so 0–500°C gives about 0–20.5 mV. To use most of a 0–3.3 V ADC range: gain = 3.3 / 0.0205 ≈ 160 (use 150 for a little headroom).
A general-purpose op-amp with 2 mV offset would give an error of 2 mV / 41 µV = 49°C — useless. A zero-drift op-amp with 10 µV maximum offset gives an error of 0.25°C, which is less than the thermocouple's own tolerance. (In practice, dedicated thermocouple interface chips combine the amplifier, cold-junction compensation and ADC in one package.)
Gain Staging, Grounding and Shielding
Put the gain first. The first stage should have as much gain as the largest expected signal allows, so that the noise of later stages becomes insignificant. Ten dB too little gain at the start costs ten dB of noise performance that no later stage can recover. Equally, too much gain causes clipping on loud signals, which is why microphone preamps have a gain control at the first stage.
Keep the input impedance low where you can. Every resistor in the input path adds thermal noise. Feedback resistor networks in low-noise stages use low values (tens to hundreds of ohms) for the part in series with the input.
Ground and shield carefully. At microvolt levels, the 50/60 Hz magnetic field from a mains transformer, a ground loop through a signal cable, or switching noise on the supply can easily be larger than the signal. Use balanced or shielded inputs, a single ground point for the input stage, filtered and well-decoupled supplies (the Bypass and Decoupling lesson), and keep the preamplifier physically close to the sensor.
Preamplifier Noise Calculator
Preamplifier Noise Calculator
Enter the source resistance, the amplifier's voltage and current noise densities from its datasheet, and the bandwidth. The calculator gives the noise contributions, the total input-referred noise, the noise figure and the equivalent input noise in dBu.
Frequently Asked Questions
Will phantom power damage a dynamic microphone?
Not if the microphone is a balanced dynamic type wired correctly. Phantom power puts the same +48 V on both signal wires, so no voltage appears across the microphone's coil and no current flows through it. Problems arise with unbalanced microphones, miswired or damaged cables (which can put 48 V across the coil), ribbon microphones (whose fragile ribbons can be damaged by a fault or by plugging in with phantom power on), and equipment that is not designed to receive phantom power on its output. The safe habit is to switch phantom power on only for the channels that need it, and only after the cable is connected.
Why does my preamplifier hum?
Hum at 50 or 60 Hz (or a buzz rich in harmonics of it) almost always comes from one of three places: a ground loop, where two pieces of equipment are grounded both through the mains and through the signal cable shield; magnetic pickup from a nearby mains transformer by the input wiring; or ripple on the preamplifier's supply. Try unplugging every other connection to find a ground loop, move or rotate the preamplifier relative to transformers, and look at the supply rail with an oscilloscope. Balanced inputs and a single star ground for the input stage cure most cases.
Why not just use the op-amp with the lowest voltage noise?
Because voltage noise is only part of the story. Op-amps with the lowest voltage noise use large bipolar input transistors running at high current, which gives them high current noise and input bias current. With a low-impedance source such as a dynamic microphone or a strain gauge, they are the best choice. With a high-impedance source — a piezo pickup, a photodiode, a pH electrode — their current noise flowing through the source impedance overwhelms everything else, and a JFET or CMOS op-amp with higher voltage noise but almost no current noise is far quieter overall. Always calculate all three contributions for your actual source.
Should the preamplifier be close to the sensor?
Yes, wherever possible. The signal is at its weakest and most vulnerable between the sensor and the first stage of gain: every meter of cable there can pick up hum and interference, adds capacitance that loads high-impedance sources, and (for low-impedance sources) adds resistance and thermal noise. Once the signal has been amplified to line level or converted to a current loop or a digital signal, it can travel a long way with little degradation. This is why condenser microphones contain their own impedance converter, industrial sensors often include a 4–20 mA transmitter, and many modern sensors put the ADC right next to the sensing element.
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