What Are FETs & What Is Their Role In Microphone Design?


When looking through catalogs of condenser microphones, it’s fairly common to see the terms FET or solid-state used to describe the microphone. Many condensers on the market today have FETs or JFETs in their design.

What are FETs & what is their role in microphone design? FETs (field-effect transistors) are active electrical devices that use an electric field from a microphone capsule to control a flow of current that is ultimately the mic signal. FETs take the high-impedance signal from mic capsules and output a usable and proportional low-impedance signal.

In this article, we’ll describe microphone field-effect transistors in greater detail and discuss the microphones that require them along with those that do not.


What Is A Field-Effect Transistor?

A field-effect transistor (FET) is a type of transistor that utilizes an electric field to control the flow of current. In simpler terms, an FET uses an input signal to modulate an output signal.

Let’s back things up a bit and describe what a transistor is before diving further into FETs.

A transistor is an active semiconductor device that is used to amplify (pseudo-amplify) or switch electrical signals and electrical power.

In many cases, transistors are used for on/off switches and have been essential for binary digital processing (1s and 0s). This is the case with many digital audio devices. In the case of analog FET microphones, the transistor acts to convert signal impedance and boost the signal (though this is not true amplification).

To learn more about microphones and their roles with analog and digital audio, check out my article Are Microphones Analog Or Digital Devices? (Mic Output Designs).

Transistors are composed of semiconductor material (typically silicon) with at least three terminals that connect to an external circuit.

Applying a voltage or current to one pair of the transistor’s terminals will control the current through another pair of terminals. In this way, we can take an “input” signal at one pair of terminals and use it to modulate an “output” signal with greater voltage and/or lower impedance (pseudo-amplification).

Microphones that utilize field-effect transistors typically use JFETs or junction-gate field-effect transistors.

A JFET is perhaps the simplest FET design and performs the task described above. Its “input” signal (voltage between gate and source) modulates a proportionate “output” signal (voltage between drain and source). So with an FET, we can take a low-level signal at the input and turn it into a high-level signal at the output.

The input and outputs of the field-effect transistor are called terminals. Each JFET has 3 terminals, which are called:

  • Gate
  • Source
  • Drain

Here is a simple diagram of a microphone junction-gate field-effect transistor:

As we apply a voltage between the gate and source (some would call this the input) of the FET, the transistor alters the conductivity between the drain and source. With the proper DC biasing voltage, we get an output voltage between the drain and source that is proportional to the input signal at the gate/source.

So basically the high-impedance capsule output signal goes to the gate and source terminals and effectively modulates a lower-impedance (and often higher-voltage) signal between the drain and source terminals.


What Are FETs Used For In Microphones?

FETs are used primarily as impedance converters in condenser microphones.

The condenser microphone capsule works as a transducer, converting sound waves (mechanical wave energy) into audio signals (electrical energy). The electrical audio signals (AC voltages) a condenser capsule outputs have incredibly high impedances and drive barely any current.

For more information on microphone capsules, check out my article What Is A Microphone Capsule? (Plus Top 3 Most Popular Capsules).

This is where the impedance converting field-effect transistor comes into play.

FETs, by design, have extremely high input impedances at their gates. The impedance at the drain, however, is much lower and actually allows for current to flow.

So the capsule’s output signal is sent directly to the gate of the FET. This AC signal alters the conductivity between the drain and source terminals and, therefore, alters the current at the drain and, ultimately, the “output” voltage of the FET.

In other words, the FET takes a high-impedance signal at its input and uses it to modulate a low-impedance signal at its output. This output signal is then capable of traveling through the rest of the microphone’s circuitry; the mic output, and through a mic cable to a microphone preamplifier.

Here is a simple diagram of an FET condenser microphone:

As we can see from this simple diagram, the FET requires some DC biasing voltage from a power source to function.

Note that the capsules of “true” condensers also require external power in order to become polarized.

The FET takes the high-impedance signal from the capsule and lowers the impedance to usable levels before the signal is sent to the mic output.

In most cases, including microphones, the role of the field-effect transistor used to be fulfilled by vacuum tubes. Transistors are generally much smaller; require less power to run (phantom power or DC biasing rather than dedicated power supplies), and are less costly to manufacture and implement.

To learn more about properly powering microphones, check out the following My New Microphone articles:

Do Microphones Need Power To Function Properly?
Do Microphones Need Phantom Power To Work Properly?
Will Phantom Power Damage My Ribbon Microphone?

Although there are differences in the sound of FETs versus vacuum tubes (audiophiles would definitely argue), nowadays FET microphones and tube microphones can be produced with the same quality standards.

It’s also important to note that FETs have become standard in condenser microphones. What I mean by this is that if a condenser mic has a tube, it will be referred to as a “tube condenser,” whereas an FET condenser will typically be referred to simply as a “condenser mic.” That is unless the prefix “FET” distinguishes the mic from a tube version of that same mic.

For a detailed read on the differences between FET and tube microphones, check out my article What Are The Differences Between Tube & FET Microphones?


What Microphones Do Not Require FETs?

Not all microphones require field-effect transistors. In fact, FETs are really only used in certain condenser microphone designs and sometimes in active ribbon microphones.

Let’s look at the microphone types that do not require FETs.

Passive Microphones

FETs are active devices. They require DC biasing in order to function properly. Therefore, passive microphones, by the simple definition of being passive, do not have FETs in their designs. Let’s look at dynamic and ribbon microphone types, both of which work on passive electrical principles.

Dynamic Microphones

Moving-coil dynamic microphones work on electromagnetic induction and do not require any active components.

Their capsule (cartridge) output signals are low-impedance and can be sent directly to the microphone output connection (though they are often sent through an output transformer first).

To learn more about moving-coil dynamic microphones, please consider reading my article Moving-Coil Dynamic Microphones: The In-Depth Guide.

Ribbon Microphones

Ribbon microphones also convert sound to audio via electromagnetic induction.

Their “capsules” (known as ribbon elements or baffles) output low-impedance signals that do not require an impedance converting FET. Ribbon mics are designed with transformers to help protect their fragile ribbon diaphragms from potential shorting of DC voltage.

For more information on microphone transformers, check out the following My New Microphone articles:

What Are Microphone Transformers And What Is Their Role?
Do All Microphones Have Transformers And Transistors? (+ Mic Examples).

Active ribbon microphones could potentially have FETs in their designs. These designs would have high ratio step-up transformers between the ribbon baffle and the FET to boost the relatively low voltage of the ribbon’s output.

These step-up transformers also boost the impedance of the signals and so FETs are sometimes beneficial to bring the impedance back down to useable levels without bringing the signal strength down as well.

To learn more about ribbon mics, consider reading my article Dynamic Ribbon Microphones: The In-Depth Guide.

Tube Microphones

Vacuum tubes essentially fulfill the same role as field-effect transistors in microphones. That is, they convert impedance from high-impedance capsule signals and act as pseudo-amplifiers.

Let’s quickly look at a diagram of a triode vacuum tube (the simplest tube for a microphone) and list its components:

  • H is the heater
  • K is the cathode
  • A is the anode
  • G is the grid

A power source heats up the heater which then causes a steady flow of electrons (an electrical current) from the negatively charged cathode to the positively charged anode. This is similar to the current flow between the source and drain terminals of the field-effect transistor.

The high-impedance capsule output is connected to the high-impedance grid (input) of the triode vacuum tube. The AC voltage at the grid of the tube modulates the flow of electrons between the cathode and anode. In other words, the high-impedance input signal at the grid controls a low-impedance (and often higher voltage) signal at the tube’s output. This is somewhat analogous to the gate terminal of the FET.

So although tubes are very different than transistors, they can be thought of as analogous to FETs in the following ways:

  • Heater = DC biasing circuit
  • Cathode = source terminal
  • Anode = drain terminal
  • Grid = gate terminal

In fact, early condenser microphones required vacuum tubes to convert the high impedance signals from their capsules. The transistor was only invented in 1947 and the FET/JFET only made its debut in commercial microphone technology in 1964.

To learn more about microphone history and the technological advances that made modern microphones possible, check out my article Mic History: Who Invented Each Type Of Microphone And When?


Related Questions

What is a microphone capsule? The mic capsule is the part responsible for the conversion of sound waves into mic signals. Capsules always feature diaphragm(s) and the housing for those diaphragms. The capsule, in its entirety, acts as the transducer of the microphone, turning sound into audio.

What does a microphone measure? A microphone essentially measures the sound pressure variations at its diaphragm within a range of audible frequencies. As the sound waves cause varying pressures around the mic diaphragm, the mic produces a coinciding electrical audio signal.

For more information on microphones, sound, and audio, check out my article What Do Microphones Measure And How Do They Measure?

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