How to Test a FET Transistor with a Digital Multimeter

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Do you often look through directories of condenser microphones? It is fairly typical to see the terms ET or solid-state utilized to describe the microphone itself. Most condensers available in the market today have FETs in their design.

However, what are FETs, and what is their overall role in microphone design? Take note that Field Effect Transistors (FETs) are active electrical devices that utilize an electric field from a microphone capsule to regulate a flow of current—the mic signal. FETs often take the high-impedance signal from capsules and output proportional low-impedance and usable signal. 

In this guide, we will talk about microphone FET transistors in greater detail and talk about the microphones that need them together with those that do not. 

So, what’s the FET transistor?

To explain it further, the FET transistor uses an electric field to regulate the flow of the current. In a nutshell, it utilizes an input signal to modulate an output signal. 

Do you know what a transistor is? Take note that a transistor is an active semiconductor device, which is utilized to switch or amplify electrical signals and electrical power. In most situations, transistors are utilized for off or on switches and have been a crucial part of any binary digital processing. 

That’s the case with most digital audio devices; regarding analog FET microphones, the transistor functions to transform impedance and increase the signal. 

Transistor consists of semiconductor material with at least three terminals that link to an external circuit. Applying current or voltage to one pair of the resistor’s terminals will regulate the current through another pair of terminals. In that way, you can take an “input” signal at one pair of terminals and utilize it to modulate an “output” signal along with higher voltage or lower impedance. Microphones using FET normally use junction-gate field-effect transistors or JFETs. 

What are the other uses of FETs?

FETs are utilized as impedance converters in condenser mics. The condenser microphone capsule functions as a transducer, transforming sound waves into audio signals. The electrical audio signals a condenser capsule outputs have high impedances and drive any current. 

That’s why converting FET transistors come into play. By design, FETs have high input impedances at their gates. However, the impedance at the drain is lower and enables for current to flow. 

Hence, the output signal of the capsule is delivered straight to the FET’s gate. That AC signal changes the conductivity between the source terminals and drain. Thus, it changes the current at the drain and the FET’s “output” voltage. 

In short, FET transistors take a very high impedance signal at its input and utilize it to moderate a low-impedance signal at its output. That output signal can travel through the rest of the circuitry of the microphone: the mic output and via the mic cable to a mic preamplifier. 

Bear in mind as well that FETs have become the norm in condenser microphones. What we mean by that is when a condenser has a tube, it will be called a “tube condenser,” while a FET condenser will be called a “condenser mic.” 

Here are some of the typical uses of FETs other than being used in microphones:

  • Integrated circuits

FETs are a typical transistor and are a vital part of the electrical operation of IC chips. They don’t need the same sequence of steps as bipolar transistors for P-N junction isolation on a chip. Nonetheless, they do enable relatively simple separation. 

  • CMOS circuits

A CMOS circuit is a kind of technology utilized to create ICs. This technology is utilized in the IC chips’ manufacture like memory chips, microcontrollers, microprocessors, and other digital logic circuits. 

  • Analog switches

The benefits of FETS for digital circuit integration outweigh those for analog integration. Did you know that the behavior is very much different in every case? Digital circuits could be switched on and off for most. The speed level and charge are two factors that have a bearing on the switching process.

Functionality should be guaranteed within the transition region of the analog circuit in the event that small V changes could change the output current.

  • Power electronics

FEts are utilized across a wide array of power electronics. They’re incorporated for reverse battery protection, powering down unnecessary loads, and switching power between alternate sources. The key features of compact FETs involve integrated ESD protection, a small footprint, and a high current. 

How do you test a field-effect transistor?

Here are the steps you need to follow to test your FET transistor using a digital multimeter:

  1. Remove the FET transistor you like to test from the circuit board. Or else, the digital multimeter might damage, and the proper results won’t appear. 
  1. Does your digital multimeter have a transistor testing port? Feel free to use it. Plug your transistor into the dedicated port of transistor testing. Plug the transistor based on the notation of PNP or NPN. If there’s no port for your transistor, you can test it with an ohmmeter as an alternative. 
  1. Rotate the knob to properly set the mode to test the resistor. You can utilize the symbol of hFE to obtain the gain of the transistor. 
  1. At this stage, your digital multimeter LCD will show the gain of the transistor. If you didn’t receive a reading at all, you could change the transistor’s configuration from E-B-C to the con B-C-E configuration. 

Final Thoughts

You see, testing a FET transistor with a digital multimeter does not need to be a complicated and challenging task. Just follow all the steps we highlighted above, and you are good to go. We hope our guide has been relevant and useful to you. 

In case you forget any of the things we discussed earlier, feel free to visit this article again, so you are guided. Do you have any thoughts you want to share with us about FET transistors? Feel free to share your insights with us by leaving your comments below.

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