What is FET... to improve SPL?

[nishi]

In general, smaller diameter ECMs have lower maximum sound pressure levels.
I had assumed that the sound pressure level would increase in proportion to the appearance.
However, I found an expert's explanation that SPL depends not on the diameter but on the impedance conversion circuit.

Further Googling revealed that there is a FET (2SK1109) branded for impedance transformation. It says that it has a built-in high gate resistance.

I have a few questions here.

• Is it possible to increase the SPL with a normal FET?
• Is FET also suitable for increasing the SPL of dynamic microphones?
• Is it possible to perform impedance conversion with a FET that is not branded for impedance conversion (eg 2SK208)? If so, what problems does it pose compared to 2SK1109?
• Is it possible to increase the SPL by inserting a resistor in the gate of a normal FET?

 

[user 37518]

A FET is a Field Effect Transistor, it can act as an amplifier, so yes, it can increase the level of a signal. I've never heard of a FET being branded for impedance transformation. But FETs have very high input impedance and the output impedance, depending on the configuration, is ussually low to mid, so, in a sense, they do couple impedances, but they do not transform impedances like a transformer does. Any FET has these properties, however, some might be better due to lower noise specs or other more subtle aspects. Saying that a particular FET has a built in high gate resistance is BS, every FET has this property, and not due to a built in resistance, but because that is the very nature of a FET, it is the inversely polarized PN junction (in a JFET) or the silicon oxide barrier at the gate (In a MOSFET) that gives it this property, not some magical resistance that a manufacturer decided to include as a special feature.

I don't know what you mean by raising SPL by adding a resistor at the gate, no, a resistor at the gate wont amplify a signal, the most usual configuration for a FET is the common source amplifier, the gain is determined by the ratio of the drain to source resistance. Of course, if you add feedback things will change. You have to be more specific and show a schematic or something, otherwise we are just guessing at what you want to do.

 

[Lampie519]

Adding a resistor in series with the gate will only add noise. In general the fet is fixed in the ECM and very difficult to modify, however if you could add a resistor in de source of the fet it will be able to handle higher SPL’s.
https://www.linkwitzlab.com/sys_test.htm#Mic

 

[nishi]

FET is difficult for me, but I understand a little.

The Japanese documentation says that for an ECM without a FET, a FET with a high gate resistance should be installed.
I also thought that a higher gate resistance would increase the input impedance, thus increasing the SPL. (this is my inference)

 

[user 37518]

FET is difficult for me, but I understand a little.

The Japanese documentation says that for an ECM without a FET, a FET with a high gate resistance should be installed.
I also thought that a higher gate resistance would increase the input impedance, thus increasing the SPL. (this is my inference)

A high gate resistance is desirable, but not by means of adding a physical resistor.

 

[nishi]

What happens if the gate resistance is low?

 

[user 37518]

What happens if the gate resistance is low?

A JFET has very high gate resistance, so we can't say it is low, however, the type of resistances used in condenser mics are of huge value, so even a small current can create problems like loading the capsule, creating unnecessary voltage drops, more current noise, and so on.

What exactly do you want to accomplish? you have to be more specific about your end goal