General > General Technical Chat

BJTs being phased out?

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Cerebus:

--- Quote from: EEVblog on October 15, 2020, 10:01:41 am ---
--- Quote from: Benta on October 13, 2020, 08:17:09 pm ---I'm much more worried about the JFET, which is a dying species.
--- End quote ---

I'm feeling a Save the JFET T-Shirt coming on...

--- End quote ---

The JFET Preservation Society.

Even got a ready made Society song, just mash-up "The Self Preservation Society" from The Italian Job.

bsfeechannel:

--- Quote from: rsjsouza on October 15, 2020, 07:13:13 pm ---The UJTs are a much more endangered species...  :-DD

2N2646 forever!

(I still have a bunch from the ol' days)

--- End quote ---

You're rich!

Digi-Key sell them for almost 6 bucks a pop.

tszaboo:

--- Quote from: tooki on October 15, 2020, 06:59:44 pm ---Electrets are still omnipresent in pro audio such as broadcast TV. Because of the poor performance and longevity of early electrets, pro audio distanced itself from the word "electret" and just calls them condenser mikes, because electrets are a subtype of condenser mikes. (It simply means a condenser mike that is pre-polarized, as opposed to ones that need external polarization power.) As such, I think it's eminently possible that a significant percentage (perhaps even a majority?) of "condenser" mikes are actually electret condensers.

--- End quote ---
I was under the impression, that condenser mikes are the ones, that have no built in circuits. Just a metallized sheet hanged in a frame. While electrets have a small metal tube, one end open, and typically they have a JFET or other transistor built in. Electret is actually the name of the material that is used as the dielectric, and for condenser microphones the dielectric is air. So they need more voltage.

coppice:

--- Quote from: NANDBlog on October 15, 2020, 10:06:16 pm ---
--- Quote from: tooki on October 15, 2020, 06:59:44 pm ---Electrets are still omnipresent in pro audio such as broadcast TV. Because of the poor performance and longevity of early electrets, pro audio distanced itself from the word "electret" and just calls them condenser mikes, because electrets are a subtype of condenser mikes. (It simply means a condenser mike that is pre-polarized, as opposed to ones that need external polarization power.) As such, I think it's eminently possible that a significant percentage (perhaps even a majority?) of "condenser" mikes are actually electret condensers.

--- End quote ---
I was under the impression, that condenser mikes are the ones, that have no built in circuits. Just a metallized sheet hanged in a frame. While electrets have a small metal tube, one end open, and typically they have a JFET or other transistor built in. Electret is actually the name of the material that is used as the dielectric, and for condenser microphones the dielectric is air. So they need more voltage.

--- End quote ---
Condenser mics, electret or otherwise, are impractical without some electronics close to the sensor. The impedance of a condenser mic is very high, so as soon as you pass its signal through some mic cable it would be heavily low pass filtered by the capacitance of the cable. Therefore you need electronics to bring down the impedance. The usual solution is based on a JFET.

Condenser mics usually consist of a rigid metal plate close to a thin plastic sheet coated in a thin metal film. This structure is chosen to be as light and flexible as possible. Sound waves vibrate the plastic sheet. There needs to be a polarising voltage between the two plates, which the sound wave can modulate. This is produced either by an external high voltage, or by using a plastic film consisting of highly polar molecules, which is produced in a strong electric field. This film is an electret, and has a permanent high voltage across the film. This removes the need for an external polarising voltage.

tooki:

--- Quote from: NANDBlog on October 15, 2020, 10:06:16 pm ---
--- Quote from: tooki on October 15, 2020, 06:59:44 pm ---Electrets are still omnipresent in pro audio such as broadcast TV. Because of the poor performance and longevity of early electrets, pro audio distanced itself from the word "electret" and just calls them condenser mikes, because electrets are a subtype of condenser mikes. (It simply means a condenser mike that is pre-polarized, as opposed to ones that need external polarization power.) As such, I think it's eminently possible that a significant percentage (perhaps even a majority?) of "condenser" mikes are actually electret condensers.

--- End quote ---
I was under the impression, that condenser mikes are the ones, that have no built in circuits. Just a metallized sheet hanged in a frame. While electrets have a small metal tube, one end open, and typically they have a JFET or other transistor built in. Electret is actually the name of the material that is used as the dielectric, and for condenser microphones the dielectric is air. So they need more voltage.

--- End quote ---
coppice does a great job answering in detail. But the upshot is, no, those construction details aren’t what defines an electret vs non-electret condenser mike.


--- Quote from: tooki on October 15, 2020, 06:59:44 pm ---
--- Quote from: coppice on October 15, 2020, 06:31:46 pm ---Most other mics are condenser or dynamic. There were once some very high quality electret mics, but they seem to have gone, and now we only have condenser, ribbon and dynamic mics for high quality applications. Condenser mics also use a JFET, but their volumes are millions, not the hundreds of millions we see with cellphones.

--- End quote ---
That is categorically untrue. Electrets are still omnipresent in pro audio such as broadcast TV. Because of the poor performance and longevity of early electrets, pro audio distanced itself from the word "electret" and just calls them condenser mikes, because electrets are a subtype of condenser mikes. (It simply means a condenser mike that is pre-polarized, as opposed to ones that need external polarization power.) As such, I think it's eminently possible that a significant percentage (perhaps even a majority?) of "condenser" mikes are actually electret condensers. (In pro audio, you'll see reference to "true condenser" to try and signify an externally polarized condenser mike, even though that phrase is technically imprecise since it attempts to mislead one into thinking that electrets aren't truly condenser mikes.)

--- End quote ---
It seems my hunch was right: while reading up on electrets to try and answer NANDblog’s question, I came across a statement from Shure (one of the big names in pro audio mikes) claiming that “probably 95% of the condenser microphones on the market are the electret type”.

Another source found during the same trawl says that MEMS overtook electret capsules in sales in 2014, but that most MEMS microphones are also electret-based! “ The MEMS microphone designed is largely based on the design of the electret condenser microphone capsule.” (This I did not know.)

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