What are Bipolar and MOSFET?
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- Опубликовано: 18 ноя 2017
- Most of us have heard of the different devices like Bipolar transistors and MOSFETS but what exactly are they and why would a designer use one or the other? Paul explains what each is and why they are chosen. Have a question you'd like to ask Paul? Go to www.psaudio.com/ask-paul/
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I just wish to thank you for your videos. First of all because they're so full of an all-american kindness and politeness, the feeling of a kind and generous person talking to us, trying to find simple ways to explain and answer questions that, from his knowledgeable point of view might even sound silly! Well, thanks Paul! Keep it up!
Hes like our Mr Rogers for adults in audio. Great guy. Hey paul know anything about the Acurus A150 amp? is it solid stuff
For a designer of power amplifiers and linear power supplies, one of the most important limitations of large bipolar transistors (BJT's) is secondary breakdown and the reduced safe operating area that they have, compared to MOSFETs. This stems from a fundamental difference in their physics: Bipolars are minority-carrier devices with negative temperature coefficient of Vbe and thus susceptibility to current concentration across the device die, while FETs are majority-carrier devices with positive temperature coefficient of Vgs which promotes balanced distribution of current across the die. It can be quite easy to run MOSFETs in parallel to increase the current capacity of an output stage, but bipolar transistors in parallel need emitter degeneration resistors and/or other means to ensure they share current equally.
In power amplifiers with supplies exceeding 30-40V on each polarity, it is often necessary to stack bipolar output transistors in series to reduce their operating collector-emitter voltage. But even when this is done, driving highly reactive loads (especially if a near-resonant load condition occurs) can spell trouble for a BJT output stage. If its safe operating area is exceeded (which can occur at a fraction of its power rating), a BJT can suffer a collector-emitter short, and the only protection for the speaker load will be its fuse. On the other hand, MOSFETs are only limited by their power dissipation and in that sense are more rugged devices.
Paul, thanks for the video series, it's a great resource, easily understood and very enlightening for an audiophile like me. Being able to understand some of the issues and problems designers face today both in the analogue and digital world of audio is very helpful. I appreciate the time you put into answering the questions, you're creating quite a legacy.
FETs are voltage controlled current controlling devices and NPN/PNP transistors are current controlled current controlling devices. FETs are in that way similar to tubes. People need to sit down and build some circuits with them firsthand to really get it.
Thank You Paul for taking the time to do these UTube videos. You have a wonderful way of explaining any topic that pertains to electronics.
I’m a tube guy, there’s nothing like the glow of a 6L6......lol
Again Thanks
Yep, real amplifiers glow in the dark!!
Well done, Paul. Thank you.
Just a clarification, class D is PWM, I think Paul said class B.
Regarding BJT vs MOSFET output stages, I believe the main reason for the preference to the latter is the difficulty to control the BJT at temperature, that MOSFETs are less prone to thermally get into trouble. And as Paul implied, the MOSFET is inherently more similar to the vacuum tube than the BJT. Square law vs exponential (even vs odd harmonic distortion).
However, audio signals are voltage based while mosfets are current based, so you need op amps with feedback to drive a mosfet. The deal is, although mosfets are voltage 'driven', they control the current, not voltage, so you need feedback to know how much voltage is coming out the other end.
Richard Smith If your load is purely resistive, then the voltage is proportional.
Not true saxon. Maybe if you know EXACTLY what the resistance of your pure resistive load will be, you could drive it with no feedback. Since the resistance is not static, you have to have feedback to know.
Do a tour video of your Home theater system at home!
you said you had a 12.1 system lets see it please?!?!!?!?!
Thank you for this interesting video! If MOSFETs are more tubeish is sounds, why are they not used in microphones? Can you replace the physical tube in a tube microphones with a MOSFET? Very interesting!
BJT's are two diodes placed back to back.
I love it! Great explanation I had to laugh a little bit when you raise your middle finger just a little humor dare anyways great to hear from you again Paul big fan of yours Steve Costello
Hallo Paul, what is a „musical tube-like sound“?
It is related to harmonics. Google is your guide. Give paul a break 😊
Paul McGowan Thanks for your Repliken. Sounds like the answer from Paul the listener. How does Paul the engineer explain tube sound?
reply, not Repliken, sorry.
Paul McGowan Thank you again for keeping up the contact with music lovers, audiophiles and (potential) customers. I can imagine how much effort it is.
Appreciate the breakdown
Class B is a PWM amplifier, I'm sure you meant class D, cheers
David Brown I thought I heard wrong too.
Yup, PWM amps are class D. Just a slip-up I guess.
With all my deep respect to your business.
MOSFET has a much more close to linear characteristics than BJT. (power(3/2))
So, you do not need that parasitic " inverse feedback", producing terrible harmonics all other the spectrum.
Sorry Paul ..... but bipolar transistor sound better in music aplications than Mosfets !Even Achupasse use bipolar transistors in the best line of amps !😉
Paul,
I found out that I was Bipolar, on top of having to deal with that, my shrink said, that I may be bisexual to!
I was really angry with him and, him being a full blown gay. I was about to punch him, then thought for
a few seconds, hes right... I said, well, as I fancy women, and a male, I must be a male lesbian, so in a way, I may infact be sort of gay!
One learns a new thing every day.
Now I'm going to try to find out why I stare at a turn table platter spinning for hours, and not even playing a record on it!
the amount of free time you have is worthless
Ignore that other guy. At least one person laughed at your joke, even if it was eight months later.
I'd say Bipolar was my X-girlfriend, and isn't MOSFET that rap guy?🤣