i see many power amplifer circuits,there're some good,there're some bad...

but just a little circuits used FET as a basic component

why?

(sorry for bad English,i'm Vietnammese )

 

If you mean for audio it can give reasonably good results.

http://sound.westhost.com/project101.htm

If you have a design you wish to share we'd love to see it.

I entered mosfet power amplifier into google and this is what came out
http://www.google.com/search?hl=en&q=mosfet+power+amplifier&btnG=Google+Search

Most designers choose the components they have used first. Then that is modified by availability, cost and ease of use.

 

thanks for your reply!

but you don't understand me.Follow this link:
http://sound.westhost.com/project101.htm

Q1 to Q7,all of them is Bipolar Transistor,only Q8 and Q9 is power Mosfet,and i believe that many other circuits similar itself

So i want to ask you: why they not used Fet-Mosfet for Q1 to Q7 ?

i think Fet-Mosfet have good perfomance than BJT,and bias of Fet-Mosfet is more simple than BJT,why we can't choose it ?

 

One would suspect it is more to do with "better the devil you know". BJTs excel in situations of ruggerised environments (potentially the realm of power amplifiers) and tend to be popular across many discrete circuit designs because of the wide range of established and well understood BJT devices available. The MOSFET revolution is built upon its suitablility for high-density, simple to manufacture principles - areas which are not so important for power amplifer designs.

This in part could explain the range and dsitribution of available BJT and MOSFET amplifer designs you are observing.

Dave

 

hi voduychau,

this is my simple answer, bjt's sounds better. you may agree with it or you may not. i have designed power amps using fets but they never sound better or at par with a bjts. are you an audiophile?

moz

 

I think the gain-bandwidth product and linearity of BJTs is much better than that of MOSFETs. If you used MOSFETs large enough to get open-loop gain equivalent to that obtained with the BJTs, I think the open-loop (and closed-loop) bandwidth and slew rate would be much worse, and the linearity would be worse.

 

Look at www.diyaudio.com , there you can discuss why mosfets are used mostly for output stage.

 

Hi voduychau

About the difference between MOSFET and BJT. Know that MOSFET is a component that is drived by voltage and therefore it is used frequently at the output stages where the input voltages to the gate of the Mosfet are from 5V to 20V. Therefore Mosfet is drived from min voltage 5V. Under 5V it doesn't work, it is blocked, i.e it depends from the gate threshold voltage of the Fet. At some Fets this gate threshold voltage is 2V. The BJT is a component that is drived by current. Therefore the BJT can be drived from input voltage under 5V, or with signal mV, uV. The FET can't be drived with signals mV or uV

 

somebody tell me FET/MosFET is good,and now i think they have something wrong in their benefit.

with me,i like BJT,because it's easy for me.

thanks for everybody!

 

somebody tell me FET/MosFET is good,and now i think they have something wrong in their benefit.

MOSFETs are good! As you are discovering, it is more a case of application specific requirements, where BJTs are suited to one application and FETs are suited to another. As an example of how "good" MOSFETs are, acquaint yourself with CMOS technology and its impact on the microelectronic world.

Dave

 

thank you very much,Dave!

all best to you.