Audio Amplifier. Which class should I use?

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Hi shayaan
Let me simplify it ! at first you need a background about mosfet and bjt . at mosfet if you put 15 volts across the GS junction , the DS will become short circuit . but in fact not completely short circuit . the channel will have a little value of resistance . that called Rdson . it is low . ( and sometimes negligible ) .and for bjt : if you put 0.6 volts ( approx ) across the BE junction , the CE will become short circuit . but here you will have base current ( i.e form mosfet there is not any current for gate . because it has pretty high impedance ) . so as you know IC= beta*ib . hence , when vbe is 0.6 volts , ( or a bit more ) but the base current is limited , thus the collector current is limited .
you understand it ?
 
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Hello sir.

Thanks. I knew all these. But now you told me it in more simplify so I will say yes, I have understood.

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Hello sir.

Thanks. I knew all these. But now you told me it in more simplify so I will say yes, I have understood.
 

Humm . shayaan . before continue , let me optimize a thing : one of my best friends noticed me about behavior of bjt . and i think it is better to clarifying it : CE of a bjt won't be short circuit , exactly . it will have a voltage across it self that called VCE sat . ( when we prepared BE voltage and base current .) did you know this ?
Ok now let me continue . so , as you told you are agree with me about on / off region of a bjt or a mosfet . ok ? so , see below , please :

Can you simulate this circuit in pspice and show me the voltage across the resistor and across the DS junction , please ? try to change duty cycle , ( by variation in PW ) . and show me the result . what about power across the resistor ? or power across the mosfet ? can you compare them together ?
Best Wishes
Goldsmith
 

Yes sir I know this.

show me the voltage across the resistor
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248.3V across resistor.

across the DS junction
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You also asked voltage across DS junction. But sir DS junction will be act as short.

try to change duty cycle , ( by variation in PW )
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Sir tried by changing PW, I see the clear result when I decrease PW to 700ns or further low. I have attached thumbnail.

what about power across the resistor
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I=4amp to 5amp (approx.) and V=248.3V
P=VI
P=(248.3)(5amp)
P=1241.5W
P=1.2KW

or power across the mosfet
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Sir I am confused how to calculate power dissipation across MOSFET..

How much my calculation is right? I have attached thumbnails.
 

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  • PW 2us result.JPG
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  • PW 2us.JPG
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  • PW 700ns result.JPG
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  • PW 700ns.JPG
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Hi shayaan
Good night
Of course we can suppose that DS is short circuit ( approximately ) but i want the wave form of the voltage across it ( use differential probe ) ( do you know what is differential probe in Pspice ?
Why 700ns ? of course it is low . increase it until nearby 3.8 usec or decrease it as low as 1usec .
P=1.2KW
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Shayaan , can you remember how to calculate the average of a square function ? if yes , you have to show me the average value of power ! if you show me that i will describe my reason by these actions !and about power calculation across the mosfet use a wattage probe on the surface of your mosfet . show me it's average value !
Best Wishes
Goldsmith
 
320W (approx.) is dissipated across MOSFET.

And I know how to use differential probes. I have attached result.

Sir I know the formula of average of the square wave but sir I don't know what limits should I use?
 

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  • 1's result.JPG
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Why your maximum time is 1usec in measurement ? it should be around 20usec hence we can talk about it's power ! increase maximum time of simulation as large as 20usec . and show me again the results .
BTW : why integral ?( limits ?) we have calculated a result for that integral . duty cycle* amplitude of your function
 
Why your maximum time is 1usec in measurement ? it should be around 20usec hence we can talk about it's power ! increase maximum time of simulation as large as 20usec . and show me again the results .
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Sir you mean by this PW?

BTW : why integral ?( limits ?) we have calculated a result for that integral . duty cycle* amplitude of your function
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Yes sir, we have evaluated integral. Now we have formula,

Average=amplitude * D.C.

Sir I have given amplitude with 1.2KW
But sir duty cycle??
 

Hi shayaan .
No of course i'm not referring to the PW . i referred to Edit simulation time ! ok ?
And about Duty cycle .
I think i told you at past . but no problem . again : Ton/T T is 1/period time . ok ?
Best Wishes :wink:
Goldsmith
 
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No of course i'm not referring to the PW . i referred to Edit simulation time ! ok ?
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Sir I don't know how to edit simulation time. how much time should I set, I don't know.

I think i told you at past . but no problem . again : Ton/T T is 1/period time . ok ?
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Sir still confusing.

Average=amplitude * D.C.
Average=1.2K * 1/period

Like this?
 

Sorry if my descriptions weren't as clear as enough .
see below , please . i told you what step needed ! increase it as large as 20 usec :



And about duty cycle , my mean was not that ! let me give an example : consider that we have a square wave , with amplitude = 50 volts and frequency = 250KHZ and D.C=0.5 ( it means ton =2usec ) . t=4usec . and ton =2 usec .
D.C= ton/T ======> 2/4=0.5 . ok ? so can you find your own ?

Don't lose your hope ! ( i believe that you will learn these things very soon and then you will laugh to these simple problems ! :smile
Good luck
Goldsmith
 
Thanks for reply sir.

Now I have understood but with little confusion.

1) You told me to increase step as large as 20usec. But how would I know how much I should increase step? Because every time you don't have me to tell me.

2) I know the formula for D.C. But how to know the value of ton and T? as you have used ton=2 and T=4. What should I use? Is it given to me anywhere?
 

1) You told me to increase step as large as 20usec. But how would I know how much I should increase step? Because every time you don't have me to tell me.
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there isn't any special methodology ! simply , i told you to do that because thus we can see some cycles of this square wave ! is it unclear ?
2) I know the formula for D.C. But how to know the value of ton and T? as you have used ton=2 and T=4. What should I use? Is it given to me anywhere?
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You can measure it from your waveform ! but don't worry about it ! you will select it at future ! don't forget that i still didn't tell you what is my purpose by these things ! i bet after some day you will be curious and wonderful about what i have tried to do with your mind !
 
there isn't any special methodology ! simply , i told you to do that because thus we can see some cycles of this square wave ! is it unclear ?
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Yes sir, because by increasing step I can't see any cycle further. here is attachment with 20usec step.


So by this time can I use D.C.=0.5?

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here is attachment
 

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Sorry sir... I have managed pulse width..
Now I have result... attached.
 

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Hi shayaan
Well , now it is good .calculate average power of RL and average loss power across the DS and show me the result of your calculations .
 

Hello sir..

Average for RL's power:

Average=amplitude*D.C.
Average=1.2K*0.5
Average=600W

Average of Ploss across DS:

Average=amplitude*D.C.
Average=320*0.5
Average=160W

Here is my calculation.
 

Yes sir I can,

efficiency = pout/(pout+ploss) *100
efficiency = 600/(600+160)*100
efficiency = 78.94%
 

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