Audio Amplifier. Which class should I use?

[goldsmith]

shayaan . do you want to create a class B amplifier or AB ? of course AB . so what is the condition of class AB ?

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But this is a coupling capacitor. If I don't use it then DC voltage will make disturbance with next stage or I will get shifted output. Isn't it?

You can change it's place ! series with out put of each transistor ! did you get the idea ?

 

[godfreyl]

So its mean base should not be ground. Base must be at some potential. I think must be positive potential.

Can the collector be at -28V if the base is at a positive potential?

 

[Shayaan_Mustafa]

Well conditions for class AB. Honestly, I am blank from this side. I just know some little things about this class, its efficiency is 78.5, it is between class A and class B. It is near to cutoff. But don't know conditions.

I told you earlier that I would learn many new things during this project which I have never read before.

You can change it's place ! series with out put of each transistor ! did you get the idea ?

Yes I think I got idea. look an attachment.

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Like this.

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Can the collector be at -28V if the base is at a positive potential?

I think. No.

But if base is at a negative potential then collector be at negative too. Isn't it?

 

[goldsmith]

How collector can give you the out put , without any series resistor ?
And an important law for class AB : biasing BE junctions .

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BTW ; do you know about crossover distortion ?
And another thing : the 78.5 percent can't obtain in practice ! perhaps 68 percent or 72 percent !

 

[Shayaan_Mustafa]

godfreyl goldsmith You both experts said me to simulate my circuit.

So I will give you result by tomorrow of my simulation of the circuit that I have posted in post#144

Kindly suggest me better simulating software too.

Thanks. Good night. Its i 3:18AM here.

I will post result tomorrow.

Thanks guys. You both on this forum are so much helpful.

 

[goldsmith]

Shayaan , the Pspice would be fantastic ! BTW : which country you are in ? here in my country time is 2:58 morning !

 

[Shayaan_Mustafa]

How collector can give you the out put , without any series resistor ?

You mean I should place a resistor between bases of the two circuits. Right?
See attachment please.

And an important law for class AB : biasing BE junctions .

I don't know this law. Thanks for this. Now it is in my knowledge that BE junction should be bias for class AB.

BTW ; do you know about crossover distortion ?

Yes I know about cross over distortion. To prevent cross over distortion we use diode bias class AB. Right?

And another thing : the 78.5 percent can't obtain in practice ! perhaps 68 percent or 72 percent !

Yes I know this. 78.5 is the theoretically efficiency.

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I am from Pakistan. I know you are from IRAN. Here is 3:24AM

 

[goldsmith]

You mean I should place a resistor between bases of the two circuits. Right?
See attachment please.

No , between collector and vdc . why you used those capacitors in base ? the bse needs DC bias too ? it isn't enough ! .

I don't know this law. Thanks for this. Now it is in my knowledge that BE junction should be bias for class AB.

Do you know the reason of this law ?

 

[Shayaan_Mustafa]

No , between collector and vdc

See attachment. I have modified it.

why you used those capacitors in base ? the bse needs DC bias too ? it isn't enough ! .

These are bypass capacitors. I used them so that AC signal could by pass easily.
As you said base needs DC bias too. So it means I should use resistors in individual bases?

Do you know the reason of this law ?

No. Don't know the reason.

 

[Shayaan_Mustafa]

goldsmith you told me to simulate circuit and give result to you. So here it. I know it is completely wrong. Because I don't know correct values of components i.e. resistors and capacitors. Also I don't know which transistor should I use and how much input AC should I use?

Input AC may be 2V.

 

[goldsmith]

Shayaan , your circuit has many problems . such as : why you used Vac as input ? it is for frequency response measurement , not sinusoidal wave ! you should use vsin .Why your capacitors are about 1n ? the capacitance of them is so high you should select around 1000 u ! why you used , those capacitors in series with base ?? thus the base has not any DC biasing ! so it can't work !

 

[Shayaan_Mustafa]

OH OK OK.. Now I understood why should not I use capacitors at base. If I use voltage divider then I can use capacitor. But here is not any connection between base and vdc. So, capacitors are not necessary.

Can you tell me library for vsin in pspice?
In source library I found vac. There was not vsin.

Also why should we use capacitors of such high values?

 

[goldsmith]

i think you forgot to add all of the libraries . press P , when you are in capture CIS , and then select add library . and select all of the thigs that are in your page . and after that , repress P . and then select vsin , easily .
Best Wishes
Goldsmith

 

[Shayaan_Mustafa]

OK thanks. May I ask what is the difference in vsin and vac source. While both are AC sources.

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One more thing. Sir kindly tell me about why could we chose high value capacitors? i.e. 1000n

 

[goldsmith]

OK thanks. May I ask what is the difference in vsin and vac source. While both are AC sources.

Vac , will vary the frequency , in the boundary that you'll select at AC sweep . but vsin will give you sa sine wave with your desired specifications .

One more thing. Sir kindly tell me about why could we chose high value capacitors? i.e. 1000n

I think your mean is 1000 u ? because as you probably know each capacitor has a reactance . that will given by XC=1/2*pi*F*C (F*2*pi called omega ) . the aim of that capacitor is to be short circuit instead of AC signal . but if the value that you have selected will be low , it won't be as short circuit as enough .

 

[Shayaan_Mustafa]

Yes I mean 1000u. And yes of course I know we have to short circuit AC signal. But don't know how much value is required to short AC. Thanks. You make me understand. I will give you result by tomorrow with modifications. Thanks once more sir.

 

[Shayaan_Mustafa]

Hi goldsmith
Here is modified schematic. I have used 5v amplitude and 20KHz frequency. And 2500u capacitors. Two +-30V dc.

 

[goldsmith]

Hi shayaan . What happened ? you changed your circuit with a CE ?? you was working on CB ! where is your input coupling capacitor ?

 

[Shayaan_Mustafa]

Hi shayaan . What happened ? you changed your circuit with a CE ?? you was working on CB ! where is your input coupling capacitor ?

Yes you give me a CE circuit that's why I think to try CE first. If I success in this then I will move to CB.

See attachment please.

I have placed coupling capacitors. Now I think circuit's components are all placed. But don't know which values should I use for components. For example, I am using resistors of 1K. Don't know how much values should I use. Help me. I want to complete my project.

 

[godfreyl]

Hi Shayaan

There is a problem with your simulator settings.

Your input is 20KHz.This is fine but remember the period for a 20 KHz wave = 1 / 20KHz = 50uS.

You are only looking at the output for 1uS, so you see only a small piece of the waveform.

If you let the simulator run for 200uS, then you will see a few waves at the output, and get a good idea what the circuit is doing.

Regards - Godfrey