Why does every school project here have a student without a text book and without a teacher??

That's the point.
I do have books (Sedra/Smith and Razavi). I don't have the time to read them thoroughly since other exams keep copming, one after the other... It's like a war against time.
Regarding the teacher's work I prefer not to speak.
I just try my best...

I played with Q5 B-C resistor and the output transistor Emitter resistors.
The red trace in for one transistor and the green trace is current in the other transistor. Blue is output voltage.
Note at zero voltage out the current in each transistor is 100mA. This could be set with the input signal=0V.
I used chose 1.6 watts in each transistor when nothing is happening. (lost power but needed)
Both transistors need to be a little bit on at 0V or there will be cross over distortion. (bump in the signal at 0V)

Whoa! Thanks for the circuit suggested.
Do you think I can get even less than 100 mA in the output transistors at 0 voltage out? I think that in such a way the distorsion increases, right?
Do you think that I can get rid of R11?
Why did you put C1 in the retroaction?
What is R12 at the input for?
Thanks a lot

By measuring the quiescent current in the output transistors.

It's less than 0.65*4 (my initial thought) because not all 4 darlington transistors are full power at the same time, right?

 

It's less than 0.65*4 (my initial thought) because not all 4 darlington transistors are full power at the same time, right?

The quiescent current is the current through the output transistors with no input signal. For class AB this should be small but not zero.

 

If you have time try to read this:
https://forum.allaboutcircuits.com/...ign-basic-questions.104719/page-6#post-804580
https://forum.allaboutcircuits.com/...o-amplifier-calculations.160221/#post-1397602
http://www.kennethkuhn.com/students/ee351/power_amplifier1.pdf

 

Lucky-Luka, did you notice in posts #12 and #19 that the output transistors are not darlingtons with 4 base-emitters in series but instead they are Sziklai pairs (look in Google) that have only 2 base-emitters in series?

 

Lucky-Luka, did you notice in posts #12 and #19 that the output transistors are not darlingtons with 4 base-emitters in series but instead they are Sziklai pairs (look in Google) that have only 2 base-emitters in series?

Post #19 uses the standard darlington arrangement.

 

The parts in post #19 are so far apart they are almost too small to see the details.

 

Lucky-Luka, did you notice in posts #12 and #19 that the output transistors are not darlingtons with 4 base-emitters in series but instead they are Sziklai pairs (look in Google) that have only 2 base-emitters in series?

Is this reply related to the voltage value at the end of the Vbe multiplier?

 

Is this reply related to the voltage value at the end of the Vbe multiplier?

Yes.

 

I was looking at the circuit AlbertHall posted in his #10 post.
It is written that the quiescent current is 15 mA but in the graph I see near zero value for the same current. Shouldn't the small signal sit onto the biasing value?
I don't quite understand this concept...

 

The quiescent current is 15 mA only when Vin = 0V and both BJT's are ON (Class A). But for positive voltage at the input NPN will start to conduct more current. But at the same time, PNP will start to decrease his current. And at the time when IL = 2 * Iq = 30mA, the PNP will be completely off. And only NPN will provide the current onto the load (Class B).
https://forum.allaboutcircuits.com/...ed-on-push-pull-amplifier.101749/#post-766570

 

It is written that the quiescent current is 15 mA

I think it is a little hard to hit 15mA. Over temperature it will change. Just saying it seems small to me.
I don't really know what value to use. It is a trade off of distortion vs power loss.
I can not find a answer on the internet and my audio amp design books are in my office.
Several sources say to set the quiescent current just to the point where crossover distortion can not be seen on a scope.

 

I think it is a little hard to hit 15mA. Over temperature it will change. Just saying it seems small to me.
I don't really know what value to use. It is a trade off of distortion vs power loss.
I can not find a answer on the internet and my audio amp design books are in my office.
Several sources say to set the quiescent current just to the point where crossover distortion can not be seen on a scope.

I have tried to replicate your circuit... Something went wrong. There must be an error but I can't see where it is.

 

I don't really know what value to use. It is a trade off of distortion vs power loss.

Here you can find the answer
http://hifisonix.com/wordpress/wp-c...oss-Over-Distortion-in-Class-B-Amplifiers.pdf

I have tried to replicate your circuit... Something went wrong. There must be an error but I can't see where it is.

Change C1 value from 33pF into 33μF

 

C1=33uF That is what I have.

 

Now I'm more confused...
I've tried to ground the input as Jony130 said.
I get 87mA... not 15mA...
What's going on?
Is the DC point mentioned in spice wrong?

 

Everything is alright and stop blaming the LTspice. Notice that when Vin = 0V the V4 DC voltage is equal to -1V.

 

The point you chose to ground is not right. It is about 0.9V away from "Vo". Just like grounding the Base of Q2 is also about 1 volt away form Vo. The voltage from B to E varies with load current. (and temperature)

 

Everything is alright and stop blaming the LTspice. Notice that when Vin = 0V the V4 DC voltage is equal to -1V.

ehm.. I've never blamed LTSpice... just questions... And i'm grateful for all the answers...
But... I haven't understood... sorry.

 

But... I haven't understood... sorry.

What you do not understand?
You want to understand how push-pull amplifier work or what?

 

When you said "The quiescent current is 15 mA only when Vin = 0V and both BJT's are ON (Class A)." I thought that grounding the input would give me those 15mA.
I haven't considered the fact that Vin has to be 0V as Vo has to be 0V.
Maybe I'm starting to undertand what you said before.