# Power Amplifiers

Discussion in 'Homework Help' started by zdzislavv, Jun 26, 2009.

1. ### zdzislavv Thread Starter Member

Jun 5, 2009
22
0
Hello!
http://images47.fotosik.pl/152/9af0765afc52047e.jpg
I want to solve this exercise about power amplifier. I notice that for ideal amplifier current entering minus is equal to zero so I can short R2 (there is no voltage on R2). The other thing is E=6V so I guess there can be nothing above +6V and nothing below -6V on the graph. Can you give me any advices how I should proceed with solving this exercise?
Greetings

File size:
86.8 KB
Views:
43
2. ### Audioguru Expert

Dec 20, 2007
10,788
1,212
The exercise does not spec an opamp part number so we don't know what is its output saturation voltage loss.
It also does not spec part numbers for the transistors so we don't know their saturation voltage loss.
The speaker impedance is not spec'd so we don't know its current.

It is a horrible circuit with extreme crossover distortion. Teachers should use better circuits.

3. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,667
1,311
This circuit work with 100% negative feedback, and this mean that crossover distortion will be reduce by a factor of Aol- Open loop gain.
If crossover region is Δ1.4V Then after apply 100% negative feedback the crossover region will be reduced to ΔU=1.4V/Aol

4. ### zdzislavv Thread Starter Member

Jun 5, 2009
22
0
Hello!

Thanks for your reply! We've got group of tests after all laboratories and I haven't passed two of them (from about twelve) - Sine-wave oscillators and Power amplifiers. On Tuesday there'll be last possibility of passing it so I though it may be good idea to try to solve exercises from the previous year - it is what I try to solve in this post.

I tried to simulate it in Multisim 7 (because I cannot use Pspice). But I don't know how to apply those +E and -E in the circuit in this application. If it wasn't horrible circuit, I wouldn't ask you for help .

I include picture with schematic in Multisim 7 (http://images48.fotosik.pl/152/8835afd9983bfffd.jpg), materials which I found (8power.zip) and Multisim file with schematic (8power01.zip with .ms7 file).

But the most important thing for me is not to simulate it but to be able to solve it manually on sheet of paper. I can also notice that emitters of those two transistors are connected together. But still I don't know how to solve this exercise.

Greetings!

EDIT: You were 2 minutes earlier than me, Jony130 .

File size:
4 MB
Views:
16
File size:
9.2 KB
Views:
11
File size:
98.4 KB
Views:
31
5. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,667
1,311
All you have to do is to look at "power amp.jpg"

6. ### Audioguru Expert

Dec 20, 2007
10,788
1,212
Most people can hear 10kHz. But the crossover distortion produces a lot of 10khz.
The open-loop gain of an opamp drops above only 5hz so that its gain at 10khz is only 80 for a 741 opamp. There will be plenty of crossover distortion.

7. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,667
1,311
Yes, you are absolute right. This is only a theoretical circuit, with almost non practical application.