# Audio Amplifier

Discussion in 'Homework Help' started by sbin19, Dec 6, 2012.

1. ### sbin19 Thread Starter New Member

Dec 6, 2012
8
0
Hello everyone,

I hava a critical question related to my project. In our project, we have to amplify a signal 2V AC to 10V AC sinusoidal. How can we do this? The problem is that I know that MOS and BJT amplifiers have a limited input voltage like 5mv or 200mv at most. Their maximum gain is also limited. Then how I can get a 10V sinusoidal by implementing them? It seems impossible to me. If I have a 5mV signal ( that is the maximum input voltage that BJT can accept for small signal condition) , then also I have -100 gain, then the output is 100x5mv= -0.5V AC.. It is much smaller than 10V AC.. Can you help me at this point, thanks a lot?

( Project: Audio amplifier. We need to drive 8ohm speaker at 6W. The input is a mp3 player, can supply 1V-2V Ac sine wave. Any other recommendation related to project will be fine also - use bjt mos amplifiers and class ab output amplifier)

2. ### MrChips Moderator

Oct 2, 2009
17,326
5,335
Go buy a PC or mp3 audio amplifier.

3. ### sbin19 Thread Starter New Member

Dec 6, 2012
8
0
It is a term project, I dont think this solution is a valid one, do you ?

So the question again:

Hello everyone,

I hava a critical question related to my project. In our project, we have to amplify a signal 2V AC to 10V AC sinusoidal. How can we do this? The problem is that I know that MOS and BJT amplifiers have a limited input voltage like 5mv or 200mv at most. Their maximum gain is also limited. Then how I can get a 10V sinusoidal by implementing them? It seems impossible to me. If I have a 5mV signal ( that is the maximum input voltage that BJT can accept for small signal condition) , then also I have -100 gain, then the output is 100x5mv= -0.5V AC.. It is much smaller than 10V AC.. Can you help me at this point, thanks a lot?

( Project: Audio amplifier. We need to drive 8ohm speaker at 6W. The input is a mp3 player, can supply 1V-2V Ac sine wave. Any other recommendation related to project will be fine also - use bjt mos amplifiers and class ab output amplifier)

4. ### MrChips Moderator

Oct 2, 2009
17,326
5,335
You didn't say this was a school project.

Then build a 6W audio amplifier.

5. ### sbin19 Thread Starter New Member

Dec 6, 2012
8
0
I know, I try this .. I asked a question though. Are there any one who can answer my question? But I need real answers.

Thanks

6. ### MrChips Moderator

Oct 2, 2009
17,326
5,335
What is the question?

7. ### PackratKing Well-Known Member

Jul 13, 2008
849
219
how many stages are you allowed ?? Google : "push pull transistor audio amplifier" and take your pick

8. ### bertus Administrator

Apr 5, 2008
18,684
3,634
Hello,

I moved the thread to the home work section.
Show us what you have done up-to now and we will help you to improve the project.

For information on audio amplifier theory, take a look in the links of the following page:

Bertus

9. ### #12 Expert

Nov 30, 2010
18,090
9,663
Oops. Forgot to change the supply voltage to 15 volts. Remember that when you look at the drawing.

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10. ### sbin19 Thread Starter New Member

Dec 6, 2012
8
0
Yes , this is a good idea, thanks. Do you know whether this configuration has a small signal condition or not? For example can I apply 5mV at the input and I get 0.005*ß1*ß2 Volt gain. ( ß1, ß2 : Beta of Transistors)

11. ### sbin19 Thread Starter New Member

Dec 6, 2012
8
0
I do not know much about audio amplifiers. I actually want to create one by using what I know already . At first I thought following block diagram:

1V AC input --> Voltage division --> Voltage buffer --> Voltage amplification --> Class AB amplifier-->8 ohm Load (6W)

At least 6W is needed. So say 10V AC is needed for Class AB amplifier input. It means voltage amplification part has to provide 10V AC. However, I could not design this "voltage amplification part" by using common-emitter amplifier, common-source amplifier, and their combinations. The problem is that this amplifiers cannot give 10V AC at the their output, as they have input signal limitations. I only wonder how I can achieve this??? This was what I did, and what I asked for..

(darlington seems possible by the way, any other solution?)

Thank you ver much..

12. ### #12 Expert

Nov 30, 2010
18,090
9,663
I just gave you the circuit that will make 10 volts peak to peak from 2 volts peak to peak. No you can not get B1xB2 out of this circuit. It has negative feedback that forces the gain to be 5.

Think for a while. You are missing the obvious.

Last edited: Dec 6, 2012
13. ### Papabravo Expert

Feb 24, 2006
11,737
2,483
Where did you get the idea that the input was limited to a couple of milli-volts? If you look at a proper DC load line you can establish your Q-point for class A (linear) operation. From the DC load line and a set of characteristic curves you should be able to establish an AC load line and figure out how big the input voltage can be while maintaining linear operation.

If you don't like class A operation you can move on to class B or Class AB. You have not said what power supplies are available for this project. For 10V P-P out I'd start with a 20VDC supply.

#12 likes this.
14. ### sbin19 Thread Starter New Member

Dec 6, 2012
8
0
Maybe I did not express myself clearly.

1V AC input (music is given from mp3 player or computer ) .

We need to amplify this sound as we drive 8 ohm speaker ( at least 6W is wanted)

I am sure that at the output stage will be Class AB amplifier. Thats my choice as it has no distortion. However, at the input of Class AB ( or any Class x) I need 20Vpp AC signal so that at the load 6Watt condition is satisfied.

The problem is that I do not know how to amplifiy input signal ( from mp3) to 20Vpp AC by using standart CE or CS amplifiers. I do not think it is possible because according to my knowledge CS amplifer has a small signal condition. If input is larger than some value, say 200mV , then distortion occurs. Am I wrong?

I need another way maybe..

The project is here , 7th project, if you interested.

http://www.mems.eee.metu.edu.tr/courses/ee313/Project combined.pdf

Thank you all..

15. ### MrChips Moderator

Oct 2, 2009
17,326
5,335
Hence, reduce your input signal to below 200mV.

16. ### Audioguru Expert

Dec 20, 2007
10,605
1,182
The beta of a transistor is its current gain, not its voltage gain.
Have you heard of "negative feedback"? It reduces voltage gain and reduces distortion.

A transistor can have a voltage gain of about 180 without any negative feedback. But then its distortion is 40% or higher.
If you add some negative feedback to a single transistor then its voltage gain and distortion are reduced. An amplifier made with 2 transistors can have a reasonable voltage gain (from 1 to 200) and have very low distortion.

17. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
510
You did not give the ground rules for the project. You could use an IC like TDA2003 to do this pretty easily. There are tons of fully integrated solutions to do this.

http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXutuqt.pdf

If you are not allowed to use a fully integrated solution, use an op-amp like a TL-071 followed by a discrete driver stage.

I have a design I did for exactly that, but I don't think posting a finished design would cause you to learn anything.

18. ### #12 Expert

Nov 30, 2010
18,090
9,663
Repeating myself: I just gave you the circuit that uses only bipolar transistors and amplifies 200 mv with very low distortion. Now that you have raised the voltage required to 20 volts peak to peak, you can change the votage supply to 30 volts and change the feedback resistor to get a gain of 100. That would be: change the 4 k resistor to 99k and change the base bias for an idle current of 150 ua at the 1k resistor.

Why do you think you can only use one common emitter stage to do this? Why are you concerned about distortion when I have given you a circuit that will do this with low distortion?

19. ### #12 Expert

Nov 30, 2010
18,090
9,663
Is this what you're thinking you both have to do and won't work because of distortion?

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Dec 6, 2012
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