# clipping op amp

Discussion in 'Homework Help' started by leftbehind5805, Mar 13, 2010.

1. ### leftbehind5805 Thread Starter New Member

Mar 12, 2010
8
0
attached is a circuit im working on and i am having terrible clipping at the output. My input is 7v. any suggestions on how to fix this.

Thank you.

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2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Do you mean 7V peak-to-peak or 7V peak input.

With a gain of 2.5 a 7V peak would clip. Try reducing the input to see when the onset of clipping occurs.

3. ### leftbehind5805 Thread Starter New Member

Mar 12, 2010
8
0
its a 7V peak input and i need the output to stay 7v since that is the output of my other circuit. i need the power across the load to be in between 3A and 25A. Is there a way to amplify the current even further with the transistors?

4. ### Bychon Member

Mar 12, 2010
469
41
The gain of this circuit is 6. The ua741 has a "typical" rating of +/- 13 volts (worst case is +/- 10 volts) and you're going to lose a Vbe in each polarity. Typically, you can get 12.3 volts each way, or 24.6 P-P. The maximum voltage at the input is 4.1 P-P just because you ran out of voltage.

Then there is the fact that the transistors are To-92 package...625 milliwatts each with a 200 ma limit on the collector. They can survive 200 ma (peak) if you give them a heat sink. Two hundred ma into 8 ohms is 1.6 volts peak. 3.2 volts P-P at the 8 ohm resistor divided by a gain of 6 = .5333 volts P-P at the input.

That's your final answer. The input voltage has to be .533V P-P or less to avoid clipping under typical conditions. I think I showed you enough math to figure where the clipping point is if the ua741 is in its worst case...20 volts P-P minus 1.4 for Vbe losses etc.

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
You certainly wont get anywhere near 25A (let alone 3A!) from the 2n3904 / 2n3906 combination. They are small signal general purpose transistors.

If the preceding circuit stage has 7V peak output then you could interpose a voltage divider to limit the peak input to the stage currently under consideration. This stage has a gain of 2.5 with the given components. You really need an overall gain of unity [true?]. So rather than using a voltage divider you could use a unity gain feedback configuration with the output feeding directly back to the op-amp negative input terminal.

6. ### Bychon Member

Mar 12, 2010
469
41
I just now saw your second post. I'm having trouble understanding it. You seem to be saying your input is 7 volts peak and your output needs to be 7 volts peak. To do that, you have to short out the 5 k resistor to make the gain of the amp 1 (one).

As for having a power rating of 3A to 25A, power is not labeled as "amps". If you meant "watts", the To-92 transistors can only deliver .16 watts into 8 ohms because of the 200 ma current limit. You need some way fatter transistors to deliver 3 amps OR 3 watts.

7. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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How do you figure that?

8. ### Bychon Member

Mar 12, 2010
469
41
The gain of a positive op-amp is 1 + Rf/Rc
1 plus Resistor (feedback) over resistor (to common).
In other words, if the voltage at the positive input is one volt, how much voltage will have to be at the 8 ohm resistor to get the negative input to one volt?
Positive 6v will send 1 ma through 6k.
5k will have a voltage drop of 5 volts and the 1 k resistor will have a voltage drop of one volt.
Presto! The voltage at the negative input is 1 volt.

9. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Are you referring to the circuit (aac.pdf) in post #1?

The values you mention are not shown in that post.

10. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Ah yes - I see the 8Ω [load]. But your analysis is incorrect.

Simulate the circuit if you don't believe me.

11. ### Bychon Member

Mar 12, 2010
469
41
I'll show you mine if you show me yours.
Oh wait..I already showed you my math.
Now that I've tried being cute, I don't have a circuit simulator and I don't know how to use one.
There are only 3 resistors in this circuit and one of them is not in the feedback path. How about you tell me how you get to a gain of 2.5 using 5k and 1k?

12. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,400
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You can get a free circuit simulation at ti.com Search for TINA on their website.

You can find alot of free simulators. pSpice and LTSpice are others.

13. ### hgmjr Moderator

Jan 28, 2005
9,030
214
bychon,

The gain is actually 2.5. If you look carefully at the feedback resistor is it 1.5K not 5K.

hgmjr

14. ### Bychon Member

Mar 12, 2010
469
41
Finally, a human pointed out my error. Thank you!

I thought the bit of blue under the resistor symbol was a stray piece of connector line like the blue ends on the op-amp connectors. (Good thing we have computer programs that can draw symbols on top of labels.) If I only knew how to use a simulator program, I would have entered the wrong values and "proved" the gain was 6!

OK. The gain is 2.5 and the maximum input voltage is 1.28 P-P for the current those transistors can carry. If the current carrying ability of the transistors was large enough, the supply voltage would not be part of the clipping problem.

Again, thanks for helping me read the labels.

15. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Sorry Bychon,

I should have realised you saw the resistor in question as 5k rather than 1.5k making the gain 6x in your case. Actually I did realise later, but had already retired to bed and couldn't be bothered getting up to resolve the matter.

I'm not sure how the OP is going with their design issues. Like a lot of these things they get an answer and move on or go to another forum.

Apologies once again for my insensitivity.

Regards,

t_n_k

16. ### Bychon Member

Mar 12, 2010
469
41
I think I saw that you have answered more than a thousand questions and the number of, "thank you's" is about zero. That clears up any delusions I might have had about being appreciated, and it tells you why I was afraid you might get impatient with me. You didn't. Thank you.

I shall now resume giving away free answers with no expectations except having fun exercising my mind...and maybe learning a few things along the way.

loqwan likes this.
17. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
That's great - you obviously know your electronics. Hope you enjoy the forum & I look forward to your contributions.

loqwan likes this.
18. ### retched AAC Fanatic!

Dec 5, 2009
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Speaking of which, Is the thank you button new? Within the past month?

19. ### leftbehind5805 Thread Starter New Member

Mar 12, 2010
8
0
Thank you for looking at my circuit and helping me. I dont check this site that often and that is the reason for the later thank you. And you are right that the problem is not with my op amp but with the transistors. I failed to check the capabilities of the 3906 and 3904. My final design will consist of the 3904 and 3906 feeding into tip 31 and tip32. A group of mine is using this circuit for an assignment where you have to design and build a karaoke machine. If would like we have created a simple website to show our process that you can see at teamfor.info. There you will be able to see a more recent update on the power amplifier but still does not have tip 31 or tip32 yet. You should be able to see where i will be making the substitutions.
once again, thanks for your help.

20. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
I'm seeing R10 as a 1.5K value. Not 5K