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.

 

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.

 

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?

 

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.

 

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?

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

What is your intended load?

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.

 

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.

 

The gain of this circuit is 6.

How do you figure that?

 

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.

 

Are you referring to the circuit (aac.pdf) in post #1?

The values you mention are not shown in that post.

 

Ah yes - I see the 8Ω [load]. But your analysis is incorrect.

Simulate the circuit if you don't believe me.

 

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?

 

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.

 

The gain of this circuit is 6.

bychon,

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

hgmjr

 

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.

 

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

 

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.

 

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.

That's great - you obviously know your electronics. Hope you enjoy the forum & I look forward to your contributions.

 

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

 

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.

 

I'm seeing R10 as a 1.5K value. Not 5K