# Op Amp problem, no negative output signal

Discussion in 'Homework Help' started by yeyeb, May 3, 2010.

1. ### yeyeb Thread Starter New Member

Nov 16, 2009
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i re-posted my comment from
due to a member calling my post "hijacking".

Hello everyone, i am currently having the same problem. My op amp isn't generating negative part of the square wave coming from the sound port of my computer. Infact, although sounding ridiculous, i am using an analog multimeter to measure this and at 1Hz i don't see the needle staying at, lets say, 5V. It only changes in positive going tick (PGT) and goes to zero. You would probably say i shouldn't be using the signal generator form my comp but in any case can you explain why this is happening. Also how can i get my op amp to generate the negative part of my square wave? i am using inverting config with R2= 100,000ohms with R1=100ohms for maximum gain (gives me 0.7v input to 6v output) however going R2=1,000,000 ohms gives me 1v, i don't understand why.

Quote:
Originally Posted by mik3
I Also, you will need to bias the non-inverting input at 2.5V. This can be done with two 10K resistors forming a voltage divider assuming you are going to use 5V for the op amp supply
mik3, can you please post a circuit diagram of that and elaborate on what you mean by baising the non inverting input and why do you need to do that.

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
You need to post a schematic of YOUR circuit as it now exists.

As a wild guess, you are either trying to use a single supply opamp and the input signal is going below the opamp's negative rail, or you are using an opamp like an LF353, TL082, TL072 that can't "see" within 3v of the negative rail.

3. ### yeyeb Thread Starter New Member

Nov 16, 2009
7
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I am using LM324N op amp and i posted the schematic which is rather simple. i am generating square wave of 0.7 to 1Vpp from my comp through a program called SOUNDCARD SCOPE

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4. ### t06afre AAC Fanatic!

May 11, 2009
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As sgtWookie says you must have a dual supply voltage. If you do not have such device you can create one with two power supplys. Or use two 9 volt batteries.

5. ### yeyeb Thread Starter New Member

Nov 16, 2009
7
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can you provide a schematic with two power supply that is relevant to my schematic?
Also i am having problem with deciding gain of the op amp. i am using r2=100k ohm and r1=100 ohm to get very high gain but i don't see much difference if i use r2=10k and r1=100 ohm. What is the limit of gain i should be focusing on? and will same combination of R1 and R2 work with any input signal and any gain?

6. ### ELECTRONERD Senior Member

May 26, 2009
1,146
16
For audio your op amp should provide a reasonable bandwidth and dynamic range. A important notion to consider is the fact that: As frequency increases, gain decreases. The gain you choose will limit your bandwidth, and so it would be best to look at the specs of your op amp for maximum performance.

Austin

7. ### t06afre AAC Fanatic!

May 11, 2009
5,939
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Before we go any further can you please tell us
1)what you want to achieve.
2) What is your supply voltage
3) complete schematic
4) Why this high gain

8. ### yeyeb Thread Starter New Member

Nov 16, 2009
7
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thx electronerd
1)all i want is to amplify the signal from my audio port to about 5-6Vpp from i think about 0.7Vpp input.
2) my supply voltage is 6V heavy duty battery that i am using but if there is a need i can get 9V battery
3) i uploaded a new schematic called "op amp 2.jpg"
4) I thought more gain would give more output volts. And even though i am using R2=100k and R1=100 ohms which equals 1000 gain, i am not seeing any significant output (i.e 6 v) also making R2=10k and R1=1k didn't give me any significant boost in volts. i don't understand why ppl use ,such as, R2=230k ohms and R1= 11.5k instead of R2=2k and R1=100 ohms to get 20 gain.

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9. ### t06afre AAC Fanatic!

May 11, 2009
5,939
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Have you download the LM324 data sheet? In this data sheet you will find many circuits suggestion. Pick one of this circuits that is most close to what you want to do. And then we can use that as starting point. Because now I am somewhat confused about your goal. Your output voltage will also be limited by your battery and the LM324. I think the max output from LM324 is VCC-2 volt approx. And with only positive supply voltages you can not have AC voltage as input. Only a negative voltage. As your circuit is now. And that will produce a positive output voltage

Nov 16, 2009
7
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11. ### t06afre AAC Fanatic!

May 11, 2009
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You connect the posetiv supply rail to V+ and the negative rail to GND (pin 11). I remember somewhat wrong the max output voltage is VCC-1.5 volt. I am not sure for the negative rail.
Also take a look at this data sheet http://www.national.com/ds/LM/LM124.pdf It can give you some design ideas. Tell us which figure that is nearest to you design goal.

12. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Sgtwookie correctly identified your problem. One way to get around the problem is to create an artificial voltage reference that is equal to your input power supply voltage divided by 2. This can be accomplished by using a couple of equal valued resistor that forms a voltage divider off of your incoming positive voltage supply. In your case you could use a couple of 2K resistors to form the artificial reference voltage that would be 3V since your input power is 6V. Then disconnect the opamp's positive input from ground and connect it to this artificial 3V reference.

hgmjr

13. ### yeyeb Thread Starter New Member

Nov 16, 2009
7
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Thx hgmjr and very thx to sgtwookie, lol sorry that my questions had to make u say the same thing twice. After snooping at some other forums, folks there have been suggesting to use voltage regulator to make negative power supply (connecting its ground to Vcc and its Vcc to ground). I just want to make sure if you guys suggest that it would work.

14. ### Audioguru New Member

Dec 20, 2007
9,411
896
An LM324 with a gain of 1000 has an output up to only 300Hz if the supply is 10V to 15V. Frequencies above 300Hz will not have as much gain.

A voltage regulator will not generate a voltage, it simply regulates a voltage that is already there.
A voltage regulator does not work when it is connected upside-down.