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silly question about op amp

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blibala

New Member
sorry..i know this is a silly question.but i face a problem with the op amp.
i just want to know the proper way of connecting the power supply to the op amp.
some ppl telling me to connect the positive terminal of the supply to the postive terminal and negative terminal to the other end.but when i connected like this,the output of the op amp(LM741 for example) is not correct.
how should i connect it?
 

crust

Member
Although there are exceptions (such as single ended amps) -- What you probably need to do is have a bipolar power supply. Your positive power side goes to +V and your negative power side goes to -V. your ground reference (0V) is in the middle. Note that I am referring to the positive and negative power terminals NOT the inverting and non-inverting signal inputs of the op-amp.
 

blibala

New Member
how to do that?i confuse with this one,although i know this is a esay job.can show me the wiring diagram?

can i use the same power supply for the op amp and whole circuit?

or anyone can explain in more detail?
 

crust

Member
You can use the same power supply for the whole circuit, though the opamp often needs a negative supply as well. I found this diagram which shows the amplifier.

For instance, if you are using +/- 15V rails, your +15 goes to the red terminal, your -15 goes to the green. There is a difference of 30V between the red and green terminals. You would use two 15V supplies in this case. The ground is considered the point where the + terminal of the negative supply is connected to the - terminal of the + supply.
 

Kane2oo2

New Member
hello

talk about negative voltages confuses me... how can a voltage be negative? i dont understand how you can have a negative of something that isnt there!

:?

Kane
 

crust

Member
Voltages are arbitrary, the voltage is the measure of potential between two points in the circuit. I can assign any point to be 0 and measure the potential between that point and any other in the circuit. For instance, if 0 is the + terminal of a battery, then the voltage at the minus terminal with respect to the positive terminal is negative.
 

mozikluv

New Member
supply for 741

:D hi kane,

the 741 is a dual supply op-amp and what i understand is you still want to use your single supply. it can be done, you have to have a voltage divider. sorry i could not show a simple diagram, my scanner has gone kaput for the moment, anyway here's how to do it;
1. from your V+ series a 47K resistor and connect it to the non-inverting input or pin 3.
2. from your pin 3 connect a 47K resistor and connect to ground.
3. from your pin 3 connect a 0.1uf capacitor and connect to ground.
4. input your signal to the inverting input or pin 2
5. connect pin 4 to ground

hope this can help you :)
 

hughmcc

New Member
negatice voltages

I have also wondered about negative voltages and why they exist since a voltage is a relative mesurement. For example, I see requirements for a supply having a +12 and a -12 supply. This means the 2 "rails" have a difference in potential of 24 volts. Would you be able to use two, 12 volt batteries hooked in series? For those devices that require "+12", could you not use the combined batteris' +24 with those devices and have them "grounded" to batteris-in-series +12 point. And for those requiring "-12" , use the combined batteries "0"volts as the supply and "ground" these to the "+12"?. Then can you then build a supply powered by AC the same way? Or is there something unique about the symmetry of using the AC's positive half for the "+" and the negative half for the "-". Perhaps its easier to create 2 opposite polarity sources this way and have mirror image outputs which also reflect the same imperfections.
Anyway, I am curious why there is no discussion of the "need" for negative voltages.
Thanks
 

Optikon

New Member
Re: negatice voltages

hughmcc said:
I have also wondered about negative voltages and why they exist since a voltage is a relative mesurement. For example, I see requirements for a supply having a +12 and a -12 supply. This means the 2 "rails" have a difference in potential of 24 volts. Would you be able to use two, 12 volt batteries hooked in series? For those devices that require "+12", could you not use the combined batteris' +24 with those devices and have them "grounded" to batteris-in-series +12 point. And for those requiring "-12" , use the combined batteries "0"volts as the supply and "ground" these to the "+12"?. Then can you then build a supply powered by AC the same way? Or is there something unique about the symmetry of using the AC's positive half for the "+" and the negative half for the "-". Perhaps its easier to create 2 opposite polarity sources this way and have mirror image outputs which also reflect the same imperfections.
Anyway, I am curious why there is no discussion of the "need" for negative voltages.
Thanks
You "need" negative supply voltages if you want or "need" your output to swing negative. Also, if you want your output to be at zero volts you usually need a negative supply because the way most opa amps are built their output stage cannot get to exactly zero volts so a slightly negative supply rail gives them the headroom they require to output zero. There are exceptions. Ones that advertise rail-to-rail output and "single" supply types can get to within millivolts (under light loads) of zero.

And yes, you can construct +/- supplies with 2 batteries.

Your statement about it being easier to create opposite polarity voltages and reflecting imperfections is not at all clear and doesnt seem related.
 

eemage21

New Member
I'm not getting the confusion?

Like someone said - it's all relative. Let's say you make your measurements relative to ground (0v). Now connect positive terminal of your 9V battery to ground - what's on the other side of the terminal? 0-12V=-12V.
 
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