op amp power question

Discussion in 'General Electronics Chat' started by count_volta, Aug 3, 2010.

  1. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    I have a simple question about power supplies for op amps. The only place I ever used them was in lab at university. They didn't really explain why op amps have + 15V and -15V for example except that it means the output can go as high as +15 and as low as -15.

    In lab we have power supplies that have a positive and negative terminal as well as ground. We always used all 3 terminals for op amp circuits.

    So now I want to hook up an op amp at home, and all I have is a power supply that can go between ground and +15V. I want to amplify a sine wave from the new function generator kit that I just put together in order to test the function generator.

    If I connect Vcc to +12V and Vee to ground on a 741 op amp, will that mean that only the positive half of my sine wave will be amplified and the negative part cut off?

    If that is true, how do I fix that if all I have is the power supply with +15V and ground terminals?

    Thanks
     
  2. BillB3857

    Senior Member

    Feb 28, 2009
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    You could use a voltage divider with equal value resistors in series. For any external measurements and for input "ground reference", use the center connection of the two resistors. This assumes that the power supply output is isolated from earth ground since most 'scope grounds are tied to earth through the connector. Your 741 will be operating on +/- 7.5 volts.
     
  3. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    Ermm that sounds a bit confusing. Don't quite get it.

    Well, you are saying the voltage divider node will be ground, the ground lead will be -7.5 and positive lead +7.5?

    Well, lets say I have my power supply, or battery. It has a positive terminal and ground. Lets say I'm connecting it to a zener diode through a resistor. If I connect the ground terminal to the anode, and positive 5V terminal to the cathode I would have -5V across the diode.

    So could I just connect the power supply backwards for the Vee part of the 741? The only thing that is confusing is what the heck will ground be then.

    Negative voltages have always confused me. Its always negative with respect to ground and positive with respect to ground. Then what the heck is the ground? Its hard when all you have is a battery.

    I could use another power source like a battery, but real circuits have just one power supply and still manage to get negative voltages. How?

    Please help me out guys.

    Thanks
     
    Last edited: Aug 3, 2010
  4. SgtWookie

    Expert

    Jul 17, 2007
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    The 741 opamp was great 40 years ago when it was introduced. Nowadays, academia is about the only place they're used. They are very slow and have crossover distortion. They can't help but make the output from even a mediocre signal generator worse.

    Why don't you post what kind of outputs you expect to see from your signal generator, so that we can even begin to suggest a suitable amplifier?

    In the meantime, if you have a spare ATX form factor computer power supply sitting around, you can make a pretty decent bench power supply from it, on the cheap. Google "ATX bench supply" for lots of ideas.
     
  5. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    Heh, I'm just using the 741 for testing purposes. We built many circuits with it in school. I saw the limitations of it, but its good enough when learning.

    That isn't my question though. My question is really the following sentence, "how to make a negative voltage AND positive voltage AND ground with a single battery?"

    I mean many real circuits use a single battery and have negative voltages and positive voltages at the same time.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    A battery is a truly independent "floating" power supply.

    In any given circuit, it is convenient to declare some point in the circuit as having a potential of 0v, or ground. Most other voltage measurements are referenced to this 0v point. Otherwise, you could wind up with ridiculously large numbers, which would complicate the math considerably.

    In the old days (prior to around 1960) most vehicles used positive ground/positive earth, up until the point where it was discovered that ignition systems worked better if the negative terminal was ground/earth. The only real problem with converting from positive to negative earth was the radios of the day.

    If you are going to be powering your circuit from a battery, you can make a voltage divider from two equal-value resistors that will allow a 0.1mA to 1mA current to flow between them; the junction will be the battery voltage/2. For a 12v battery, that would be roughly (12v/2)/0.5mA = 12k each, for a total of 24k Ohms. You can use an opamp as a voltage follower to establish an active assertion on the ground point.

    If you are using a power supply that is referenced to earth ground via it's plug, you will need to AC couple your input signal using capacitors to block the DC level, and bias the input around the 0v point. You will also need to AC couple the output.
     
  7. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    Dang, how come nobody teaches us this stuff in EE school? This is the first time I ever heard of such a thing. They don't teach us many things in fact. And since half my classmates could care less I can see what wonderful engineers they will become. ;)

    Edit: Found a nice thread on the forum discussing this issue.
     
    Last edited: Aug 3, 2010
  8. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    Could you please explain the above quote in a little more detail. How exactly does this work? How do you know this, and why don't they teach this in Circuits 1?

    They always told us, pick a point in your schematic and call it ground. I always noticed that point is connected to the negative side of the battery. So minus side of the battery is 0V by default?

    A better question is, how do volt meters know when a voltage is positive or negative?

    I regret associating this thread with op amps, because its negative voltage that I'm actually confused about. But its op amp applications that forced me to think about it.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    Any opamp will work from a single polarity supply if its input voltage is at half the supply voltage and the input, output and feedback resistor to ground are capacitor-coupled. Teachers never learned this.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    I've been through Circuits 1. And 2, and... let's just say I'm a few weeks ahead of you. ;)

    Have a look at the attached schematic and simulation using one channel of an LM353 dual opamp. It is essentially the same as a TL082 that you can buy at Radio Shack, if you have no other source. Digikey sells them for far less, and will ship via USPS 1st Class for SMALL orders.

    Note that I have declared the non-inverting input to the opamp as ground. Note that R1, R2 are equal. This ensures that the power "rails" will remain equidistant from each other.

    Note that C1, C2 are equal; they keep the "resistor noise" at a minimum. Think of resistors generating noise like the nozzle of a garden hose. Think of capacitors as buckets of water - you toss the garden hose nozzle in the water bucket, and the noise goes away.

    Also note that the VGND output (Virtual GND) is not precisely 0v; this is due to the input offset of the opamp itself. For an application like this, it is not critical - however, it must be kept in mind for when you get into precision opamp circuits, particularly where amplification is used - as the input offset is amplified along with the input signal. This can result in gross errors in precision applications.

    The inverting (-) input tries to make the output do what the non-inverting (+) input is doing. This is a key point. If you don't understand this, you will never understand opamps.

    LF353/TL072/TL082 are very old opamps as well, but far outclass the tired and ancient 741 and derivatives. They have their own shortcomings; they cannot "see" within -3v of Vee, (negative supply), nor within 1.5v of Vcc.

    Some manufacturers' versions of these opamps are prone to "latch-up" aka "phase inversion", and some are not. You will have to review datasheets to determine which are vulnerable. If they do not specifically state they are not vulnerable to "latch-up" or "phase inversion", assume they are. You're in school, you need to research these things. You will be researching these things for the rest of your career.

    So what they are telling you agrees with what I am writing to you. Nothing conflicting so far.

    In automotive and many other battery-powered systems, this is generally true. However, many audio systems require dual supplies. This is where things can get confusing. This is also why you can't connect the speakers of many automotive sound systems to "chassis ground", or you will blow the amp.

    It's the direction that the electrons are flowing. If they are coming in the + probe and going out the - probe, it'll show a negative voltage.

    Don't let this minor point confuse you. Negative voltage is simply a voltage that is a lower potential than what has been declared to be the 0v reference point.

    Well, I hope you're a bit more sorted on it. Might take a few read-throughs to make sense. Don't let it throw you.
     
    Last edited: Aug 3, 2010
  11. SgtWookie

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    Well, it's more a question of what's the voltage, what opamp, and how much did you expect to get out of it?

    A TL071 will work on 5v, if you AC couple the input, and bias it at (((Vcc-1.5)-(Vee+3))/2), but don't expect more than a 0.5v P-P output swing when used as a voltage follower.
     
    Last edited: Aug 4, 2010
  12. gootee

    Senior Member

    Apr 24, 2007
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    Believe it or not, they did give you the mental toolkit that you need to understand this, in EE school. They just didn't have much occasion to make you think about it, at the time. (You should find that they made you build a pretty good toolkit for yourself, and that you will be able to learn to use the tools to learn to perform almost any EE task that might arise, in various jobs you might get, or projects that you pursue. They couldn't teach you how to do each of those tasks, because there are far too many different possible EE tasks that you might need to perform. So they did the next-best thing for you, or, rather, they helped YOU to do it.)

    Voltage is (truly, and only,) RELATIVE. It's always (always) really the relative voltage between TWO points that is being called "a voltage". It's a "difference in potential". There really is no "zero volts" point, since one point alone cannot define a voltage, because voltage is, by definition, only defined between two points.

    When considering the output terminals of a single battery, or a single "floating" DC power supply, of say "15 volts", we _only_ know that one terminal is 15 volts above the other terminal, or vice versa.

    "Ground", or "zero volts", is basically an artificial concept. It just means zero volts relative to itself, and ONLY(!) because we are thinking of some other point as having a voltage relative to it.

    For a battery, or a floating DC power supply, we could JUST as easily and just as correctly call the "ground" a negative voltage and name the positive terminal as ground! Let that sink in.

    An unconnected battery's terminals are neither positive nor negative. There is ONLY "a voltage difference" between the two terminals. One is more positive than the other, and vice versa. So in layman's language we might call the terminals "positive" and "negative". But they are not. They are only "more positive" and "more negative", when compared to each other.

    So you could take two (identical in this case) batteries and connect the more negative terminal of one to the more positive terminal of the other. If you then call the most negative terminal of that assembly "ground", then you have a power supply with two other terminals, say +V and +2V. But if you call the center connection, i.e. between the two batteries, "ground", then you have a power supply with two other terminals, -V and +V. And if you call the most-positive terminal in the assembly "ground", then you have a power supply with two other terminals, -V and -2V. It REALLY IS just that simple!

    And if you have a 1000-Volt DC terminal, relative to, say, earth, and you connect any ONE of the terminals of your stack of two batteries to the 1000-Volt terminal, then you would STILL have a power supply with the same voltages as in the examples above. And you could still connect a little +/-15v opamp circuit to it and it would work fine. BUT, you wouldn't want to try connecting the battery-powered circuit to anything using earth as its ground reference! It's all truly relative. But you sometimes have to remember what each thing is relative TO.

    Floating DC power supplies can usually be connected together in exactly the same ways as in the battery examples, above, to create multiple relative power supply voltages and a "ground" defined wherever you choose.

    It can be a little bit confusing, at first, when you throw in the idea of NON-floating DC supplies. Those usually have one terminal (or possibly some internal circuit point) physically connected to the "ground" wire of your AC wiring. The terminals are still only relative voltages. But what they are relative to is more pre-defined and you don't usually have the freedom to connect them any way you like, any more.

    There are actually several easy solutions, for what you are wanting to do:

    Option 1: Get two identical regular transformer-type 15V wall warts (or other isolated DC supplies) and connect the positive terminal of one to the negative terminal of the other. That will be your new "ground". The remaining + and - leads or terminals will be your new +15v and -15v. (If I was doing it, I might get two >= 18V wall warts and then add voltage regulator circuits to the + and - to get a nice, clean +/-15V.) You can often find wall wart power supplies dirt cheap at garage sales. But mouser.com, for example, has tons of different ones, for reasonable prices, and up to some decent power levels. (They also have lots and lots of fairly nice little multi-output DC power supplies, cheap.)

    The wall warts must be the transformer type, and should NOT have three prong plugs. In the USA, the properly-isolated types should say something like "Class 2 Transformer", somewhere on the outside.

    Option 2: You could also use a 30V (or whatever) wall wart (or other DC supply) and use a "rail-splitter" voltage divider circuit, similar to the one in the LM675 datasheet that you can download from national.com . For low current/power you can just use an opamp instead of the LM675 power amp chip. And sometimes you can just use the two resistors in series, by themselves, and call the point between them "ground". This option would work with your existing power supply. But then you'd only end up with half of the original magnitude at each of the (positive and negative) terminals.

    Option 3: There are also fairly-easy ways to invert a single positive voltage (probably with a small switch-mode power supply) to give you both + and - outputs relative to the original "ground". But the really cheap/easy ways are usually only viable for very low power output levels.

    Option 4: There are ways to double the output amplitude of the transformer (in a wall wart, for example) and then divide that into + and - DC supplies.

    But Option 1 is usually the easiest and fastest. There are many other options, actually. But they might typically include designing and building a multiple-output power supply, either of the switch-mode type (SMPS), or of the "linear" transformer/rectifiers/smoothing-caps/regulators type. Using old ATX computer power supplies, as already mentioned by another poster, is often a very good way to go.

    Cheers,

    Tom

    "Current does NOT take the path of least resistance. It takes ALL paths, in inverse proportion to their resistances."
     
    Last edited: Aug 4, 2010
    count_volta likes this.
  13. marshallf3

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    Jul 26, 2010
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    Depends on how critical your ouput requirements are. Seems to me at least half the op amps on the market now are either designed for single rail operation or will operate as such. I have no problem running the TL08x series off of a single supply.

    BTW: THe more you come to know me the more you'll find that I highly recommend http://www.mouser.com over DigiKey or anyone else. They'll ship via USPS but if you get something sent UPS (their preferred method) they pass on every discount they get to you so $4 isn't unusual for shipping a small package.

    Besides that, being in Oklahoma City and them in the Dallas area, if I order something by 8 PM at night I have it the next day via regular UPS Ground.
     
  14. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    Thanks gootee that really does help. I will think about it a little more.

    How about the physical reason behind what is going on? One side of a battery has a deciency of electrons. Another side has an abundance of electrons. We call one side positive and the other negative respectively. So is a voltage positive or negative only with respect to "Conventional current"?

    And once again, how does a volt meter know if I connect a battery backwards to attach the minus sign. I sort of know how a volt meter works. Its an ammeter with a resistor, and it basically uses ohms law to give you the voltage.

    So I assume if current is going one way through the resistor its positive voltage, if current is going the other way, its negative voltage. But the designers of the volt meter had to choose which direction is which didn't they? Is that choice made based on conventional current?
     
  15. BMorse

    Senior Member

    Sep 26, 2009
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    "Real" circuits use a Negative Voltage regulator .... The LM79xx series of voltage regulators put out a negative voltage (LM7915 is a -15 volt regulator)
    B. Morse
     
  16. BillB3857

    Senior Member

    Feb 28, 2009
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    The 79XX series regulator does not produce a negative source. It can only regulate an already existing source. There are, however, circuits that can take in a single DC source and produce both positive and negative outputs. DC to DC converters are quite common that could meet the OP's needs.
     
  17. gootee

    Senior Member

    Apr 24, 2007
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    Yeah, it's merely a "sign" convention that was adopted, long ago. They got it backwards, actually, but the math works either way and I guess people decided that it was too late to change it once they figured out that it was backwards.

    But voltages don't depend on currents, for their existence, or their sign convention (maybe it's the other way around, though). There can be voltage between two points without any current flowing. But the sign conventions for each do have to be consistent with each other.
     
  18. marshallf3

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    Jul 26, 2010
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    We tackled that one long ago in the world of physics. Electrons flow from negative to positive, holes flow from positive to negative.
     
  19. Wendy

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