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Opamp Power Supply

Discussion in 'Power Electronics' started by electronicsLearner77, Jul 2, 2015.

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  1. electronicsLearner77

    electronicsLearner77

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    Jul 2, 2015
    I have seen the power supply to op amp sometimes to positive/ negative and some cases Positive/ Ground. Please help me to understand when should i go for +/- Supply and when should I go for Positive/Ground supply. I know I am not asking very good question. But please tell me one instance to get an idea that a positive/negative supply can achieve which is not possible with positive supply alone.
     
  2. Gryd3

    Gryd3

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    Jun 25, 2014
    Depends entirely on the purpose.
    ...That and when using a positive supply only, you can make a 'virtual' ground to use with an opamp to 'fake' a dual +/- power supply
     
  3. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Traditionally, op-amps were bi-polar devices, early op-amps operating with ±100 V DC outputs provided by vacuum tube electronics. Later solid-state op-amps settled on ±15 V DC power supplies to produce ±10 V DC outputs. As "head room" improved, power supplies were reduced to ±12 V DC for the same ±10 V DC output. Since the original purpose of op-amps was to solve differential equations, both positive and negative output signals were mandatory. Today there are many circuits where bi-polar outputs are perhaps convenient but not necessary. As @Gryd3 stated, it depends entirely on the purpose.

    If you are contemplating using a circuit that produces, or accepts, both positive and negative signals with respect to some common or "ground" potential, then ±DC power supplies should be your first choice, typically ±15 V DC. However, many modern op-amps work at reduced voltages, and if you don't need a full ±10 or ±12 V DC "swing" in the outputs, a lower voltage, dual-polarity, power supply such as ±5 V DC or even ±2 V DC might be appropriate for you.

    When can you "get by" with using a single power supply to power an op-amp? Just about any time you wish, although it could make signal processing more difficult. The op-amp itself doesn't know squat about where its power comes from: it has a positive power supply terminal and negative power supply terminal. Most op-amps don't have a "common" or "ground" terminal, so as long as the voltage difference between the +V and -V supply terminals is within the specified operating range of the particular device you are using, it doesn't make any difference to the op-amp where that voltage comes from. It can and will make a lot of difference in how you connect external circuitry to the inverting, non-inverting, and output terminals of the op-amp, but that's another problem.

    If you do need a bi-polar signal and you have only a single power supply, you can create a "virtual ground" anywhere between the extremes of the two power supply terminals. That means if you have, say, a 24 V DC power supply you can create a "virtual ground" half-way between the output terminals so the positive terminal is +12 V DC with respect to the "virtual ground" and the negative terminal is -12 V DC with respect to the "virtual ground." But you could also move the "virtual ground" toward either the positive or negative terminal, producing for example +20 V DC and -4 VDC power supply terminals with respect to the "virtual ground." This could be advantageous if you need mostly a positive supply (to run a motor perhaps), but also need signals that reach zero potential or go slightly negative with respect to the "virtual ground."

    When you create a "virtual ground" to allow a single power supply to appear as if it is two power supplies of opposite polarities, there are limitations that you don't encounter with two real power supplies of opposite polarities. The most obvious is source impedance. Two real power supplies will each have very low source impedance, meaning you can pretty much draw whatever current you want from them (up to their maximum current rating) without any effect on their output voltage. It is difficult to create a "virtual ground" that shares this property with respect to its positive and negative terminals on a single power supply. It may be worth the trouble of doing so if you only need, say, a small negative power supply to power up a transducer or something and have only a humongous positive power supply (a car battery for example) available to power everything. But there is always more than one way to do anything. Instead of creating a "virtual ground" you could instead build a charge-pump that converts a positive voltage to a negative voltage. Easy enough to do with modern integrated circuits and power transistors; not so much thirty years ago.
     
  4. LvW

    LvW

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    Apr 12, 2014
    In short:
    Each opamp needs a voltage between the power pins in order to work properly - that means: To be able to amplify in its linear transfer characteristics.

    1.) Then, the best quiescent operating point (DC output voltage) is in the middle of this region. Then, we have equal room (above and below this DC voltage) for the amplified output signal.
    Normally, we wish that this operating point is at VDC=0; in this case we do not need any coupling capacitors and the device can be used to amplify also dc signal or signals with a very low frequency. In this case, we need a split supply (midpoint app. zero).

    2.) In some cases, there is only one single supply available. In such a case, the opamp must be biased in such a way that the dc output voltage is at app. 50% of the suplly. This is called a "virtual ground" because all changes are referred to this quiescent point.
     
    hevans1944 likes this.
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Or a so-called rail-to-rail opamp has to be used and one has to be aware of the limitations as these opamps can reach the raisl (0V, +Vcc) only to within a few millivolts and the voltage drop increases with output current.
     
    hevans1944 likes this.
  6. LvW

    LvW

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    Apr 12, 2014
    Harald, perhaps the following applies only to me - however: To me, this sounds as if a so called "rail-to-rail opamp" would be an alternative solution to single-supply operation. I am sure, that´s not what you wanted to express, correct?
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Yes, you're correct, that's not what I mean.
    No, a rail-to-rail opamp is a way of using a single supply while still being able to have inputs and outputs at (or at least very close to) 0V and Vcc.

    Of course, if the input signal is bipolar, you will still have to add an offset (e.g. by a voltage divider) and couple the signal through a capacitor (your post #2). You will also have to decouple the output to restore the AC-only signal.

    For me this is a bit different from a true virtual ground (although I admit that this could be a point for discussion). The difference is that with a true virtual ground you connect the AC source to the virtual ground (which is at Vcc/2) whereas with AC coupling you connect the AC source to ground.
    See the diagram for a (hopefully) much clearer explanation:
    [​IMG]
     

    Attached Files:

    Arouse1973 likes this.
  8. electronicsLearner77

    electronicsLearner77

    152
    1
    Jul 2, 2015
    Wow very great explanations, it will take some time for me to understand the points raised. Thank you very much.
     
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