Negative Voltage

Discussion in 'General Electronics Chat' started by PeteVee, Apr 21, 2017 at 2:41 AM.

  1. PeteVee

    PeteVee

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    My question is about the concept of negative voltage in general, although the context from which it arises is to do with vacuum tubes in a guitar amplifier, biasing tubes in particular. This page http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm states that "Bias is a negative voltage applied to a power tube's control grid, to set the amount of idle current the tube draws". At this juncture my confusion has less to do with the concept of biasing and more to do with what the term "negative voltage" means. My first inclination is to think "how can a voltage be negative?" From my understanding, voltage (through current in a loaded circuit) is the notion of electrons moving in a circuit to do some sort of useful work, like lighting up a light bulb for example. So what could the term "negative voltage" be possibly describing? The "unability" to do some sort of useful work? That is obviously not what it means, but you can see where my confusion begins. Many moons ago I came across this terminology in the context of digital electronics, where a transistor might be said to be "on" at +5V and "off" at -5V. The explanation I remember for that type of scenario was that the -5 was in reference to some benchmark, lets say 20 V, so that -5V actually means 15V and +5V means 25V. I could be way off on this and if so I would love to be set straight. Anyway, to make a long story short, if someone could help me understand what it means when a "negative voltage" is applied to a power tube's control grid, I would be right chuffed. Thanks in advance.

    PV
     
    PeteVee, Apr 21, 2017 at 2:41 AM
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  2. PeteVee

    davenn Moderator

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    it's just that, a negative value voltage relative to the 0V rail.

    Think of a split rail PSU that supplies - 12V, 0V and +12V

    In AC the voltage is cycling between negative and positive values, above and below the 0V reference line

    in digital it's usually 0V and +5V


    yes, as I said earlier, usually the 0V rail



    as a more negative value of voltage is applied to the grid, more of the electrons ( negative charge) emitted by the cathode are repelled by the grid and don't get through to the positive anode plate

    so increasing or decreasing the negative voltage on the grid allows more or less electrons to get through to the plate


    Dave
     
    Last edited: Apr 21, 2017 at 3:19 AM
    davenn, Apr 21, 2017 at 3:14 AM
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  3. PeteVee

    Ratch

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    I wish all questions were this easy. Voltage is always measured from a reference point. If the voltage to be measured has a higher value than the reference point, the voltage is positive. Otherwise negative or zero.

    Voltage is not the ability to do work. That is the definition of energy. Voltage is the energy density of the unit charge. A negative voltage means there is less energy density per unit charge than the reference point.

    Applying a negative voltage to the grid usually means the grid is negative with respect to the cathode. This inhibits the electrons from traveling to the plate.

    Ratch
     
    Ratch, Apr 21, 2017 at 3:43 AM
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  4. PeteVee

    hevans1944 Hop - AC8NS

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    Voltage is always measured between two points. Think of a common dry-cell where one end is positive and the other end is negative. A typical alkaline cell produces 1.5 volts between its terminals. Whether you consider that voltage to be positive or negative, depends on where you decide to measure zero voltage. Clearly either end of the cell provides zero voltage with respect to anything else connected to that same end of the cell.

    If the end you choose for your "zero reference" is the positive terminal of the cell, then all voltages the cell can produce will be negative voltages. Flip the cell around and it now produces positive voltages because your "zero reference" is now the negative terminal of the cell.

    In vacuum tube circuits, the grid voltage is measured with respect to the cathode of the tube. The cathode may not be at "circuit ground" but the grid voltage is still measured between the cathode and the grid, and the grid voltage is usually negative with respect to the cathode.
     
    hevans1944, Apr 21, 2017 at 3:54 AM
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  5. PeteVee

    AnalogKid

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    The thing to remember is that there are very few absolutes in physics, and this ain't any of them. Positive and negative are 1) completely arbitrary terms, just like up and down; 2) they are relative terms, and are meaningless without a reference point. A voltage is not positive or negative, it is positive or negative with respect to the voltage somewhere else. That is why a volt meter has two leads. Connect them one way to a battery and it reads -9 V; reverse the connection and it reads +9 V. Nothing about the battery changed; you were right there, you would have seen it. What changed were the relationships among the meter, the reference connection, and the measurement connection.

    To your specific example, a tube is biased into its linear operating region when the control grid is negative *with respect to the cathode*, neither of which are required to be connected to ground (the circuit's arbitrary "0 V" point). If a 12AT7 has the cathode at +999 V and the grid at +995 V, it's happy.

    Early Ampex video recorders had video processing circuits powered by +300 V and -300 V (with respect to ground). The output video signal was centered at ground, created by two vacuum tubes connected in series just like a totem-pole output stage in an audio power amp quasi-complimentary output stage. The output point was the cathode of the upper tube connected directly to the plate of the lower tube. Linear as all get-out, great bandwidth, and designed by a college kid - Ray Dolby.

    ak
     
    AnalogKid, Apr 21, 2017 at 5:19 AM
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  6. PeteVee

    BobK

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    Actually, the ability to do negative work. Work can also be negative. When a weight is lifted in the earth's gravitational field, it requires positive work. When it is lowered, requires negative work, which is to say that it does work instead of requiring work done on it. Charges (electrons for example) behave the same way with respect to voltages. If a positive charge goes from a lower voltage to a higher voltage, you must push it, doing work on it. If it goes from a higher voltage to a lower voltage, work is done by the charge, possibly heating up a resistor or wire.

    bob
     
    BobK, Apr 21, 2017 at 8:47 PM
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