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Vcc, Vdd, Vss

Discussion in 'General Electronics Chat' started by daviddoria, Apr 16, 2004.

  1. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,


    What i think happened was that early circuit designers/drafters didnt
    want to use terms like Vc and Vd for supply voltages because they
    were already used to represent other things like Vc=Collector Voltage
    and Vd=Drain Voltage, so they simply doubled the second letter so
    it would not be confused with other terms.
    The original motivation for Vcc was probably "Supply voltage for a
    Common Collector transistor circuit" but then that idea of doubling
    the second letter was probably taken further for the other terms too.

    It is reasonable to do this because Vc usually means "Collector Voltage
    from the Collector to ground" but was not called Vcg at that time
    because it was most likely deemed too complicated for a simple
    voltage like the collector voltage so Vc alone was used. This of
    course meant that it could not be used for other things like supply voltage.
    Same for Vd turning into Vdd and Vs turning into Vss etc.
    Doubling the second letter simply shows what it is while avoiding the
    possibility of confusing it with a different measurement.

    For an example of a simple NPN transistor amplifier with emitter to
    ground, collector to a 1k resistor and that 1k resistor to the plus
    voltage supply and the base the input, the plus voltage supply
    would be called "Vcc" and the voltage at the collector (referenced
    to ground) would be called "Vc".
    Another source of confusion then is "Vss", which could be ground
    even though there is no MOSFET in this circuit, or "Vee" which
    makes a little more sense. Vss would mean ground *as if* there
    *were* an N-MOSFET in the circuit, even though there *is not*.
     
    Last edited: Feb 16, 2009
  2. geeks

    geeks New Member

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    return of the king

    Vcc is Voltage Collector Collector
    Vdd is Voltage Drain Drain
    Vss is Voltage Source Source
    Vce is Voltage Collector Emitter
    Vbe is Voltage Base Emitter
    Vec is Voltage Emitter Collecter

    and Vdd is drain drain

    these are just the terms which were used by the electronic engg and later became universal terms in electronics
     
  3. unclejed613

    unclejed613 Well-Known Member

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    Vcc = collector supply voltage
    Vdd = drain supply voltage
    Vee = emitter supply voltage (may also be ground in most cases)
    Vss = drain supply voltage (may also be ground)

    the reason it's not Vc or Vd is that those voltages are the voltages directly at the device. if you have a common emitter amplifier with a collector load resistor, the supply voltage is Vcc, and the voltage at the collector (on the low side of the collector load resistor) is Vc. the big difference is that in a circuit with multiple stages, Vcc or Vdd can be the supply for the whole circuit, but each device in the circuit has it's own Vc or Vd. the choice of Vcc or Vdd in mixed circuits usually depends on the output stage of the whole circuit, so you can have an amp with BJT (bipolar transistors) all the way through except the output transistor which is a MOSFET, and it's called Vdd. or you can have an amplifier that's all FETs, except the output stage which is a BJT, and it would be called Vcc.

    for instance in an audio amplifier with bipolar output transistors and a complementary output stage, you have 2 Vcc supplies, one positive and one negative, called +Vcc and -Vcc. with MOSFETs in the output stage it's +Vdd and -Vdd, even if the driver and other previous amplifier circuits are bipolar transistors.
     
  4. dave

    Dave New Member

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  5. farhad0

    farhad0 New Member

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    hi guys,
    These notations are used in describing voltages at various common power supply terminal of a given circuit.
    these common voltage terms map to transistor technology as follows:
    BJT FET "Vxx" meaning
    Vcc Vdd Positive supply voltage
    Vee Vss Negative supply, ground
    good luck
    F. Sheikh hosseini
     
  6. mneary

    mneary New Member

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    This is the thread where single posts have gathered since 2004.....
     
  7. Electronworks

    Electronworks New Member

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    I cannot believe 12000 people have viewed this post. The power of the internet!
     
  8. Hero999

    Hero999 Banned

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    Vdd and Vss (for source and drain) don't make any sense for CMOS because the drains of the P-channel MOSFETs connect to the 0V and the drains for the N-MOSFETs connect to the positive rail.

    The terms probably originate from NMOS which didn't have an P-channel MOSFETs, just N-channel or it could refet to electron flow, with source being the source of electrons (-V) and drain being the sink of electrons (+V). It doesn't matter either way as long as you remember that Vdd is +V and Vss is 0V.
     
  9. unclejed613

    unclejed613 Well-Known Member

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    as i mentioned earlier. in a MOSFET output amplifier with a bipolar power supply, you can have +Vdd, Vss (ground) and -Vdd, similarly for a bipolar supply for BJT outputs, you have +Vcc, Vee (gnd) and -Vcc.
     
  10. KGbeME

    KGbeME New Member

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    "Source" actually refers to the source of electrons, whereas "drain" is the place they go. Positive current is opposite the flow of electrons, so current flows into the source from the drain. It's residual nomenclature from the early days of electronic experimentation.
     
  11. Martel

    Martel New Member

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    I saw, a long time ago a PMOS device (i don't even remember what the device was...) where Vdd was NEGATIVE and Vss POSITIVE. But that was a single device in many many others.

    At first glance, in CMOS devices, having a Vdd in a nonsense but since the VAST MAJORITY of FET devices are of N-channel types, the Vdd (positive) and Vss (negative) terminologies for supplies are the standard today.

    Why the letter repetition ? Probably to distinguish the supply lines from the signal lines. That dates back from the vacuum tube (valve) days ((Ebb for plate (bb meant Battery-battery for plate voltage), (Ecc meant control-control for the negative control grid supply) and B++ (plus-plus for HIGH VOLTAGE !!!) )
     
  12. Martel

    Martel New Member

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    I saw Vee for negative and Vcc for positive.

    The europeans are more clear on this topic:

    +Ve for positive supply
    -Ve for negarive supply and
    0V for ground.
     
  13. unclejed613

    unclejed613 Well-Known Member

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    yes the convention of the double letters follows the format of the engineering shorthand of tube technology. but do you know where the B+ termnology came from? very early in tube days you needed 3 batteries to run a radio set, A, B and C. A was the filament battery, B was the plate battery, and C was the grid supply. the A battery was usually a large 1.5V cell about 2" in diameter and 5 or 6" tall, the B battery was usually anywhere between 45 and 90V, shaped like a small brick, and the C battery was usually a 22.5V battery, also rectangular, but smaller than the B battery. the terminals on the radio were in pairs marked A, B and C. for the most part, in later days, the B, denoting the plate supply was the only one that really stuck, and was also applied to any DC plate supply, not just battery operated equipment. you would see A and C in schematics once in a while, but the B (and since it was always a positive voltage B+ came into common usage) stuck, and you even see it sometimes in solid state schematics (more commonly in the RF business).
     
  14. vlad777

    vlad777 Member

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    There is minimum two collectors for multi transistor circuit, hence Vcc.
    I you have million transistors then it's Vccccccccccccccccccccccccccccccccc....
     

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