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I've seen lots of schematics use \$V_{CC}\$ and \$V_{DD}\$ interchangeably. For extra credit: Why \$V_{DD}\$ and not simply \$V_D\$? |
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Back in the pleistoscene (1960s or earlier), logic was implemented with bipolar transistors. Even more specifically, they were NPN because for some reasons I'm not going to get into, NPN were faster. Back then it made sense to someone that the positive supply voltage would be called Vcc where the "c" stands for collector. Sometimes (but less commonly) the negative supply was called Vee where "e" stands for emitter. When FET logic came about, the same kind of naming was used, but now the positive supply was Vdd (drain) and the negative Vss (source). With CMOS this makes no sense, but it persists anyway. Note that the "C" in CMOS stands for "complementary". That means both N and P channel devices are used in about equal numbers. A CMOS inverter is just a P channel and a N channel MOSFET in its simplest form. With roughly equal numbers of N and P channel devices, drains aren't more likely to be positive than sources, and vice versa. However, the Vdd and Vss names have stuck for historical reasons. Technically Vcc/Vee is for bipolar and Vdd/Vss for FETs, but in practise today Vcc and Vdd mean the same, and Vee and Vss mean the same. |
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You already know from the other answers that for bipolar
Likewise, for CMOS
For bipolar logic like TTL this is correct; even for push-pull outputs ("totem-pole") only NPN transistors were used and \$V_{CC}\$ is indeed connected to collectors.
Due to the symmetry it's actually connected to the source of the P-MOSFET. This is probably an inheritance from NMOS, CMOS's predecessor, where \$V_{DD}\$ was indeed the side of the drain (with a resistor in between).
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Vdd is usually used for CMOS, NMOS and PMOS devices. It stands for voltage (at) drain. In some PMOS devices it is negative, but pure PMOS chips are rarely (if ever) found today. It's usually the most positive voltage but not always, for example a motor controller might have a Vs pin for the motor voltage, or a processor might use a core voltage and an IO voltage. Vss stands for voltage (at) source; PMOS devices might be positive, but again, PMOS is a relic, so for all intensive purposes it is the most negative voltage available. It's often tied to the substrate, so it must be the most negative, or the chip won't work properly. Vcc stands for voltage (at) collector and is primarily used for bipolar devices, although I have seen it used with CMOS devices, probably out of convention. Vee stands for voltage (at) emitter and is usually the most negative. I've also seen Vs+ and Vs-, as well as V+ and V-, but V+/V- can be confused with the input pins on op-amps/comparators and other amplifiers. |
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The convention of letters VAB for voltage means the potential between A and B. Voltage is a potential measured with respect to another point in the circuit. For example VBE is the voltage between base and emitter. Ground does not have a specific "letter". So the convention of repeating letters is used, like VDD or VEE to refer to the point relative to ground. Using single letters in this context adds more confusion since Vs may refer to the voltage of a source "s" (which may be different than VSS if there are multiple sources in series, etc) and not the voltage between a transistor's emitter & ground. Even without transistors in a circuit, voltages can be referred to with the style VAB or V12 to refect potential between A and B or point 1 and point 2. Obviously order is important, since for two points in the circuit A and B, VBA = -VAB. Bibliographic reference: "If the same letter is repeated, that means a power supply voltage: Vcc is the (positive) power-supply voltage associated with the collector, and Vee is the (negative) power-supply voltage associated with the emitter". Text abstract from Paul Horowitz and Winfield Hill (1989), The Art of Electronics (Second ed.), Cambridge University Press, ISBN 978-0-521-37095-0. Chapter 2 - Transistors, page 62, Introduction. |
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What they said, most of the time, but there are still occasions where the differences are real and/or useful: There are a small proportion of devices which use multiple supplies relative to ground and in some of these it may make sense to use eg Vee gnd or Vss. In other cases there may be multiple supplies or grounds which are at the same potential but separated for system reasons. eg
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It's \$V_{CC}\$ rather than just \$V_{C}\$ because the C stands for collector. But \$V_{CC}\$, though a collector-side positive voltage in an NPN transistor circuit, is not the voltage at the top of the collector, \$V_C\$! There is usually a load resistor or some other device between the collector and \$V_{CC}\$. The doubled-up C indicates that it's a higher voltage beyond that which appears on the collector and is clearly distinguished from from \$V_C\$. The letters denote transistor parts: source, drain, gate, collector, emitter, base. When there are two different letters, the meaning is different: it means the voltage between those terminals of the device, like \$V_{BE}\$: base to emitter voltage of a BJT. This is possibly why a doubled-up letter was chosen for \$V_{CC}\$. Let's invent a rationale. Suppose want a name for a voltage associated with the collector which is not the voltage at the collector. Suppose we want the name to be as short as possible, but we want to include the letter C to clearly associate it with the collector. This means that the name will be two symbols long: C plus another character. The other character will be a letter, number or some other kind of glyph. A number would look like a voltage, so the choice is between using a glyph like ampersand or hash, or a second letter. If it is going to be a second letter, then it cannot be any other letter beside C, because then it looks like the \$V_{XY}\$ notation denoting a voltage between two points. If the C is repeated, then we know it cannot be the useless designation of the voltage from C to C, which reminds us that the notation has another meaning. If the second character is going to a glyph, then it should probably be something other than So the shortest possible way to denote the collector-side supply voltage is either something glyph-based like \$V_{C@}\$ or else \$V_{CC}\$. Clearly, an argument can be made that \$V_{CC}\$ was a sober, well-considered choice to express what the inventor of the notation wanted to express, which caught on. |
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