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Vcc refers to the voltage applied to the COLLECTOR of a transistor. Most commonly in digital electronics that is POSITVE.

Vdd is the voltage applied to the Drain of a FET.  The DRAIN is where the 'Holes' filled by electrons come from. It is positive  (While electrons flow from negative to positive, In the HOLE theory, Holes flow from Positive to Negative.  The ‘Hole’ theory was the accepted version of electronics flow before it was discovered that electrons were the moving entity and that they were negative.  Both theories work equally well unless you mix them, I am an electron flow follower, so for me Diodes and transistors all point the wrong way but I just think of them as pointing toward the SOURCE of the flow not away from it.)

Vss is the SOURCE of a FET that is where the Electrons come from. IT is Negative

You may also see Vee.  It is the Emitter of a Transistor, Usually referred to as Negative.

These notations are normally used when marking integrated circuits or parts on data sheets.  Some people also use them as a general indicator of POSITVE or NEGITIVE voltage on circuit diagrams.  While this is not technically accurate as some of these circuits don’t even have a discrete transistor or FET in them. No one worries about it too much, we all just accept that in general (Vcc and Vdd are Positive and Vss and Vee are negative).
There are situations where, for instance, the Vcc of a transistor will be connected to ground or even a negative supply voltage.    It depends on what the function of the part is within the circuit.  In general if you analyze that circuit you may find that even though it is connected funny, it is still functioning the same way it normally would.

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Different kinds of IC technology use different kinds of transistors. TTL ICs, such as the 7400 series, use bipolar junction transistors (BJT). CMOS ICs, such as the 4000 series logic chips, use field effect transistors (FET). Generally, each stage in a logic circuit has an output section consisting of a pair of transistors in push-pull configuration - that is, one at the top, connected to the positive power rail, and one at the bottom connected to the negative power rail.

BJTs have three connections: Emitter, base and collector. The 'c's in 'Vcc' refer to the collectors of all the positive-half (top) output BJTs.

FETs have drain, gate and source connectors. The 'd's in 'Vdd' refer to the drains of all the positive-half (top) output FETs. The 's's in 'Vss' refer to all the sources of the negative-half (bottom) output FETs.

Thus in general Vcc/Gnd are the labels used to denote the power supply connections on Bipolar TTL IC, and Vdd/Vss are the labels denoting the power connections on a CMOS IC.

For bipolar logic ICs, Vcc must be about 5V higher than Gnd.
For CMOS logic ICs of the 4000 series, Vdd can be up to 15V higher than Vss.
For CMOS logic ICs of the 74HC series, Vdd can be 2 to 6V higher than Vss.

There are of course variations on these themes, and you should check the datasheets for an IC before you power it up.

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