What is Vdd, VD, Vs, and Vss?

Discussion in 'Homework Help' started by DaConfusedOne, Jan 31, 2018.

1. DaConfusedOne Thread Starter New Member

Jan 31, 2018
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I'm new to circuits. I'm taking a digital logic class, and my textbook just went over the 4 terminals: Vdd, VD, Vs, and Vss. The textbook isn't explaining what they are or what they do. The only thing it explained is that VDD is the high voltage and Vss(Gnd is the low voltage). I also know that VD and VS are the two pins that are connected to the gate of the transistors. What are they, and what do they do? Can someone give me a good understanding of them? Googling them isn't helping me.

2. WBahn Moderator

Mar 31, 2012
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7,302
There is a widely used convention for naming differentials between things (in this case voltage, but can be height, pressure, or many other things) called the "double subscript" convention.

Let Va be the voltage at point 'a' and Vb be the voltage at point 'b'. Then, by this convention

Vab = Va - Vb

Or, in words, Vab is the voltage at 'a' relative to the voltage at 'b'.

But what would Vdd then be? It would be Vd - Vd which is always 0 V. So a voltage using a repeated subscript is useless according to this convention and would never be used. But that allows us to extend the convention and agree to use voltages with repeated subscripts to mean something else entirely. This extension says that Vxx represents the power supply voltage most closely associated with node x.

So Vdd is the supply voltage associated with the drain node of an NFET circuit and Vss is the supply voltage associated with the source node of an NFET circuit.

Not everything follows this convention and you need to learn to interpret things in terms of the schematic you are working with. Vs, in particular, is a common notation for a signal source. Similarly, Vi is often the input signal while Vo is the output signal.

What VD means depends on how it is used. It likely means the voltage at the drain of a FET.

3. dl324 AAC Fanatic!

Mar 30, 2015
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Welcome to AAC!

It would if be helpful if you posted an excerpt from the book so we have some context.

It would also behoove you to be more precise. There are multiple devices that can be called transistors, but not all transistors have a source/drain. You could be referring to a JFET or a MOSFET.

4. MrChips Moderator

Oct 2, 2009
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5,649
VC is the voltage at the collector of a bipolar junction transistor (BJT) with respect to GND (or COMMON).
VE is the voltage at the emitter.
VB is the voltage at the base.

VCE is the voltage at the collector with respect to the emitter.
... and so on...

VD is the voltage at the drain of a field-effect transistor FET.
VS is the voltage at the source.
VG is the voltage at the gate.

VGS is the voltage at the gate with respect to the source, ... and so on...

VDD is the drain or positive supply voltage.
VSS is the negative source or ground supply voltage or return node to the supply.

5. DaConfusedOne Thread Starter New Member

Jan 31, 2018
2
0
I meant to talk about NMOS and PMOS