Power rail names

[Tha fios agaibh]

It seems odd to me some of the identifying terms used for power rails; like Vcc or Vdd.

Yes, It makes sense when more than one collector or drain are similarly connected to the positive rail, but what about when you have a circuit with both mixed bjt and fets, or mixed N type and P type transistors on the same rail? (For instance both a drain and a source connection to the same rail) What's the criteria for properly identifying them?

John

 

[davenn]

It seems odd to me some of the identifying terms used for power rails; like Vcc or Vdd.

Yes, It makes sense when more than one collector or drain are similarly connected to the positive rail, but what about when you have a circuit with both mixed bjt and fets, or mixed N type and P type transistors on the same rail? (For instance both a drain and a source connection to the same rail) What's the criteria for properly identifying them?

John

hey John ... Mainly because it isn't really used as a device specific thing as you are assuming
that is ... Vcc is very commonly used to denote the positive rail regardless of the circuit type or components within

have a read through .....
https://en.wikipedia.org/wiki/IC_power-supply_pin

here's an interesting response on another forum .....

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.

am sure there's more comments out there

 

[Tha fios agaibh]

Thanks Dave, makes sense.

 

[OBW0549]

What's the criteria for properly identifying them?

No criteria, really, nor any "proper" usage; they are just a commonly-used naming convention that helps us quickly and intuitively determine what kind of voltage, positive or negative, they represent. When I see "Vdd" or "Vcc" I expect a positive supply voltage; and when I see "Vss" or "Vee" I expect a negative voltage.

There's really little more to it than that.

 

[Tha fios agaibh]

When I see "Vdd" or "Vcc" I expect a positive supply voltage; and when I see "Vss" or "Vee" I expect a negative voltage.

There's really little more to it than that.

Thanks. I was wondering because It's possible to have the opposite polarity depending on transistor type.

Another thing that seemed odd to me, is how a N type mosfet has its "source" connected to Negative and its drain hooked to a positive source.

I will try not to overthink this stuff.
Lol.

 

[OBW0549]

I will try not to overthink this stuff.
Lol.

Yup. Just accept the conventions for what they are: conventions. (It's probably a good idea to follow them, too; if someone sees a negative supply voltage labeled "Vdd" their head will probably explode.)