MOSFET schematic symbol

Thread Starter

AnalogKid

Joined Aug 1, 2013
8,487
The original bipolar transistor symbol had a circle around it because that's what vacuum tube symbols had. Nowadays, the symbols in most cad libraries do not have the enclosing circle.

What about MOSFETs? I've seen it both ways. Do you think the "standard" MOSFET schematic symbol includes an enclosing circle? What's in you're library?

ak
 

kubeek

Joined Sep 20, 2005
5,724
I draw both mosfets and bipolar without the circle. I think the circles stopped showing in schematics that I saw somewhere in the nineties, yet I think that especially in sloppily hand drawn schematics it helps distinguish the mess, and in such case it really helps with the mosfet as it is just a mess of unconnected lines withou the circle.
 

dl324

Joined Mar 30, 2015
10,932
It's now typical for the circles to be dropped.

When I was working on microprocessor designs (90's thru 2013), we didn't use the typical MOSFET symbols that show the bulk connection; we didn't even use arrows. We used a bubble on the gate for PMOS.
inv.jpg
When drawing by hand, the square corners by the gate were rounded so they looked more like semicircles. We could draw schematics faster that way...

EDIT: When I was in school (in the 70's), text books didn't have circles on transistors.
 
Last edited:

DerStrom8

Joined Feb 20, 2011
2,390
BJTs I often still draw with circles. MOSFETs I generally draw without. I guess I never really thought about why I did or did not include the circle. Just things I picked up over the years I guess....

Matt
 

cmartinez

Joined Jan 17, 2007
6,992
Hey... I learned something new today... just like DerStrom8 said, I've been doing things without even thinking of their origins. I always saw the need of a circle as something that delimited the part being drawn... now I know better, thanks!
 

shortbus

Joined Sep 30, 2009
7,756
While not on the same level as the rest of you, I use the circle. Until recently all of the beginner books and many website show it. But it's a pain when doing it in hand drawn schematics. Will stop doing it now though.:)
 

#12

Joined Nov 30, 2010
18,210
Good point. When I'm drawing it, I don't use the circle because I already know what it's doing. When I read Other People's schematics, the circles are helpful in sorting the clutter.
 

Thread Starter

AnalogKid

Joined Aug 1, 2013
8,487
That's where I was headed with this. I never draw the circles, but they're in every FET in my CAD libraries, including the native ones from the publisher. OTOH, none of the BJTs have circles.

ak
 

ISB123

Joined May 21, 2014
1,236
I draw schematics in a way it would look most pleasing to the viewer.So If I have a bunch of connections going around the transistor then I would make sure it gets noticed,so I would just a draw a circle around it or a dashed box.
 

dl324

Joined Mar 30, 2015
10,932
It seems that the transition away from drawing circles for BJTs started in the 60's and 70's. I have a reference manual that's a compilation of information from various sources. Some of the schematics from the 60's with transistors have circles and some don't. I have a bunch of databooks from the late 70's and none of them had circles.
 

DerStrom8

Joined Feb 20, 2011
2,390
It seems that the transition away from drawing circles for BJTs started in the 60's and 70's. I have a reference manual that's a compilation of information from various sources. Some of the schematics from the 60's with transistors have circles and some don't. I have a bunch of databooks from the late 70's and none of them had circles.
That would be consistent with the idea that prior to the 60s and 70s switching was done with tubes, hence the circle around it. I suppose it was intended as an easier transition from the older tube symbols to newer transistor symbols--You can see that a transistor is doing the same thing a tube would be. Tubes were phased out in the '60s or '70s
 

#12

Joined Nov 30, 2010
18,210
I first repaired transistorized TVs at Sears in 1971 or 1972. Still mostly tube sets in production.

The first problem was that all the training manuals assumed transistors never fail, so they were useless to the technicians. You could go around in circles for days because the answer never landed on, "replace a transistor". We also didn't have transistors in the trucks, so all transistorized TV's got a ride to the shop where the department manager could argue with the manufacturer to send replacement transistors. Great way to start transistor TVs with a reputation for being unreliable and un-serviceable! No matter how many fewer transistorized TVs failed, paying in-shop prices and waiting for weeks gave a sour memory to many people. Now, everything has a microprocessor, or ten or twenty of them, and replacing the microprocessor still won't repair the machine. That's progress! :)
 

MaxHeadRoom

Joined Jul 18, 2013
20,874
I have some old British Mullard Manuals from the 60's and others that show bi-polar transistors with a circle, also the 90's IR Mosfet manuals show circles also.
Max.
 

wayneh

Joined Sep 9, 2010
16,398
I found a reference here to a standard, IEEE Standard 315/ANSI Y32.2/CSA Z99, that seems to endorse the circle. But that same link points to an international version of the same standard, sans circles.

I think the rule that form follows function applies: If using the circles improves the readability, they should be used for that reason alone. If they don't help, omit them.
 

Russmax

Joined Sep 3, 2015
82
There's a very simple rule of when to use a circle and when to not, and it has nothing to do with MOS or BJTs.

If it is a discrete device then it goes inside a circle. If it is part of an integrated circuit, then it doesn't. A discrete darlington will have both symbols inside a single circle. Yes, the use of circles fell out of favor in the 60s and 70s, because that's when designers switched from designing with discrete transistors to designing integrated circuits.
 
Top