Understanding the Op Amp Minus Sign Symbol

[Mindscrape]

What does it mean when an op amp has only a minus sign in its symbol? Is the positive terminal at ground, or is it at some arbitrary voltage?

 

[edmondng]

can you clarify? pic maybe?

 

[berkeman]

What does it mean when an op amp has only a minus sign in its symbol? Is the positive terminal at ground, or is it at some arbitrary voltage?

I'd guess that it is just drawn incorrectly, or else it is not just an opamp. A unity-gain buffer might just be drawn with a + input, for example.

 

[dlgoff]

Could it be a logic gate, like an inverter?

 

[Mindscrape]

If anyone has the Franklin book on control theory, he does this for the lead, lag, and quad circuits in the chapter 2 or 3 problems. This is what it looks like (with a wire going in at the center and coming out at the center). *dots should be white space, and the two hyphens represent the minus

|
|..|
|...|
|.--...|
|...|
|..|
|

 

[berkeman]

Hmmm. No, Franklin is not in my bookshelf, and my google of the book got me some tiny excerpts at amazon.com, but no joy on the opamp symbol.

Any chance that you could scan a page showing this symbol in a schematic? I'll wave the copyright stuff for now, until we can help you figure out the symbol. As you can tell, from your description of it, it is a non-standard symbol.

 

[Mindscrape]

Here is the lead circuit, sorry for the cartoony look. I solved for it if v+ were some arbitrary voltage because that seemed to be give the most useful transfer function, but I just don't have any idea what the symbol was supposed to mean.

 

[edmondng]

if i were to guess it would be non-inverting.

 

[berkeman]

Here is the lead circuit, sorry for the cartoony look. I solved for it if v+ were some arbitrary voltage because that seemed to be give the most useful transfer function, but I just don't have any idea what the symbol was supposed to mean.

Do you have a part number for the device, so that we can look at a datasheet? What is the source for this symbol? What are the pin numbers of the device as shown in your circuit? Do you know what power rails it uses?


Edit -- I'll check with some of the other engineers here at work today to see if anybody has the Franklin control theory text.

 

[rbj]

Here is the lead circuit, sorry for the cartoony look. I solved for it if v+ were some arbitrary voltage because that seemed to be give the most useful transfer function, but I just don't have any idea what the symbol was supposed to mean.

if i were to guess it would be non-inverting.

no, i think it's inverting. the + terminal is connected to ground and it is not shown in the pic. I've seen this convention before and i have never liked it.

 

[berkeman]

no, i think it's inverting. the + terminal is connected to ground and it is not shown in the pic. I've seen this convention before and i have never liked it.

Ohhh. That would make sense. + input grounded, with split supply rails. Yeah, that's a pretty limiting and strange convention... more idealized than real-world. It better be a FET-input opamp! (Quiz Question -- why?)

 

[edmondng]

oops you're right inverting. i must be sleeping replying that early in the morning :)