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open collector

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blibala

New Member
what is open collector output?how to we know that the op amp has opne collector type output?is it stated in the data sheet?
 
to best descibe open collector, think of a Transistor. It has a base, emmiter and a collector. Typicaly the base is input, the emmiter is is connected to ground and the collector is connect to + through a load. Open collector is were the collector is floating. ie. not connected to + through a load. and yes the datasheet will call it out. op-amps usually are not open collector
 

blibala

New Member
i couldnt find in the data sheet stated that it is open collector(LM339).can u show me how?
if it is open collector, what should i consider if i want to connect it to the circuit?
 

crust

Member
You should just put a pullup resistor (perhaps a 4.7K) from the output of the comparator to your Vcc (or depending on your part, perhaps a higher or lower voltage).
 

Russlk

New Member
My (old) data sheet does state: The output is the uncommitted collector of a grounded-emitter NPN output transistor." And, if you look at the equivalent schematic, the output collector is clearly shown. Since the output can only pull down, the load has to be from output to positive voltage.
 

ivancho

New Member
The datasheet doesn't say it outloud.... :? wierd because most of them do. But there are hints that tell you it is.

Refered to the attached tables from the datasheet. In here the Electrical caracteristics of the output voltage, it tells you that it sinks..... but it does not give you a Output High voltage characteristic.

Then on the typical application schematic. They show a pull up resistor, that tells you right away that the output needs help to show a logic 1, therefore making it a open collector. (open collector just means that the chip is able to output a 0, but not a 1...... in other words it can only sink current not source)

Ivancho
 

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StupidDum

New Member
In open collector, the collector is not connected to anything. Therefore the transistor is.. not operating. To complete the circuit, a resistor should be connected to collector end, to Vcc. This is called "pull-up resistor". Otherwise, the transistor couldnt change state.

Well, normally application notes are included in datasheet of op amp. By referring to them, you should have idea if it is an open collector.
 

blibala

New Member
for the comparator, what happen if the noninerting input's input is greater than the inverting side?does the output voltage go high?
 
ivancho...good call. if it only gives you a sink current, then it's IS open collector. And to ansewr the last quention. If your neg is more then the pos. the output will swing neg. and vise versa
 

Roff

Well-Known Member
Down near the end of the National datasheet, there's another really good clue, if you can read schematics...
BTW, the LM393 is a dual version of the same part, for those who don't need 4 comparators.
 

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