Open Collector Op.amp

[kidi3]

Could someone explain me how a open collector Op.amp work.. And how a pull -up and down resistor works, and how i can use them on my open collector Op. amp. ..

 

[WimRFP]

See https://en.wikipedia.org/wiki/Open_collector.

The purpose of the pull-up resistor (resistor between output and positive supply) is to get high output voltage when the open collector NPN transistor is in the off state. A well-known open collector comparator is the LM 339. The datasheet contains the internals, so you can see the NPN open collector transistor at its output.

 

[HTA]

An output stage of a operation amplifier or comparator has normally a push-pull output stage to drive an active high or low level on the output. On an open-collector output the high side output transistor is missing. Thus you can only pull the output to a low level and you need an external pull-up resistor, e.g. to +5V. There are comparator with open-collector outputs. An operational amplifier needs an active push-pull output to amplify an input signal. A pull-down resistor(to 0V or negative) is needed if an output has only the high sider driver transistor or on input to avoid floating inputs. You may search for "NPN/PNP/PP outputs" to find examples.

Enjyo your design work!

 

[kidi3]

How do determine the value of the resistor..

And how does the Pull up resistor and down work.. ?

 

[WimRFP]

Designing the output circuitry for an open-collector output depends on what you want. For example a relative low value for a pull-up resistor may result in steep edges (so you get a fast circuit), but increases power consumption.

Could you narrow down your question as driving a mosfet, large NPN BJT, relay or lamp require different solutions?

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For general info on pull-up resistor see for example: https://en.wikipedia.org/wiki/Pull-up_resistor

 

[kidi3]

Imagine i want to build a comparator with an LM339, where e+ is input and e- is grounden.Since it's Open collecter at the output i need to suply the put an pull resistor and the the supply at the output of it..
I only want value 0 and 15...

how would i then choose the value of the resistor??
What is the criteria?

 

[FvM]

how would i then choose the value of the resistor??
What is the criteria?

Output load and intended switching speed (output rise time).