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Comparators with diodes on the output why?

Davewalker5

Sep 20, 2014
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I don't understand why there is diodes on the output of these comparators? any reason

Is it about sinking and sourcing current?

Because a comparator with switch from a low state to a high state, so the diode does what?
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Because the design requires it. The outputs are "OR"ed together.
 

Davewalker5

Sep 20, 2014
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The outputs are "OR"ed together.

ORed together how so ? i don't see it

The comparator goes high or low, how does the diodes make the comparators output ORed?
 

Supercap2F

Mar 22, 2014
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Lets say that you drop the diodes from the circuit, so you have two comparators outputs tied together. Now if one of the comparators output goes high, and the other ones output is low, your going to get a short. But if the diodes are there, only high voltages will pass through them eliminating the shorts that could happen on the output.

Putting the diodes there instead of a OR gate is just to save IC packages count. Some people call this technique "Mickey Mouse Logic" or M2L.
Dan
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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I'm sorry, you'll have to figure that out for yourself.

If you wonder why, you should google "askhole"
 

Davewalker5

Sep 20, 2014
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Now if one of the comparators output goes high, and the other ones output is low, your going to get a short.

Yes I understand

But if the diodes are there, only high voltages will pass through them eliminating the shorts that could happen on the output.

So both comparators outputs can be HIGH and it won't short out? or damage the output stage internally of the op amp?


Now I do see how the diodes are a OR, when you "TIE" both outputs of op amps together
 

Davewalker5

Sep 20, 2014
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So the voltages don't add or the current add when the 2 outputs are HIGH?

I guess since both outputs are at the same potential they don't add together
 

kpatz

Feb 24, 2014
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The diodes are needed since 741s can both source and sink current, meaning if one is "high" and one is "low" current would flow between them in a nearly short circuit.

If comparators with open-collector outputs like the LM339 were used, the diodes wouldn't be needed, since these devices only sink current, they can't source it.
 

davenn

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nice answer Kpatz :)

ya taught me something
 

Davewalker5

Sep 20, 2014
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The diodes are needed since 741s can both source and sink current, meaning if one is "high" and one is "low" current would flow between them in a nearly short circuit.

How can a 741 op amp source current?

The diodes are in series , so how can a diode in series sink current?

I can understand if the diode was in parallel with the output, it would sink the output

I'm confused about how it's sources or sinks current

Since op amps aren't open collector outputs, what is their internal output called?

Open collector outputs can only sink current , they can't source current because of why?
 

davenn

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Open collector
that shows you why they don't source current

if you don't understand what to sink current or to source current means then google those terms

its pretty straight forward

An IC with an output that is a current sink will have its NPN transistor with emitter connected to the internal GND (0V) and current is sourced from external circuitry into the collector

An IC with an output that is a current source will have its PNP transistor with emitter connected to the internal V+ and current is sourced from the collector
 
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kpatz

Feb 24, 2014
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As davenn explained, an op-amp or comparator IC can be designed to source current (PNP transistor output, so a positive current flows from the V+ rail to the output), or sink current (NPN transistor output, so a current flows from the output to GND). An op-amp IC typically has both the PNP and NPN outputs together, so it can source or sink current depending on what it sees at its input. This is also known as a push-pull configuration.

Op-amps are push-pull so they can output a variable voltage (as the "push" and "pull" sides can act as a variable voltage divider) as an amplifier is designed to do, while comparators are used more in a "digital" fashion (on or off) so they are usually open-collector so that the outputs can be tied together without needing diodes. This is probably the biggest design difference between comparator and op-amp ICs.
 
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