Can i use a comparator as an Opamp ??

[BGAmodz]

Hello everyone .

Am trying to make a circuit with this IC Low offset voltage dual comparator lm393 .
Can i use it for this circuit ??

 

[ericgibbs]

hi BG,
Dont forget the LM393 is an open collector output pin, needs a pull up resistor.
The LM393 can be configured a low frequency OPA, its in one of the datasheets how to do it
E

 

[BGAmodz]

Hi eric .
Why do i need to make a pull up at the output ?

 

[ericgibbs]

Hi eric .
Why do i need to make a pull up at the output ?

hi,
Look at this image, note the output pin is the collector of a transistor.

I will look for the OPA version circuit.
E

 

[ericgibbs]

hi BA.
Look a this image for a LF OPA circuit.
E

 

[BGAmodz]

ok like this ??

 

[MikeMl]

Using a comparator (with a pullup resistor) as an opamp is a great way to build an oscillator. There is no compensation network for closed-loop stability in a comparator like there is in an opamp...

 

[ericgibbs]

hi,
With a 10K load you will only get about 1mA thru the LED, very dim light!
You could try a 2k2 for a brighter led.
You must add the capacitor and other components as shown on the first circuit.

 

[BGAmodz]

Ok this ???

 

[ericgibbs]

Ok this ???

hi,
You need the R2 and C2 components, for negative feedback and stability.

 

[audioguru]

Why do you need the opamp? (and the resistors and the pot and the transistor?)
To adjust the brightness all you need is this:

 

[BGAmodz]

Why do you need the opamp? (and the resistors and the pot and the transistor?)
To adjust the brightness all you need is this:

Lol this is what i was wandering ( the pull up stuff ) , to be honest i don't get a damn think on how an opamp works internally or externally ( i may open a new thread to discuss it later )
As for the schematic i have just being using this circuit to check if the Opamps i got are working or not , i wanted to apply this same circuit on a standart voltage comparator 'lm393' , to check if its working the same.


Eric has suggested to make a pull up on the comparator , do you think its necessary ??

 

[audioguru]

Oh, you use the circuit to test opamps?
A comparator is completely different from an opamp.

The circuit in your first post will light the LED brightly when the pot is turned to maximum and will dim as the pot is turned down until around half-way the LED turns off with some opamps.
If the pot is turned down more then some opamps will suddenly light the LED bright again.
Is that what you want?

EDIT: A comparator does nothing without a pullup resistor on its output. When it has the resistor and has negative feedback then it oscillates at a high frequency.

 

[BGAmodz]

Oh, you use the circuit to test opamps?
A comparator is completely different from an opamp.


Is that what you want?

Notrmally when there is a bad opamp , the led stays fully on even on low pot resistance , or there is no output at all , this is what i base my test on .

I have another case ( LM741 ) where the light gets dimmed just slightly but never turns off when i fully lower the pot resistance , do you think those are good though ??

 

[audioguru]

I think you are testing opamps that were abused or that got overheated.
I designed a complicated audio equalizer using 4 opamps. Tens of thousands were tested and sold. None failed.

A lousy old 741 opamp was designed 46 years ago to use ONLY a 30V supply. Some work from a 12V supply but the inputs and outputs do not work within a few volts from the positive supply and 0V in your circuit.

 

[BGAmodz]

OK i will try them with a 30 v supply and see what this will give

 

[audioguru]

With a 30V supply, your circuit will overheat the opamp, the 330 ohm resistor and might burn out the LED. The datasheet for the 741 shows that its inputs will not work when they are more than 27V or less than 3V and the output will not reach higher than about 8V or lower than 3V. The LED will never get very dim.

 

[BGAmodz]

With a 30V supply, your circuit will overheat the opamp, the 330 ohm resistor and might burn out the LED. The datasheet for the 741 shows that its inputs will not work when they are more than 27V or less than 3V and the output will not reach higher than about 8V or lower than 3V. The LED will never get very dim.

Do you have any test circuit for the lm 741 ??

 

[audioguru]

It seems that you are simply testing the output of the opamp to see if it is shorted to the positive supply or is shorted to ground. Both defects are very unlikely unless the opamp has been overloaded or overheated.

Here is a very simple opamp tester for the old 741 opamp. It will work with most opamps:

 

[BGAmodz]

I appreciate your help audioguru .

I gues you use use them a lot for your audio amplification ?