Electronics > Beginners

External pull up/down/GND resistor's on op-amp output's ?

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MathWizard:
With jellybean op-amps, is it good practice or not, to have some dummy load, like 10k or 100k, on the output's ? If I was making some peak detector, I might not want to, so I know not all circuits would benefit.

So when is it a benefit for the op-amp itself? Or is there any with common modern jellybean op-amps ? Or is it usually all taken care of internally, and usually doesn't need extra DC loading. Then AC loading and compensation, well that's more complicated than I'm thinking about, like DC or low frequency logic circuit's.


As an example, if the output of 1 op-amp was only connected to an input of another op-amp, I would place some resistor between them, but I would also think I should have some much larger external shunt resistor to GND. I'm sure I've seen circuit's where it's left out, maybe for cost reasons, or it didn't matter in that circuit.

I know a common-emitter amplifier should have low AC output impedance, and pretty low DC output resistance, so maybe most op-amp's never need it, IDK.

RoGeorge:
Most opamps don't need a pullup or pulldown resistor at all, but some opamps models, by design, have crossover distortions.  These need to be helped with an output resistor, to mitigate the crossover, for example LM358 or LM324:

#215: Basics of crossover distortion | LM358 op amp example
w2aew


MathWizard:
OK I use LM358 a lot, I know CO dist. makes them bad for audio, but I didn't know adding increasing the load can help, I'll try and remember that.

And then there's the LM393 comparator, it took me a few years to remember or find out it needs a pull up resistor to work.

BillyO:
The LM328/324 only need the pull up/down if you use them with a split supply.  The design is optimized for a single supply.

Oh, and it's not increasing the load.  It's providing a current path when the output would normally cross zero.

David Hess:
The feedback network counts as a load on the output also.

For precision operational amplifiers, a heavy load on the output will spoil precision because of self heating of the output transistors.

As mentioned, operational amplifiers with class-b output stages can have their crossover distortion removed by forcing the output into class-a with a load to one supply rail.

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