@Tonyr1084 I believe the comments are valid and meant well. I used to balance out the 741 in class every time because I thought they needed it. It worked well enough as long as you understood that they’re not perfect and there was some offset. If you’re concerned about offset it makes sense to use better ones. There are a number of uses where it doesn’t matter enough to make a difference. Oh the instructors explanation for using 741 was so we can see the imperfections with the equipment we had in the labs.

 

A future project, the one I mentioned which is decades old, is to power a Laser Diode (Laser pointer) to shine upon a wall. Its beam is either reflected by a hexagonal rotating mirror or on an angled rotating mirror. With the hex, the beam sweeps like an oscilloscope left to right or right to left, wouldn't matter. There's no Y deflection but there is a Z axes pulse. The pulse is an audio signal. Amplitude controls the brightness of the dot. The frequency of the audio signal (music) as it sweeps across will pulse with every positive going audio sine wave and extinguish with every negative going wave. At a given sweep rate and a given frequency dots or dashes will appear on the surface. Those dots can be moving from one side to the other either way, depending on the sweep and frequency. As the audio signal (again, music) changes the patterns on the wall will change. Low frequency (base) notes will produce long dashes whereas high frequency notes will produce small dots and many of them. With the complex signals going on in an audio wave dots can be seen going both directions and can mix in with dots and dashes of all lengths.

We used to put an LED on an old LP (record) mounted on a motor shaft. Make brushes and transfer an audio signal directly to the spinning disk. Depending on how fast the disk spun the circling LED would produce varying patterns. The problem was back then old LED technology didn't produce very much light, and had to be viewed in the dark for the greatest affect. That's where the Laser and the offset mirror comes into play. It will bounce the laser beam on the wall in a circular pattern, so that when certain notes were played it could carry all the way around the circle.

Anyway, that's a possible application of the 072's (dual op-amp). Left and right stereo signal. I may even split the audio into several signals; low, mid and high (left) and low, mid and high (right). But right now ambition is somewhat on the low side. But for sure, all the advice I'm getting is appreciated and meaningful. Thank you all.

 

So how do you actually adjust the offset? Like, what is the generally accepted proper way to do it?

Say I have an op-amp, power supply in front of me, a multimeter and the adjustment pot. What's next?

 

Here’s TI’s documentation
https://www.ti.com/lit/an/sloa045/s...78876&ref_url=https%3A%2F%2Fwww.google.com%2F

 

So how do you actually adjust the offset? Like, what is the generally accepted proper way to do it?

For the circuit I posted in post #7, I grounded R1 and R2 and adjusted the pot until the output read as close to 0mV as I could get. The 200mV range on my meter is 0.1% of the reading plus 0.2mV (with current calibration).

With the values I used, an adjust range of +10mV to -20mV was sufficient.

 

DC offset adjustment of op-amp

There are two ways to adjust the DC offset. If the op-amp has offset null pins, then you can use the configuration as set out in the datasheet. In essence, a 10kΩ trim-pot is wired between the two offset null pins and the sliding wiper is connected to V- supply.



If the op-amp does not have offset null pins you can inject a small trimming voltage into either the inverting or non-inverting input, depending on your circuit configuration.






Offset null adjustment is important in DC measurement applications and not critical in AC signal applications.

 

Vo goes to the non-inverting input to the circuit in place of ground.

Vo = Ri Rc ( 1/(Rc+Rb))-(1/(Ra + Rb))
Using this circuit to create the offset voltage allows the offset to be added to the non-inverting input in place of ground, having virtually no effect on gain as offset is adjusted. It is particularly useful in cases in which you only have one regulated voltage from which to generate the offset voltage.

 

This might be a bit of a digression, but I was reading the latest edition of Doug Self‘s book on power amplifiers. He says that the distortion increases when the long tailed pair is out of balance. So, the offset adjustment on the TL071 puts unequal loads on the output of the long tailed pair in order to correct the offset. Is the offset adjustment putting the LTP back in balance or putting it out of balance to correct the offset?

 

For differential pairs the source of offset can be from
mismatched resistors (collector and/or emitter)
mismatched transistors (BJT, JFET, MOS)
Offset will introduce distortion due to errors in bias
correction using null will minimize this error in all cases, by increasing emitter resistance (picture a common emitter amplifier and the effects of Re) we effectively reduce gain giving transistors more stability and less reliance on beta or externally trimming the resistance.

 

@MikeA You had me confused. I thought this was my thread. Instead of hijacking mine, perhaps you might start your own? Would be appreciated. Thanks Mike.

 

Instead of hijacking mine, perhaps you might start your own?

My question was on topic and very relevant to the theme of the thread.

It's a safe bet that in the future, anyone performing an internet search about op-amp offsets, will find all posts in this thread useful.

 

@MikeA I hope it's useful too.

You've been a member of AAC for quite some time and by now you should know about hijacking someone else's thread. All I'm saying is you could have asked your own question and other members could answer your question directly. As for my thread - I get notifications when there's a response. Which in this case I thought it had been resolved. For someone else to start asking questions within my question is considered hijacking.

I'm not offended or annoyed. Just pointing out that posting your own question is the proper way to address your needs. You can refer to, even tag this thread. But you will get direct answers to your question instead of having me get notified every time someone addresses your embedded question.

For my part I've gotten my answer.

 

I agree that @Mike A 's pot https://forum.allaboutcircuits.com/...-what-are-the-offsets-for.174552/post-1574520 on "How to" is completely relevant.

Personally, I want to thank you @Tonyr1084 for posting your fundamental question. You could be complaining about all of the different ways of adding offset. Why aren't you? It's nice to have everything in one place.

Your answer could have just been:

The DC offset voltage is also amplified with the gain of a DC amplifier. For DC amplification offset can be a significant contribution of the gain error.

Q.E.D.

An I-V converter, transconductance amplifier, is really sensitive to both Ib and the offset voltage and it's a function of temperature.

 

I'm simple minded. Complex answers go over my head - as you can see from the lack of fur on top of my head.

 

So there's a distinction between

Can you tell me what they're for and how to use them?

and

So how do you actually adjust the offset? Like, what is the generally accepted proper way to do it?

Perhaps I should write about designing laser sound-to-light effects as that seems to be allowed.

 

A future project, the one I mentioned which is decades old, is to power a Laser Diode

These are cheap low power laser pointer modules that can be had on Amazon, 10 for less than $3.00. Not talking about things that can start fires or blind persons or pets in an instant. Even still, those cheapies DO require caution when using them.

Now, I HAVE seen much more powerful lasers available, but those are definitely not in line with AAC guidelines. And they are NOT the ones I have for the use.