crossover disturtion in Op Amp - Electronic Circuits Projects Diagrams Free

froten · 13th May 2008, 01:08 PM

Hello all,

I would like to understand an issue with the Op Amps. The inverting and non-inverting inputs of many of the Op Amp circuits like the summing circuit of the 741 Op Amp are not biased. Is there a crossover disturtion in the out put of the circuit? How can it be corrected if any? If not, how the crossoverdisturtion was eleminated ? when I design a signal amplifier using Op Amp, do I have to consider the biasing and crossover disturtion?

thank you very much for any help.

**Nigel Goodwin** · 13th May 2008, 02:40 PM

Pick the correct opamp for the job (don't even consider the antique 741), and you don't have anything to worry about.

audioguru · 13th May 2008, 03:18 PM

Most opamps have their output transistors biased into class-AB so the crossover distortion is very low.

Except the LM358 dual and LM324 quad opamps are low power so they use not enough bias current in their output transistors. They operate in class-B. When they have 100% negative feedback their crossover distortion is reduced to "only" 3%. They recommend a load resistor to one supply voltage so that one output transistor operates in class-A for no crossover distortion.

arhi · 13th May 2008, 03:26 PM

Quote:

Originally Posted by Nigel Goodwin

Pick the correct opamp for the job (don't even consider the antique 741), and you don't have anything to worry about.

Nigel, that's always a good advice (right tool for the job) but for us digital people (I'm sw developer, and as for the hw, I understand zeros and ones pretty good, everything other then 0/1 is big problem) maybe you can give some pointers in "how to pick the correct op-amp for the job" by pointing us to the right thread or some external link ..

**Nigel Goodwin** · 13th May 2008, 04:05 PM

It obviously depends what you're wanting to do with the opamp, but the TL0xx series are pretty useful for most purposes.

ericgibbs · 13th May 2008, 04:27 PM

Quote:

Originally Posted by arhi

Nigel, that's always a good advice (right tool for the job) but for us digital people (I'm sw developer, and as for the hw, I understand zeros and ones pretty good, everything other then 0/1 is big problem) maybe you can give some pointers in "how to pick the correct op-amp for the job" by pointing us to the right thread or some external link ..

hi,
Look here:
http://focus-webapps.ti.com/general/...put=New+Search

Enjoy.

speakerguy79 · 13th May 2008, 04:41 PM

The big factors:

Single supply or dual supply?

What's the allowable voltage supply range? (0-12, -18/+18, etc).

How many op-amps in the package? (1,2 or 4)

Rail-to-Rail on the Input? Output? Is it needed?

How much current can it drive? Some can only do 10ma, others can do >100ma.

Quiescent current draw - do you need it to be low power consumption?

Gain Bandwidth Product (Is it fast enough? 1-10MHz at unity gain is typical).

Is it compensated or not? (essentially means - is it stable at near unity gains?) If uncompensated, you usually have to use a gain of 5 or more.

FET or Bipolar input type - FET inputs have extremely low input bias current. Bipolars have higher input bias current. Each type has advantages and disadvantages, but you are safer just picking a FET type.

There are lots of others. These are just some real basic criteria.

ETA: You should *never* have to worry about biasing a modern op amp. Anything from the mid 80's on should be fine.

arhi · 13th May 2008, 04:45 PM

Quote:

Originally Posted by ericgibbs

Look here:
http://focus-webapps.ti.com/general/...put=New+Search

Thanks

Risking the drift into off-topic, I hope the OP will not mind and that mod's will intervene if needed, thing is, whoever did the "real" electronics use op-amps as easily as resistors or capacitors, for "0|1" people like me, op-amps are "difficult" to select, I use one op-amp in 99% cases (TL072) and that is only because I can get it in local store, saw it working somewhere, tried it, it worked. Now, I do this for fun, not for bread&butter so for me it really does not matter if the part is 1$ od 10$ .. if it could be smaller or more efficient, but, I would like to look at the page stating something like: "there are xyz important properties of op-amp, they are explained like this, op-amps are mostly grouped by this set of properties, these ones are best to use here and here, those ones are best for use in this and that type of circuitry etc... typical representative of this group is xyz###, typical representative of that gropu is xxy###..."... I was googling for it

but no luck

Anyhow, I do not have a big range of op-amps available in local stores so for those I can find I mostly know what is to know (they are all ooooooold and data sheet's are available everywhere) .. but if anyone know a page like the one I just described I will be joj.

ericgibbs · 13th May 2008, 05:08 PM

hi arhi,
Perhaps this may be more useful:

http://www.national.com/apnotes/Tran...rrays__v1.html

Goto to this link and download AN31 application note.

arhi · 13th May 2008, 05:11 PM

Quote:

Originally Posted by ericgibbs

hi arhi,
Perhaps this may be more useful:

http://www.national.com/apnotes/Tran...rrays__v1.html

Goto to this link and download AN31 application note.

now, that's what I needed, THANKS.

/me jumping out of joy

ericgibbs · 13th May 2008, 05:20 PM

Quote:

Originally Posted by arhi

now, that's what I needed, THANKS.

/me jumping out of joy

hi,
This is also a useful pdf.

arhi · 13th May 2008, 05:30 PM

great one too ... now I can turn my head from tuple's, singlton's, mediator's and other "friends" of mine and learn bit about OP's

I might even understand how some of mine projects do what they do

thanks again, now I have reading material for few days

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13th May 2008, 01:08 PM	(permalink)
froten	crossover disturtion in Op Amp Hello all, I would like to understand an issue with the Op Amps. The inverting and non-inverting inputs of many of the Op Amp circuits like the summing circuit of the 741 Op Amp are not biased. Is there a crossover disturtion in the out put of the circuit? How can it be corrected if any? If not, how the crossoverdisturtion was eleminated ? when I design a signal amplifier using Op Amp, do I have to consider the biasing and crossover disturtion? thank you very much for any help.

13th May 2008, 02:40 PM	(permalink)
Nigel Goodwin	Pick the correct opamp for the job (don't even consider the antique 741), and you don't have anything to worry about. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

13th May 2008, 03:18 PM	(permalink)
audioguru	Most opamps have their output transistors biased into class-AB so the crossover distortion is very low. Except the LM358 dual and LM324 quad opamps are low power so they use not enough bias current in their output transistors. They operate in class-B. When they have 100% negative feedback their crossover distortion is reduced to "only" 3%. They recommend a load resistor to one supply voltage so that one output transistor operates in class-A for no crossover distortion. __________________ Uncle $crooge

13th May 2008, 04:05 PM	(permalink)
Nigel Goodwin	It obviously depends what you're wanting to do with the opamp, but the TL0xx series are pretty useful for most purposes. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

13th May 2008, 04:41 PM	(permalink)
speakerguy79	The big factors: Single supply or dual supply? What's the allowable voltage supply range? (0-12, -18/+18, etc). How many op-amps in the package? (1,2 or 4) Rail-to-Rail on the Input? Output? Is it needed? How much current can it drive? Some can only do 10ma, others can do >100ma. Quiescent current draw - do you need it to be low power consumption? Gain Bandwidth Product (Is it fast enough? 1-10MHz at unity gain is typical). Is it compensated or not? (essentially means - is it stable at near unity gains?) If uncompensated, you usually have to use a gain of 5 or more. FET or Bipolar input type - FET inputs have extremely low input bias current. Bipolars have higher input bias current. Each type has advantages and disadvantages, but you are safer just picking a FET type. There are lots of others. These are just some real basic criteria. ETA: You should never have to worry about biasing a modern op amp. Anything from the mid 80's on should be fine. Last edited by speakerguy79; 13th May 2008 at 04:44 PM.

13th May 2008, 05:08 PM	(permalink)
ericgibbs	hi arhi, Perhaps this may be more useful: http://www.national.com/apnotes/Tran...rrays__v1.html Goto to this link and download AN31 application note. __________________ Eric "Good enough is Perfect" PIC tutorials: Gramo's: www.digital-diy.net/ Bill's: www.blueroomelectronics.com/

13th May 2008, 05:30 PM	(permalink)
arhi	great one too ... now I can turn my head from tuple's, singlton's, mediator's and other "friends" of mine and learn bit about OP's I might even understand how some of mine projects do what they do thanks again, now I have reading material for few days __________________ http://www.it4um.com