distortion in inverting opamp

BkraM

Member

• Oct 25, 2015

• #1

Hi all,

I've build a basic inverting opamp circuit where I'm using a single power supply.
An artificial ground is created by a resistance divider.

similar to this image:
**broken link removed**
with R1 = 1k, R2=2k R3=R4=10k
I'm using a LM324 and a 7V source.

The result I get is that each time the input signal crosses the artificial ground level, there is a (small) distortion on the output:
**broken link removed**

red=input, blue = output

What is causing this distortion?


Thanks,

 

K

kubeek

Well-Known Member

Most Helpful Member

• Oct 25, 2015

• #2

It´s called crossover distortion, and it is caused by that lousy 40 years old LM741. Either use something more modern, or add a resistor from the output to one of the supply rails, which will bias the output stage into class A instead of class B.

 

BkraM

Member

• Oct 25, 2015

• #3

Thanks,

found a good tutorial on that subject :

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Oct 25, 2015

• #4

But as he said, DON'T use 741's

 

BkraM

Member

• Oct 25, 2015

• #5

switched to a TL074

 

MikeMl

Well-Known Member

Most Helpful Member

• Oct 25, 2015

• #6

The other problem is that a '741 is very marginal at a single-supply Vcc of only 7V.

Which you might have realized if you had read the '741 datasheet, for which no values are even given for any supply voltage lower than 20V.

 

BkraM

Member

• Oct 25, 2015

• #7

I was originally using a LM324, but i guess that did't make much difference

 

MikeMl

Well-Known Member

Most Helpful Member

• Oct 25, 2015

• #8

BkraM said:

I was originally using a LM324, but i guess that did't make much difference

Click to expand...

The LM324 and LM358 are notorious for that type of cross-over distortion. You can sort-of cure it by putting a 4.7K pull-up resistor between the the output pin of a '324 or '358 to its Vdd pin.

 

audioguru

Well-Known Member

Most Helpful Member

• Oct 25, 2015

• #9

It is a shame that your schematic is wrong because it shows a 741 opamp that does not have crossover distortion but you used an LM324 that has 4 of the same opamps in an LM358 that do have crossover distortion.

The tutorial did not say why the LM324 and LM358 are designed to have crossover distortion. It is because these opamps are the first LOW POWER opamps so the output transistors do not have enough bias current (Class-B) so that the idle current very low which produces crossover distortion. All opamps that do not have crossover distortion have some bias current in the output transistors (class-AB).

 

Tony Stewart

Well-Known Member

Most Helpful Member

• Oct 25, 2015

• #10

With a single supply the crossover distortion load resistor must go to V/2 , so when a cheap LM324 is needed for high volume production and/or Very Low Supply Current Drain (700 μA), the fix is a pullup and pull down resistor which is effectively V/2. The external load current affects the base current which affects NPN/PNP bias Voltage towards eliminating the crossover distortion.

A pair in the range of 10k to 25k , as I recall, works well.

Using a pullup only also works but requires twice as much current and effectively puts the lower of the push-pull drivers into class A mode.