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LM324, LM358 crossover distortion


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In the datsheet of Mitsubishi's copy of National's LM358 low power dual opamp (half an LM324 quad), a very good description is made about why and how much crossover distortion is caused, and a method to reduce it.
They show an awful full output frequency bandwidth to only about 6kHz at 10% distortion just like National does, and a horrible 3% of crossover distortion buzz with a gain of only 1. The crossover distortion is even worse with more gain. :'(
http://www.datasheetarchive.com/semiconductors/download.php?Datasheet=1232026

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  • 3 weeks later...

Hi Walid,
Most opamps and audio amplifiers avoid crossover distortion in their complimentary NPN-PNP output stage by using class-AB with a couple of diodes in series between their bases to provide enough voltage for them to have idle current, for a smooth transistion from one transistor to the other.

The LM324/LM358 opamps are low-power, so they tied together the bases of the output transistors to eliminate any idle current. They use a 50uA current source to operate one transistor in class-A for low crossover distortion when the load resistance is extremely high.
Therefore with a load, the output voltage abruptly jumps 1.3V from one transistor to the other.
National recommends adding a resistor from the output to the negative supply to provide plenty of current for the NPN transistor to always operate in class-A. Mitsubishi recommends adding a resistor from the output to the positive supply for the PNP transistor to always operate in class-A. The resistor increases the circuit's idle current to be far more than an ordinary class-AB opamp. ;D

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Hi Walid,
The phenomenon of cross over distortion arises in Class -B power amplifiers  in complimentery symmetry mode.
here we use 2 transistors both biased at cutoff region.
so the base to emmiter voltage in no signal conditions will be zero
when there is some signal applied whose amplitide is less than 0.7V ie., the cutoff voltage of base emitter junction,
then the base emitter junction is reverse biased and still the transistor is in cutoff region only when the signal exceeds the 0.7V then the trasistor turns active and we get the out put

thus in Cass B operation wecan't get the output for input voltages less than 0.7V  and for i/p voltages >0.7V we get 0.7V less than what we are supposed to get when we don't have this phenomenon .
this is called as crossover distortion

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