741 cross-over distortion

[Frosty_47]

Dear Komrades,

There seems to be some cross-over distortion present at the output from my 741 at very low frequencies ( 6Hz ). Basically I have a simple integrator circuit with a 10uF input capacitor and a 260Ω feedback resistor. The crossover distortion was very bad when I used smaller capacitor and larger feedback resistor. With larger cap and smaller resistor the distortion is barely noticeable but it is still there. I suspect that the poorly designed class AB amplifier inside the 741 is the cause. I would like to know what I could do to eliminate this problem.


Thanks

 

[audioguru]

The lousy old 741 opamp is 40 years old this year. It is noisy, has a full output frequency response to only 9kHz and has distortion.

TL07x opamps are much better. They have low noise, wide bandwidth (100kHz) and low distortion (0.003%).

The new LM4562 dual opamp has only 0.00003% distortion.

None of these opamps can drive a load as low as 260 ohms.
Please post your schematic.

 

[Roff]

Dear Komrades,

There seems to be some cross-over distortion present at the output from my 741 at very low frequencies ( 6Hz ). Basically I have a simple integrator circuit with a 10uF input capacitor and a 260Ω feedback resistor. The crossover distortion was very bad when I used smaller capacitor and larger feedback resistor. With larger cap and smaller resistor the distortion is barely noticeable but it is still there. I suspect that the poorly designed class AB amplifier inside the 741 is the cause. I would like to know what I could do to eliminate this problem.


Thanks

Can you post a schematic? The circuit you described sounds like a differentiator, not an integrator.

 

[MrAl]

Hi,

Yes, it sounds like a diff not an integrator.

Also, some of the older op amps do better with a load to ground like
a 10k or maybe 1k resistor. The LM358 is famous for requiring a 10k
to ground to help keep distortion lower. Try a resistor to ground to
help, but as others have said, a better op amp is the real key, if
that is possible of course.

 

[Frosty_47]

Thank you all for the quick reply. I apologies for the long delay in my reply. At the moment and very little time in school due to unusual amount of lab work involved. Further I made a mistake by calling this circuit an "integrator" when it is actually a "differentiator" op-amp. Below you can see the schematic diagram.

**broken link removed**

 

[Frosty_47]

The lousy old 741 opamp is 40 years old this year. It is noisy, has a full output frequency response to only 9kHz and has distortion.

TL07x opamps are much better. They have low noise, wide bandwidth (100kHz) and low distortion (0.003%).

The new LM4562 dual opamp has only 0.00003% distortion.

None of these opamps can drive a load as low as 260 ohms.
Please post your schematic.

Thank you for your suggestion. Here is my simple schematic:

**broken link removed**

Perhaps I can use a smaller cap and larger resistor and hopefuly not encounter any cross-over distortion with the above mentioned op-amps.

 

[audioguru]

The values of both 260 ohm resistors is far too low. Use 2k ohms minimum so that the opamp doesn't current-limit.

 

[Frosty_47]

The values of both 260 ohm resistors is far too low. Use 2k ohms minimum so that the opamp doesn't current-limit.

Thanks. I will use a 1uF cap and a 2.6K resistor than...

 

[Roff]

Thanks. I will use a 1uF cap and a 2.6K resistor than...

I would use 26k and 100nF. Put 100 ohms or so in series with the cap so that the circuit doesn't oscillate.