I was watching a video of the legendary Bob Pease, in which he says that the regular LM324/LM358 is not a low distortion amplifier, however, if you add a 10K resistor from the output of the opamp to the negative supply rail, then, distortion is greatly reduced.
It appears that in the video they are using bipolar power supplies, so my question is: if im using an LM324/LM358 with a single supply, say 9V and ground, will adding a resistor from the output to ground also lower the distortion? I must add that im adding a 4.5V bias to the input of the opamp so the output is idle at 4.5V. The following schematic displays what I'm doing
simulate this circuit – Schematic created using CircuitLab
The video link is the following: Whats All This Distortion Stuff, Anyhow?
Best Answer
The output stage of the LM324 has low output impedance when it sources output current (when the 'push' half of the push-pull stage is active), and when it sinks output current (when the 'pull' half of the push-pull stage is active). It turns off (goes high impedance) at zero output current, and that causes a 'dead spot' in the transfer characteristic.
To reduce distortion, you must never allow zero output-stage current. $$I_{output} = -I_{load} + {V_{output} -V_R \over R} \ne 0$$ That ensures that the load current plus the addon resistor's output current is always nonzero, at all output signal voltage values.
That means a pullup resistor in conjunction with a load that has a limited ability to sink current, or a pulldown resistor in conjunction with a load that has a limited ability to source current. A resistor to 'ground' might meet neither requirement.