Consider a dual-supply op amp circuit used to process (but not amplify) line level audio; for the sake of simplicity, imagine a unity gain follower. With the input AC coupled and referenced to ground, only a small DC offset, caused by the op amp, will appear at the output and this seems negligible at the first glance.
In this scenario, is it necessary to also put a coupling capacitor at the output? Here are some of my thoughts so far.
Pros of having an output coupling capacitor:
- If a later stage is DC coupled and has gain, the coupling capacitor would prevent the DC offset from being amplified.
- If a fault develops within the device and the op amp starts outputting a rail voltage (or any substantial DC level for that matter), the output capacitor would prevent this condition from negatively affecting later stages.
Cons of having an output coupling capacitor:
- Since the input impedance of a later stage is unknown, a fairly large capacitance is required to achieve the desired low-frequency response.
- The capacitor would have to be terminated with a resistor to ground. To avoid loading the output, it would have to be a high value resistor and this would introduce additional noise at the output.
Is there anything I'm missing here? As much as I'd like to omit the cap and the accompanying resistor for obvious reasons, I really wouldn't like to design a product that causes trouble for its users and interacts badly with other devices.