Electronic – Opamp DC offset Question

dc-offsetoperational-amplifier

Just need some help understanding what's happening in removing the DC offset from two band-pass filters.

enter image description here

VG has a value of 4.5V. When I added in the virtual ground the circuit had an offset of 4.5V at the Out and Out2 nodes. I added in capacitors C7 and C8 at 1\$\mu\$F understanding that it will block the DC signal. This worked for the node at Out but not Out2. Later I saw a similar problem where the solution was to put a resistor between the output and opamp which worked but I do not understand why the resistor is needed and why Out didn't need one but Out2 did.

The resistor that was added was R7.

Best Answer

When you run a simulation, in most circumstances, the default starting voltage across a capacitor is 0 volts. This means, that at the instant of beginning the simulation, if 4.5 volts is forced onto one plate of that discharged capacitor by the op-amp output, 4.5 volts is also seen on the other plate. Without a discharge resistor (such as R7), the OUT2 node will remain with an average level of 4.5 volts.