For those who want their gain loop, to be capacitively grounded at ground, instead of 1/2 the supply voltage, this is for you.

 

What is the purpose of the 220nF capacitor?
What is the purpose of the 2.2uF capacitor?

For the most recent schematic at the time of your post (this would be a lot easier with reference designators) -

220nF - This "disconnects" the shunt feedback resistor (50K) at DC. By isolating the 50K resistor, the circuit becomes a voltage follower with a gain of 1. The purpose of this is to prevent the opamp's input offset voltage error from being multiplied by the gain of the circuit. This is not a big deal with a gain of only 4, but if the circuit gain were 100, as in a microphone preamp, the shift in the output signal's DC component could cause clipping, or be a problem depending on what the circuit is driving.

2.2 uF - output coupling capacitor. In normal operation, the output sits at 4.5 V when there is no signal. A downstream amplifier might not like this.

ak

 

Thanks for enlightening me.

 

For the most recent schematic at the time of your post (this would be a lot easier with reference designators) -

220nF - This "disconnects" the shunt feedback resistor (50K) at DC. By isolating the 50K resistor, the circuit becomes a voltage follower with a gain of 1. The purpose of this is to prevent the opamp's input offset voltage error from being multiplied by the gain of the circuit. This is not a big deal with a gain of only 4, but if the circuit gain were 100, as in a microphone preamp, the shift in the output signal's DC component could cause clipping, or be a problem depending on what the circuit is driving.

2.2 uF - output coupling capacitor. In normal operation, the output sits at 4.5 V when there is no signal. A downstream amplifier might not like this.

ak
[/QUO

For the most recent schematic at the time of your post (this would be a lot easier with reference designators) -

220nF - This "disconnects" the shunt feedback resistor (50K) at DC. By isolating the 50K resistor, the circuit becomes a voltage follower with a gain of 1. The purpose of this is to prevent the opamp's input offset voltage error from being multiplied by the gain of the circuit. This is not a big deal with a gain of only 4, but if the circuit gain were 100, as in a microphone preamp, the shift in the output signal's DC component could cause clipping, or be a problem depending on what the circuit is driving.

2.2 uF - output coupling capacitor. In normal operation, the output sits at 4.5 V when there is no signal. A downstream amplifier might not like this.

ak

I have corrected the output problem. A down stream amplifier will like the preamp output