# Newb question on capacitor

#### Graygem

Joined Feb 4, 2011
4
Newb question. On the non inverting amp below, what is the capacitor for? I see the capacitor on some circuits but not on others. Does it make a difference if the circuit is single supply or dual supply?
Thanks.

#### mik3

Joined Feb 4, 2008
4,843
For DC input signals the output follows the input.

For AC input signals with frequencies greater than 0.333Hz, the output is 100 times greater than the input.

#### Graygem

Joined Feb 4, 2011
4
That makes sense, thank you. So R2 + R1 and C create a high pass filter. Now, I have also seen a small cap, say 22pF, in parallel with the R2 resistor. What does that cap do?

#### mik3

Joined Feb 4, 2008
4,843
That makes sense, thank you. So R2 + R1 and C create a high pass filter. Now, I have also seen a small cap, say 22pF, in parallel with the R2 resistor. What does that cap do?
It decreases the AC gain at high frequencies.

#### Graygem

Joined Feb 4, 2011
4
Thank you. I appreciate your time.

#### crutschow

Joined Mar 14, 2008
30,098
Newb question. On the non inverting amp below, what is the capacitor for? I see the capacitor on some circuits but not on others. Does it make a difference if the circuit is single supply or dual supply?
Thanks.

View attachment 39297
Usually that capacitor is to allow offsetting the input DC voltage (typically by 1/2 the supply voltage) to allow an op amp that normally requires dual supply voltages to operate from a single supply. It makes the DC gain equal to +1 but the AC gain is determined by the ratio of the two resistors.