# Purpose of capacitor at inverting input of opamp ?

#### Buzz20

Joined Apr 30, 2019
4
I found this amplifier circuit. Its a headphone amplifier by Rod Elliot. What is the purpose of C2L and R3L at the inverting input of the opamp ? Most of the opamp circuits usually only has R3L. This circuit has both the resistor and capacitor together. What does it do ?

Then I found another circuit which also has capacitor and resistor at the inverting input. But this circuit has the capacitor, Ci and resistor Ri flipped. #### AlbertHall

Joined Jun 4, 2014
10,046
These circuits provide negative feedback from the output of the amplifier to the input.
Consider the amount of feedback at DC, low AC frequencies, and high AC frequencies.

#### panic mode

Joined Oct 10, 2011
1,837
R3L and R4L are feedback, they form voltage divider that sets amp gain.

#### AlbertHall

Joined Jun 4, 2014
10,046
But this circuit has the capacitor, Ci and resistor Ri flipped.
When two components are in series with nothing else connected to their junction, as in this case, it doesn't make any difference which way round they are.

#### BobaMosfet

Joined Jul 1, 2009
1,118
Additional low-level understanding info- The resistor controls current, and the capacitor is used to eliminate DC bias so that the signal going into the OpAmp is a cleaner AC waveform. R3L and R4L form a voltage divider that regulates inverting input.

#### crutschow

Joined Mar 14, 2008
25,122
The resistor and capacitor form a high pass filter which blocks DC.
The amp thus acts as a follower with a gain of one at DC to maintain the amp DC output at 0V.
Above the RC corner frequency (about 1.6Hz for the top circuit) the feedback is reduced and the amp gain becomes 1+R4L/R3L = 23 at audio frequencies.

The capacitor is not absolutely needed.
The only difference is that, without the cap, the output DC level would be equal to the amp input offset times the amp gain, instead of being equal to just the offset.
Since you want the output voltage to be as close to zero as possible to minimize DC current through the speaker/headphones, the capacitor is added.

Make sense?

The order of an R and C in a series circuit is immaterial.
The equations describing the response of a series circuit makes no distinction of the elements' order.

Last edited:
• Kjeldgaard