# Function of resistor in OpAmp circuit

I came across this circuit as part of a larger circuit in a guitar effect pedal: simulate this circuit – Schematic created using CircuitLab

I was wondering about the function of the resistor $R_1$. My calculations show that the same transfer function can be achieved with or without $R_1$ (leaving out $R_1$ just means that we need to change the other resistor values). The only difference that I can see is the change in input impedance of the circuit. Is this the only function of $R_1$ (i.e., reduce/define the input impedance), or is there anything else I'm missing?

• Does your opamp have input bias current? May 20, 2017 at 15:35
• @sstobbe: It's a real circuit with a real opamp, so yes, it does. I do not know exactly which type of opamp is used in the original circuit. May 20, 2017 at 15:38
• R1 just biases input when not connected to a low R source to prevent noise. and gives low DC current. for AC coupled inputs. R1 is larger than all the others May 20, 2017 at 15:41
• If the input on the left is ac coupled you will have to provide a path for the DC bias current I'm not sure the approach taken is the best May 20, 2017 at 15:41
• @TonyStewart.EEsince'75: The input of the circuit is connected to the output of another opamp (via an RC-low pass filter). Would your comment then still apply? May 20, 2017 at 15:46 