# Simple single-supply op amp virtual ground questions

I found this circuit online and made it and it works.

It uses an LM358 op amp to amplify an oscillation from a 555 in astable mode.

simulate this circuit – Schematic created using CircuitLab

1. How were the values of R2 and R3 arrived at? I know they should be equal but why 100k?

2. What function does the capacitor serve? And how was its value arrived at? Is there a formula for these things or just trial and error?

• Why do you need to amplify since what you get is just 0V and 5V? – Jason Han Aug 2 '17 at 23:39
• I'm not sure what you are asking. The 555 will actually be multiple 555s and the output signal is only around 3V. – thatsagoal Aug 3 '17 at 9:37

The values of R2 and R3 are chosen so that the unwanted voltage $V=G*I_{in}*R/2$ (Where $I_{in}$ the maximum input current of the opamp and $G$ is the closed-loop gain) remains acceptably low. This voltage will appear as error on the output.