# An op-amp connected to another op-amp, the one with ground and the other with virtual ground [closed]

When I have two op-amp circuits, the left one is connected to real ground and the output of this circuit is connected to the input of the right op-amp circuit which is connected to the virtual ground. How do I calculate Vo ?

• What is the potential difference between the two grounds? You can easily calculate the output when you specify that information. Dec 27, 2017 at 20:07
• Is a part of a bigger circuit. But it confuses me that I have two grounds. The virtual ground circuit is shown here: electronics.stackexchange.com/questions/346179/… Dec 27, 2017 at 20:09
• One is "ground" and one is "virtual ground". Yet there's a 10 V difference between them. Okay. Dec 27, 2017 at 20:13
• So Vo = -(R4/R3)*V2 plus 10 Volt offset? Dec 27, 2017 at 20:15
• What is "real ground" and is there a connection.. somehow.. between the two? Dec 27, 2017 at 20:15

OK Since you are not telling us where your grounds are connected I have to assume left ground is connected to the bottom of the 20V supply in your original post.

You have also not indicated which voltage U1, U2 and Uo are referenced to, so again I will make the assumption it is to the same side of the 20V supply.

As such you have the following circuit.

simulate this circuit – Schematic created using CircuitLab

The formula for the second stage is

$U_o = V_1 - (U_2- V_1).\frac{R_4}{R_3}$

$= (\frac{R_4}{R_3}+1)V_1 - \frac{R_4}{R_3}.U_2$

We know

$U_2 = -\frac{R_2}{R_1}.U_1$

So

$U_o = (\frac{R_4}{R_3}+1)V_1 + \frac{R_4}{R_3}.\frac{R_2}{R_1}.U_1$

Of course if the first of amp does not have a negative rail.. the above may be nonsense.

Ground in electronics only means 0V for some physical reference.

Virtual ground means a 0V differential reference such as in Op Amp inputs with the output in the linear range. In your case link Vcc/2 is defined as a virtual gnd to create single supply, bipolar to single sided values. If you can subtract then you properly figure this out. Include gain factors of OA.