Virtual ground

Thread Starter

pete.york

Joined Mar 15, 2011
1
Hi,
Can anyone explain, the reason for virtual ground?
Everyone always tell : " if you have negative feedback, then V+=V- in opamp" , but why? What is the reason?
How people get to this thing at the beginning?

Thank you.
 

Papabravo

Joined Feb 24, 2006
21,159
Answer: An operational amplifier amplifies the difference between the two inputs. If they are exactly equal down to a large number of decimal places then the output will be close to zero. Small deviations will produce an output that is close to one of the supply rails.

So the beginning is to look at the terminal without the feedback and ask "What is happening at this terminal?". The answer is very likely that it is connected via a resistor to ground. Question: What is the voltage drop across a resistor in which there is only a tiny current flowing, say a couple of picoamperes?

Answer that question and you can get some feel for the virtual ground. It is just a point in a circuit that is at or near ground potential without being connected to ground. In the case of the inverting opamp it is the node where the feedback and the input resistor are connected.
 
Top