Question: I came across this circuit for a non inverting op amp which is biased Vs/2. I'm wondering what the difference bettween these two circuits are? What is the purpose of R3 in the first circuit and how is it different to the second circuit?

Thank you
[![enter image description here](https://i.stack.imgur.com/OlQXr.png)](https://i.stack.imgur.com/OlQXr.png)

Answer: 
They're both the same. The first circuit assumes a Vbias supply is available independently. The second provides it from Vs as a Thevenin equivalent. In this case, R4||R5 = R3, and Vbias = Vs\*R5/(R4+R5) if the circuits are to be functionally identical.
In case R3=R4||R5=R3/2 both circuits provide the same gain - however, the first circuit uses dual power supplies (+ - Vs) and can work with Vbias=0 (ground), whereas the second one can work with one supply voltage only. Therefore, the second one needs a DC bias voltage at the non-inv. input for biasing the output DC at Vs/2 (R4=R5).

As a consequence, the input resistance of the 2nd amplifier is reduced to R4||R5.