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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

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    \$\begingroup\$ To understand how R3 is calculated from R4 and R5 (it's on the sheet!), assume both Vs and GND are at Vbias instead. \$\endgroup\$ – Janka Nov 30 '18 at 13:26
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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.

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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.

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    \$\begingroup\$ How can you determine what power supplies are being used from those circuits? Neither shows any detail at all. \$\endgroup\$ – Finbarr Nov 30 '18 at 13:49
  • \$\begingroup\$ Yes - you are right. However, what do you think could be the purpose of the additional resistors R3, R4, R5 ? To me, the answer is clear (as written above in my comment.) \$\endgroup\$ – LvW Nov 30 '18 at 16:49
  • \$\begingroup\$ Both circuits will work fine from single or split rail supplies. The bias may be needed because a single rail is being used, or it may be because whatever Vo is driving needs to have the signal biased, e.g. an A/D converter on an MCU. \$\endgroup\$ – Finbarr Dec 1 '18 at 13:41

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