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Could anyone help me out on this, why would an op amp have the curve of X1 with nothing in the circuit, it has a voltage source to the non-inverting input and negative feedback, just like a buffer. I can't figure out why it would have a 1.5v output when the input is 0v, with the shape of the curve I initially thought it was due to rail-rail issue, but that wouldn't explain the initial 1.5v?

The X2 curve is another op amp which has a typical voltage follower curve, but again it seems to have a slight initial voltage and falls just short of 5v.

Any help would be appreciated, I have been going backwards and forwards on this for quite some time now!

*It is a single rail 5v supply

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  • \$\begingroup\$ What op-amp are you using? \$\endgroup\$ – Trevor_G Apr 28 '17 at 18:06
  • \$\begingroup\$ It is a TL072 op-amp \$\endgroup\$ – Jdoe Apr 28 '17 at 18:23
  • \$\begingroup\$ TL072 isn't really a single supply device \$\endgroup\$ – Trevor_G Apr 28 '17 at 18:34
  • \$\begingroup\$ See my answer update.... \$\endgroup\$ – Trevor_G Apr 28 '17 at 18:39
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Probably because you are not using a rail to rail op-amp. Normal op-amps can only drive so close to either rail and bottom or top out.

Check the spec for the TL072

First it is not a single supply op-amp. Though you can drive it +-5V.

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From the data sheet it can not drive any closer than 1.5V to the nearest rail. This is in alignment with your chart.

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  • \$\begingroup\$ Ah, Thank you, that clears it up perfectly, so it's just taking the ground as the bottom rail and 5v as top rail, giving it 1.5v to 3.5v. It also explains the second curve as that op-amp has +-2.4v @ VCC +-2.5V. Thanks again for your help. \$\endgroup\$ – Jdoe Apr 28 '17 at 18:48

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