I have a simple non-inverting follower circuit that I'm simulating in LTspice, but the simulation shows the output is 1.6 volts when it should be 1 volt. Any thoughts on why this may be? Here is a picture of the simulation. enter image description here

Here is a description of the circuit:

  • 2V in
  • Divide in half to 1V
  • Output of the op am is 1.6V ??? which does not make sense to me. This should be 1 V

Also I tried with several other op amps with similar results.

  • \$\begingroup\$ Did you consider the input common-mode range of the LT1357 which you are simulating? \$\endgroup\$ Jan 30 '17 at 17:10
  • 1
    \$\begingroup\$ Not all opamp can go that close to the ground or rail. Pick a rail to rail opamp if you do want to be that close to thee rail. Or ground. \$\endgroup\$
    – dannyf
    Jan 30 '17 at 17:15

The LT1357 datasheet indicates that it is not a rail-to-rail op amp at either the input or the output (this is the case for most op amps, which is why you saw similar results with other op amps you simulated). It requires several volts difference between the rails (power supply voltages) at both the input and output:

enter image description here

enter image description here

However, you are using a single supply so you are expecting its input and output to be 1V above the lower supply voltage (ground, in this case).

Possible solutions:

  • Use a negative supply. For example, you could use a -10V supply to balance the +10V supply, or you can use a -5V supply to give the op amp just enough extra headroom on the negative side.
  • Find an op amp that can accept inputs and swing to within 1V of its negative supply. The key specs to look for are "input voltage range", "input common mode range", "output swing", or similar. Look for op amps marketed as "single supply" and/or "rail-to-rail".

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