I am new to LTspice and as an exercise I am building an inverting op amp. I have built this same circuit on a bread board and in every circuit and gotten the results I expected. However, in LTspice the the output voltage is always at ~1.5V no matter the resistance ratio and for most input voltages. Does anybody have an idea as to why this is? The green line is the input voltage before R1 and the blue line is the output voltage of the op amp.
Electronic – LTspice inverting op amp circuit does not function at all like expected
circuit-designinverting-amplifierltspiceoperational-amplifier
Related Solutions
If your negative power supply voltage is 0V, you can't put 2V in the inputs:
You are using +/-15V rails, the opamp will never go beyond the voltage rails, in fact the opamp you are using will never go to the full voltage rails. Lower the inputs or bump the opamp rails.
Best Answer
This is your inverting amplifier circuit: -
So, as with any inverting amplifier, if you feed the input with a positive voltage, then the output has to become negative with respect to the non-inverting input voltage in order to maintain a virtual earth at the inverting input pin.
Given that your op-amp negative supply pin is 0 volts and your non-inverting input pin is also 0 volts, the op-amp cannot fulfill this requirement. But the op-amp does what it can and, it lowers its output voltage as far as it can go. That ideally would also mean ground but, the LF411 isn't a rail-to-rail op-amp and can only lower its output pin to a couple of volts above the negative supply pin (0 volts).
Try setting input voltage V1 to a negative value like -0.1 volts. If you do that, the LF411's output should then be +1.0 volts i.e. ten times higher but inverted. With -0.2 volts at the input, the output should be about +2.0 volts.
However, another problem with the LK411 is that the minimum supply rail for the device is recommended to be 7 volts (0 volts and 7 volts or, -3.5 volts and +3.5 volts as an example). You are operating it with only 5 volts and your spice model may throw up some oddball behaviour because of this.
As with any op-amp application, virtual or real, read the data sheet. The LF411 is not suited to low power rail voltage applications and it is not a rail-to-rail device by any stretch of the imagination.