LTspice filter giving same output voltage from every input voltage and frequency?
Yes, you have build an oscillator. In general, you can enhance the selectivity (pole-Q) using positive feedback. This is a known modification as proposed by Deliyannis, long time ago. However, the pole-Q (selectivity) as well as the midband gain are both very sensitive to the ratio R4/R5. So you must be very careful during designing such a topology.
Working principle of &quot;your oscillator&quot;: We have a fixed positive feedback and, at the same time, negative feedback which is very strong for (a) very small frequencies (resistive) and (b) for very large frequencies (capacitive). In between, there is a frequency which cannot override the positive feedback and which - at the same time - has zero phase shift. Hence, we have oscillation.
By the way: A &quot;good&quot; ac response (magnitude for small-signal analysis) is NEVER an indicator for stability. Only a TRAN analysis (time domain) can reveal stability properties (or a detailed loop gain analysis, applying a stability criterion)