Here is my task:
How to solve this? I'm confused because of only one input in opamp.
Here is solution:
But I'm still confused why is there only one input to opamp
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Compare a 3rd order BJT sallen key filter with its op-amp equivalent: -
Sorry about it being a 3rd order filter but the principle is just the same. The BJT has a voltage gain of about 1 from base to emitter so treat the transistor as a gain block. If the gain increases, to keep the same type of response C2 has to lower and the other values have to increase to keep the frequency (and Q factor) the same.
It doesn't matter what is inside the gain block at all.
However, for this circuit to oscillate you have to ground the input and what you will find is that it oscillates at exactly the same frequency as a wien bridge oscillator with a gain of 3 when R1=R2 and C1=C2
Think it as like this schematic above. In the figure the opamp is demonstrated is like this. If you do the calculations you will see that you can put this instead of the gain block over there. However, it may would appeal the answer a little bit.