Hi, I'm studying the opamp based oscillator which is used in a proximity switch. I find an LC oscillator which has an LC tank and an inverting amplifier. I don't how the positive feedback happens since LC tank is only connected to the non-inverting input of the opamp. Can anyone help me analyze this circuit and explain the operation? In addition, I have built this circuit in MULTISIM to do simulation. The opamp I used is TL083 which is a general purpose opamp. When I run the simulation, I found the circuit does oscillates, the frequency is LC resonant freq, but the opamp saturates, the output is like a square wave. No matter how I changed the resistors, the circuit never outputs a sine wave, how can this problem be solved? Thanks very much!!

 

... I don't how the positive feedback happens since LC tank is only connected to the non-inverting input of the opamp...

The output being connected to the tank and to the non-inverting input IS the positive feedback. Try moving the 20k resistor (R2) so it is between the + input and the tank. The tank is inputting directly into the op-amp in the current configuration.