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update : when the two-transistor structure is replaced by a darlington model provided as spice model by the manufacturer, the gain dependence of stable waveforms disappeared. Now, without those gain setting resistors, everything is fine.

enter image description here

If the circuit is like that, when the opAmp is connected to out node with R13 and R14 gain mechanism, everything is fine. It simulates easily and plots nice waveforms.

However when I decrease the gain, by deleting all gain mechanism(making direct connection to out node from inverting input) or by just setting R13 to 1k(to lower gain again); it really stucks while simulating(transient). The simulation speed is comperably slower than the first case and also the drawings of waveforms related are like these:

enter image description here

or a close lookup

enter image description here

So, it is not about the values of resistors because I have tried with 82 and 10 and it was nice. It is about the ratio of R13 to R14, which is the gain simply if I am not wrong.

Vcc is 7 volts and the signal at the non-inverting input is a sinusoidal wave 1Vpp at 1kHz.

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    \$\begingroup\$ Your amplifier is oscillating (LTspice slows its time-step to capture the detail). Adding extra gain to an opamp in a fed-back loop often invokes dreaded oscillations. \$\endgroup\$ – glen_geek Dec 10 '19 at 17:54
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    \$\begingroup\$ And changing the feedback to decrease the closed-loop gain is what is increasing the open-loop gain and causing the problem. You need to compensate for the frequency response of the transistorized output section; an appropriately-sized cap from the opamp output to its inverting input may do the trick, but you'll have to experiment, and it'll reduce your amplifier bandwidth. \$\endgroup\$ – TimWescott Dec 10 '19 at 18:03
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    \$\begingroup\$ Loop stability is a big subject and a different topic than LTSpice speed: do some reading first. allaboutcircuits.com/technical-articles/… \$\endgroup\$ – glen_geek Dec 10 '19 at 18:49
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    \$\begingroup\$ It's a separate question; you should ask it as such. As pointed out, it's a big subject (book length, to treat it correctly), and because your output stage has considerable zero-crossing distortion the subject gets even bigger. \$\endgroup\$ – TimWescott Dec 10 '19 at 19:20
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    \$\begingroup\$ @muyustan Don't know. I didn't analyze your circuit. But in real life there is likely to be at least \$0.001pF\$ on each node. \$\endgroup\$ – Aaron Dec 10 '19 at 21:34

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