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After applying square wave as input to integrator circuit,I did not got a triangular wave as output.It was like capacitor's charging and discharging curve.What is the reason of not getting a pure triangular wave. The feedback capacitor also has a feedback resistor in parallel.

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    \$\begingroup\$ Please add a more information, such as schematic diagram. As is, there is to little information to diagnose the problem. \$\endgroup\$ – Greg Apr 22 '14 at 16:38
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If there is a resistor in parallel to the feedback capacitor you didn´t realize an integrator but a first order active low pass. Hence, the output cannot show a triangular shape. The problem with such an integrator circuit is that - as a stand-alone circuit - it cannot be operated because of missing dc feedback. Thus, for testing the circuit you need such a parallel resistor or you must carefully perform an offset compensation. But note that for many applications the integrator is used as an element within a larger loop with dc negative feedback and, thus, no parallel resistor is required.

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Probably because the integrator had a feedback resistor to set the dc operating conditions of the op-amp: -

enter image description here

Without the resistor or some other feedback mechanism, the DC conditions of the op-amp are unstable. Note how large R2 is and you should aim to make R2 as large as possible but stuff conspires against you. For instance, the input offset currents of the device may be 10nA and if you are AC coupling the input, it will produce an error voltage on the output that is \$I_{OFFSET} \times R_2\$ = 0.1V.

Alternatively, you used an imperfect integrator based on a resistor and capacitor.

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