when i am applying an ac signal on both the differential pair, the output is 90 degree out of phase and when i am giving an ac signal at non inverting terminal and grounding the other, the output is normal. Can you help me what is the reason for this? and how can i correct this?

 

You have not provided enough information for anyone to offer educated advice.
What differential pair? Where is the circuit diagram?
What is your ac signal frequency?
What are the reactive components in the circuit?

 

You have not provided enough information for anyone to offer educated advice.
What differential pair? Where is the circuit diagram?
What is your ac signal frequency?
What are the reactive components in the circuit?

the cinout voltage is 1micro volts and the frequency is 100KHz and i have designed a unipolar cmos op amp.

 

You need to answer MrChips' questions, particularly the schematic, before we can give you answers that aren't total guesses.

 

You would be able to find some answers on your own if you do some basic investigation and analysis.

Is the 90° phase shift at the output leading or lagging?
Is the phase shift the same over your entire frequency range of interest?

A 90° phase shift will come from having reactive components in your circuit.
What and where are your reactive components?
Is your signal in or out capacitively coupled? What is the reactance at 100kHz? What is the reactance at a lower or higher frequency?
Have you taken into account any input or output impedances?

 

How do you know the phase shift is 90 degrees? Is is exactly 90 degrees, or only approximately? Please post your schematic and scope photos.

ak

 

How do you know the phase shift is 90 degrees? Is is exactly 90 degrees, or only approximately? Please post your schematic and scope photos.

ak

Good question. Maybe he means 180° phase shift!