Characteristics of an ideal operational amplifier

¥ Infinite voltage gain.

¥ Infinite input impedance so that any signal source can drive it.and it can avoid loading effect also.

¥ Zero output resistance so that the output can drive infinite number of other devices.

¥ Zero output voltage when input voltage is zero.

¥ Infinite bandwidth so that any frequency signal can be amplified without attenuation.

¥ Infinite slew rate. So that output voltage changes occurs simultaneously with the changes in input Voltage.

¥ Infinite CMRR then there will be no noises in the output.

Why does an operational amplifier have high input impedance and low output impedance

   Operational amplifiers amplify the input signal they got.In order to receive the input voltage signal that should be dropped across the operational amplifier.
   As we know by Ohms law voltage drop V=IR .If resistance is more voltage drop across that device also more.Hence an operational amplifier should have high input impedance in order to drop the input voltage signal across it.
   If operational amplifier has low input impedance the input voltage signal does not drop across it and operational amplifier may not receive the signal.
  Similarity an operational amplifier should have very low output impedance so that the amplified voltage signal drop across the device which is connected to the output terminals of operational amplifier and not in the operational amplifier itself .
If the operational amplifier has high output impedance then the amplified voltage will drop across the operational amplifier itself and it will not transferred to the device which connected to the output terminals of operational amplifier.That's why operational amplifier needs low output impedance .
  Another reason for high input impedance is because of loading effect .If operational amplifier has low input impedance then heavy current will drown from the supply it may Leeds to the burning of the operational amplifier .So by using high input impedance we can avoid loading effect of operational amplifier .