Amplifier are devices that produce 1 input signals. The 2 is the amplification ratio of the output signal to its input signals. Operational amplifiers are often called 3 , they usually have 4 inputs. An op-amp has 5 input impedance and 6 output impedance. An ideal op-amp has 7 input impedance and 8 output impedance. In an ideal op-amp, we could assume that no 9 flows into the input. Op-amps are often used with 10 feedback.