Operational Amplifiers part2
Input and Output Resistances Assuming an ideal op amp with infinite open-loop gain, the input resistance of the closed-loop inverting amplifier of Fig.5.5 is simply equal to . This can be seen from Fig. 5.6(b), where Since the amplifier input resistance forms a voltage divider with the resistance of the source that feeds the amplifiers, to avoid the loss of signal strength, voltage amplifiers are required to have high input resistance. In the case of the inverting op-amp configuration, to make high we should select a high value for .However, if the required gain is also high, then could become impractically large (e.g,, greater than a few mega ohms). We may conclude that the inverting configuration suffers from a low input resistance. A solution to this problem is discussed in Example 5.2 below. Since the output of the inverting configuration is taken at the terminals of the ideal voltage source (see Fig. 5.6a), it follows that the output resistance of the closed-loop amplifier