Op-Amps: Why a high input resistance and low output resistance is required

A very high input resistance is required so that the input becomes an open circuit. A very high input resistance will imply an open circuit. As shown in the diagram below, for an open circuit, the current through Vin will be zero.

From KVL,
-Vs+VRi +Vin=0
-Vs+iRs+Vin=0
Since i = 0,
-Vs+Vin = 0
Therefore, Vs=Vin

If it is an open circuit, the exact source voltage, Vs, gets transferred to the input as Vin. This means that the Ri resistor almost did nothing. Therefore we need a very hight input resistance, Ri, to achieve a zero current, i, so that it becomes an open circuit and all the source voltage will be the input voltage.

This is also true for a voltmeter. A voltmeter should have a very high resistance so that little or no current is able to flow meaning that all the source voltage of the instrument being measured will appear at the input.


A very low output resistance is required so that all the current will be consumed by the load. A very low output resistance is required for an op-amp so that if a load RL is placed in series with it, all the current will be consumed by the load. See the figure below.

From the figure above, this is a short circuit so Vin=0. This implies that all the source current will be eaten up by the load