Ideal Op Amp
Ideal OP AmpWe know that, the input resistance of an op amp must be very high where as the output resistance should be quite low. An op amp should also have very high open loop gain. In ideal cases, the input resistance and open loop gain of an op amp should be infinity whereas the output resistance would be zero. So an ideal op amp should have following characteristics.
|1||Open Loop Gain (A)||∝|
So, an ideal op amp is defined as, an amplifier with infinite open loop gain, infinite input resistance and zero output resistance. The ideal op amp has zero input current. This is because of infinite input resistance. As the input resistance of ideal op amp is infinite, an open circuit exists at input, hence currents at both input terminals are zero.
The voltage between the input terminals is zero. This is also because infinite resistance between inverting and non-inverting terminals of the ideal op amp. As ideally there is no current through the input resistance, there will be no voltage drop, hence the voltage applied to one input terminal will appear to other. If v1 and v2 are the voltages of inverting and non-inverting terminals of op amp, then in ideal case,
This (v2 - v1) is differential input voltage. Hence, for ideal op amp, For better understanding, let us go through one example of ideal op amp.
Let us find the closed loop gain of this op amp circuit, assuming the op amp is ideal. In ideal op amp differential voltage vd=0. Hence voltage at node 2 and 3 will be same and it is zero . Hence, voltage at node 1 is also zero i.e. v=0. Now, applying Kirchhoff current law at node 1, we get,