Negative Feedback in Op amp and Closed Loop Gain of Op amp
Negative Feedback in Op ampNegative feedback in an op amp circuit is obtained by connecting output terminal of an op amp to its inverting input terminal through a suitable resistance as shown below. The gain of an op amp with negative feedback is called closed loop gain.
Closed Loop Gain of Op ampTo understand the closed loop gain of an 741 op amp, let us go through an example. The open loop gain, input resistance and output resistance of a typical 741 operational amplifier are, 2×105, 2MΩ and 5Ω respectively. Let us find the closed loop gain of the op amp when a 20kΩ resistance is connected in its feedback path as shown.
The equivalent circuit of the op amp with input source will be as shown below,
Let us assume, the voltage at node 1 is v. Now applying Kirchhoff current law at this node. we get,
Now by applying Kirchhoff current law at node 2 get,
Now, from the figure it is found that,
From equation (i) and (ii) we get,
So, the open loop gain of the op amp is, 2×105
whereas closed loop gain comes to only 2.
Let us take another example of closed loop gain of an op amp. The equivalent circuit of above 741 op amp circuit can be redrawn as, Now, consider voltage at node 1 is v applying Kirchhoffs current law at node 1. We get, Now, applying Kirchhoffs current law at node of the equivalent circuit, We get, Now, From the equation (iii) and (iv), we get, So, the closed loop gain of the above op amp circuit is 8.
So, it is cleared that by choosing suitable resistor for feedback in an op amp circuit, we can impose its desired gain.