Operational Amplifier with Feedback


The open loop configuration of Op-Amp limits the application of its use. To extend the application feedback is introduced in the open loop configuration of an Op-Amp. Based on the type and nature of the feedback, an Op-Amp can be used as Summer, Oscillator, Amplifier, Integrator, Differentiator, Multi-Vibrator etc. Here we review the basics of the feedback which will form the basis of op-amp application.

Drawbacks of Open Loop Op-Amp

Clipping of output signal at saturation.


High Distortion.


Only smaller signals of the order of microvolts can be Amplified.


High gain & hence only low-frequency signals can be Amplified.


This limits the frequency of use in which the op-amp can be used.


The voltage gain of open loop Op-Amp varies with

Temperature


Power supply variation


Manufacturing process


The manufacturing process creates a mismatch between the BJT’s. No two BJT’s are same, hence when multiple stages of the differential amplifier are cascaded, the effect of mismatching amplifies. Hence two open loop op-amps made up of the same type of BJT’s can give different gain.

What is Feedback

The signal fed from output to input is feedback.

There are two types of feedback:

1. Positive

If the feedback signal is in phase with the input signal the feedback is said to be positive.


Positive feedback is also called as Regenerative Feedback.


Positive feedback is used in Astable Multivibrators, Schmitt Triggers, Oscillators etc


2. Negative

If the feedback signal is out of phase with input signal i.e. phase difference is 180° between the input and feedback, then the feedback is said to be Negative Feedback.


Negative feedback is also called Degenerative Feedback


Negative feedback is used in Adders, Subtractors, Integrators, Differentiators etc.


Positive Feedback in Op-Amp


Output is fed-back to Non-Inverting input of the op-amp.


A basic positive feedback op-amp circuit is shown.






Since the inverting input is grounded, the output voltage depends on the voltage applied at the non-inverting terminal


Let us apply a positive voltage at the non-inverting terminal




Now since input voltage V is positive


↓


Output voltage is positive


↓


This positive output voltage is fed-back into the non-inverting input as feedback


↓


This increases the difference voltage subsequently the output voltage also increases.


↓


Finally, the op-amp is driven into saturation


↓


If input voltage is positive, op-amp goes to positive saturation


If input voltage is negative, op-amp goes into negative saturation.




Negative Feedback in Op-Amp


Output is fed-back to Inverting input of the op-amp


A basic negative feedback op-amp circuit is shown




Output closely follows the input


The above circuit is also called a Voltage Follower.


As the input voltage increases, output voltage increases


↓


This output voltage is fed back to non-inverting (V– ) terminal of the op-amp.


↓


Here the difference voltage which is (V+ -V–) does not increase


↓


Op-amp does not move into saturation, it remains in the linear region


A negative feedback system is a self-stabilizing system


Negative feedback system operates in linear mode.


 Closed Loop Gain of Negative Feedback System


Consider an amplifier circuit with open loop gain A and feedback factor β


Let us find the closed loop gain of this op-amp when it is connected to negative feedback










1 + Aβ is known as Return Ratio


Op-Amp with Feedback Study Notes – Download PDF

For better understanding let us now go through some Practice Questions.


Effect of Negative Feedback


Stabilizes the Gain


Input Resistance Increase


Output Resistance Decreases


Gain Decreases


Bandwidth Increases


Noise Decreases


Distortion Decreases


Output Offset Voltage decreases


Ques.2. Which of the following is the drawback of negative feedback?

Input Resistance Increase


Output Resistance Decreases


Gain Decreases


All of the above


 Ans: 3

Solution:

The increase in the input impedance and decrease in the output impedance is advantage of negative feedback.

The only drawback of the negative feedback is reduction in gain which may require additional stages of amplifier to be added.

Hope you liked this article on Op-Amp with Feedback. If you have any queries regarding this article do mention in the comments section.

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