Use of Feedback in Op-Amps
Operational amplifier is a very versatile device, primarily due to its very high input impedance, low output impedance, and very high gain.
But, it cannot be used without modification as an amplifier because it is not very stable in the form the two main factors which contribute to this problem are:
1. Frequency response
2. Drift.
Stated in another way, op-amp gain K does not remain constant; it can vary with frequency of the input signal (i. e., frequency response function is not flat in the operating range); and, also it can vary with time (i.e., drift).
The frequency response problem arises due to circuit dynamics of an operational amplifier.
This problem is usually not severe unless the device is operated at very high frequencies. The drift problem arises due to the sensitivity of gain K to environmental factors such as temperature, light, humidity, and vibration, and also as a result of the variation of K due to aging.
Drift in an op-amp can be significant and steps should be taken to eliminate that problem.
It is virtually impossible to avoid the drift in gain and the frequency response error in an operational amplifier.
But an ingenious way has been found to remove the effect of these two problems at the amplifier output.
Since gain K is very large, by using feedback we can virtually eliminate its effect at the amplifier output.
This closed loop form of an op-amp has the advantage that the characteristics and the accuracy of the output of the overall circuit depends on the passive components (e.g., resistors and capacitors) in it, which can be provided at high precision, and not the parameters of the op amp itself.
The closed loop form is preferred in almost every application; in particular, voltage follower and charge amplifier are devices that use the properties of high Zi, low Zo, and high K of an op-amp along with feedback through a high-precision resistor, to eliminate errors due to non-constant K.
In summary operational amplifier is not very useful in its open-loop form, particularly because gain K is not steady. But since K is very large, the problem can be removed by using feedback. It is this closed loop form that is commonly used in practical applications of an op-amp.
In addition to the unsteady nature of gain, there are other sources of error that contribute to less-than-ideal performance of an operational amplifier circuit. Noteworthy are:
i. The offset current present at the input leads due to bias currents that are needed to operate the solid-state circuitry.
ii. The offset voltage that might be present at the output even when the input leads are open.
iii. The unequal gains corresponding to the two input leads (i.e., the inverting gain not equal to the no inverting Such problems can produce nonlinear behavior in op-amp circuits, and they can be reduced by proper circuitdesign and through the use of compensating circuit elements.
