Representation of feedback configurations

An op-amp that uses feedback is called a feedback amplifier. A feed back amplifier is sometimes referred to as a closed loop amplifier because the feedback forms a closed loop between the input and the output. A feedback amplifier essentially consists of two parts: an op-amp and a feedback circuit. The feedback circuit can take any form whatsoever, depending on the intended application of the amplifier. This means that the feedback circuit may be made up of either passive components or combination of both.
A closed loop amplifier can be represented by using two blocks, one for an op-amp and another for a feedback circuit. These are the four ways to connect these two blocks. These connections are classified according to whether the voltage or current is fed back to the input in series or in parallel, as follows:

1) Voltage series feedback
2) Voltage shunt feedback
3) Current series feedback
4) Current shunt feedback

In all these configurations, the the signal direction from the op-amp is from the input to the output. On the other hand, in the ideal case the signal direction through the feedback circuit is exactly opposite: from output to input.
The voltage series and voltage shunt feedback configurations are important because they are most commonly used.


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