Op-Amp Open Loop Gain

The high open loop gain leads to the voltage rule.

Practically, the gain is so high that the output will be driven to for any appreciable difference between This is useful in the comparator. For practical applications other than the comparator, negative feedback is used to control the device gain.

Negative Feedback

The use of negative feedback reduces the gain, but most other rationales favor its use. Part of the output signal is taken back to the input with a negative sign.

For an amplifer with negative feedback thegain becomes

The large open loop gain of an op-amp makesit possible to build amplifiers whose gain isdetermined by the feedback B and not by the gain of the amplifying element itself!

Rationale for Negative Feedback

Why would you feed back a negative signal from the output which cancels part of the input, reducing the gain?

Because:

• It helps to overcome distortion and nonlinearity.
• It flattens frequency response or allows you to tailor it to a desired frequency response curve.
• It makes properties predictable, less dependent on temperature, manufacturing differences or other internal properties of the active device.
• Circuit properties are dependent upon the external feedback network and are thus easily controlled by external circuit elements.
• Circuit design can concentrate on function and not the details of operating point selection, biasing, and the other details characteristic of discrete transistor amplifier design.