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What are opamps?

Opamp/Comparator Circuit Configurations

Opamp Circuit Configuration

The internal circuit configuration of a standard opamp is shown below.
Generally, opamps are broken up into 3 stages : Input, Gain, and Output.
The input stage includes a differential amplifier that amplifies the differential voltage beween the 2 input pins, while the common-mode signal component (same voltage input to both pins with no potential difference between the two) works to counteract without amplification.
Since the gain using only the differential amplifier circuit is insufficient, the open gain of the opamp increases at the gain stage.Typically, a phase compensation capacitance is connected between the gain stage to prevent internal oscillation.
The output stage is connected as a buffer to prevent variations in opamp characteristics based on load effects (i.e. resistance connected to the output pin).
Load-based output characteristics variations (distortion, voltage drop) largely depend on circuit configuration and current capacity.
Several different types of output stages exist, classified by the amount of drive current that flows to the output circuit (different bias voltages) : Class A, Class B, Class C, and Class AB.
Typically, the different types are sorted according to the least amount of distortion: Class A, Class AB, Class B, Class C, etc.

Internal Circuit Configuration of a Standard Opamp BA4558Internal Equivalent Circuit
Comparator Circuit Configuration

The circuit configuration of a standard comparator is shown below.
The circuit configuration is the same as an opamp, but because comparators are not used to configure negative feedback, phase compensation capacitance for preventing oscillation is not built in.
Since the phase compensation capacitance limits the operating speed between the inputs and output, response time is significantly faster than opamps.
The output circuit configuration of comparators can be broken down into two types: open collector (open drain) and push-pull.
The equivalent circuit of the BA10393 is shown below. The output circuit is an open collector type.

Standard Comparator Internal Circuit Configuration Internal Equivalent Circuit of the BA10393
Opamp/Comparator BasicsApplication Notes