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Next: Analysis of Op-Amp Circuits Up: Chapter 5: Operational Amplifiers Previous: Chapter 5: Operational Amplifiers

Operational Amplifier

The circuit schematic of the typical 741 op-amp is shown below:

opamp741b.gif

A component-level diagram of the common 741 op-amp. Dotted lines outline:

Like all op-amps, the circuit basically consists of three stages:

The op-amp requires two voltage supplies $\pm V_{cc}$ of both polarities (typically $V_{CC}=15$ V).

Although the op-amp circuit may look complicated, the analysis of its operation and behaviors can be simplfied based on the following assumptions:

Based on these approximations, an op-amp can be modeled in terms of the following three parameters:

OpAmp0.gif

Also, as the output $v_{out}=A(v^+-v^-)$ is in the range between $-V_{CC}$ and $V_{CC}$ and $A>10^5$ is large, $v^+-v^-=v_{out}/A$ is small (in the micro-volt range), i.e., $v^-\approx v^+$. If, as in some op-amp circuits, $v^+=0$ is grounded, then $v^-\approx v^+=0$ is very close to zero, i.e., it is almost the same as ground, or virtual ground. The analysis of various op-amp circuits can be much simplified by this virtual ground assumption.

As $A$ is large, $V_{out}=A(v^+-v^-)$ is usually saturated, equal to either $V_{CC}$ or $-V_{CC}$ (called the ``rails''), depending on whether or not $v^+$ is greater than $v^-$. For $v_{out}$ to be meaningful, some kind of negative feedback is needed. In the following, we consider some typical op-amp circuits to show how to analyze an Op-amp circuit to find its input resistance $R_{in}$, output resistance $R_{out}$, and open-circuit voltage gain $G_{oc}$.


next up previous
Next: Analysis of Op-Amp Circuits Up: Chapter 5: Operational Amplifiers Previous: Chapter 5: Operational Amplifiers
Ruye Wang 2019-07-07