Engineers currently have access to thousands of different op-amp ICs. It is somewhat misleading to identify all of these devices using the term “operational amplifier,” because in reality they form a diverse group of components. On the other hand, op-amps consistently exhibit various fundamental characteristics, and in this way they represent a fairly unified component category.
Before we begin exploring the defining electrical characteristics of op-amps, we need to understand why these components are so popular and effective.
The simplified, idealized op-amp is a three-terminal device.
The two terminals on the left are inputs, and the terminal on the right is the output. Note that the input terminals have different labels: the plus sign indicates the non-inverting input terminal, and the minus sign indicates the inverting input terminal.
A real op-amp needs at least five terminals—two inputs, one output, and two power-supply connections:
We often omit the power-supply terminals when we draw an op-amp because we assume that the device is connected to supply voltages that enable proper operation within the context of a given application. However, it’s important to remember that the op-amp’s output-voltage range is limited by its supply voltages.
The idealized input-to-output relationship of a typical op-amp is conveyed by the following diagram:
Despite the complex circuitry that is present within a real operational amplifier, we can successfully carry out many op-amp-based design tasks by assuming that the op-amp is a voltage-controlled voltage source (VCVS). The controlling voltage is \[(V_{IN+} - V_{IN-})\], and the factor of proportionality between the controlling voltage and the voltage generated by the VCVS is the op-amp’s gain, denoted by A:
\[V_{OUT} = A(V_{IN+}-V_{IN-})\]
Op-amps have very high gain, often above \[10^5\] or even \[10^6\] . As we’ll see in a future video, this high (ideally infinite) gain is extremely important—not because we frequently need to increase the amplitude of a signal by five or six orders of magnitude, but because an amplifier that combines high gain with a differential input stage provides a convenient means of exploiting the beneficial properties associated with negative feedback.
Let’s look at a few additional characteristics that are implied by the VCVS model shown above.
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The video shows:
“ Pro-Tip
Op-amp gain (A) is typically 10^5 to 10^6 “
Nonsense and misleading. Instead:
Op-amp gain (A) is typically A(s)=G/s: (1/s)*2*pi*10^5 to (1/s)*2*pi*10^6.
Where G is the Gain-Bandwidth Product in rad/sec. A IS FREQUENCY DEPENDENT!
The model for the nearly ubiquitous “internally compensated” (for 60 years - since type 741) op-amp types is that of an INTEGRATOR. [Often A=infinity is used for an “ideal” op-amp.] Finite A (a large constant) is never used