An introduction to operational amplifiers
Engineers currently have access to thousands of different op amp ICs. Using the term “op amp” to identify all of these devices is a bit misleading, since in reality they form a distinct set of components. Op amps, on the other hand, exhibit various fundamental characteristics throughout, so they represent a fairly uniform class of components.
Why do we use operational amplifier?
Before we start exploring the defining electrical characteristics of op amps, we need to understand why these components are so popular and effective.
Op amps have been mass-produced for decades, so engineers have access to a wide variety of parts that offer both low cost and high performance.
Operational amplifiers are extremely versatile. It’s hard to think of an analog circuit that can neither be implemented using an op amp nor improved by adding an op amp.
Designing a circuit around an op amp is much easier than using discrete transistors. The electrical characteristics of op amps lead to simplified assumptions, and in many applications, these assumptions do not lead to significant differences between theoretical and actual circuits.
Operational amplifier circuit symbol
A simplified, idealized op amp is a three-terminal device.
The two terminals on the left are inputs and the terminals on the right are outputs. Note that the inputs have different labels: the plus sign indicates the non-inverting input, and the minus sign indicates the inverting input.
A true op amp requires at least five terminals—two inputs, one output, and two power connections:
The dual-supply op amp circuit (on the left) uses positive and negative supply voltages.
The negative supply terminal is grounded in a single supply arrangement (right side).
We normally remove the power supply connections when drawing op amps because we believe the device is linked to a supply voltage to function effectively in a given application. It’s vital to remember, too, that an op amp’s output voltage range is restricted by its supply voltage.
Operational Amplifier Electrical Model
The ideal input-output relationship of a typical op amp is in the following figure:
Although complex circuits exist in real op amps, we can successfully perform many op amp based design tasks by assuming that the op amp is a voltage controlled voltage source (VCVS). The control voltage is \[(V_{IN+} – V_{IN-})\], and the proportional factor between the control voltage and the voltage generated by VCVS is the gain of the op amp, represented by A: (Volt I N+-Volt I N-) (Volt I N+-Volt I N-), the scaling factor between the control voltage and the voltage produced by VCVS is the gain of the op amp, denoted by A:
Op-amps have very high gain, usually higher than \[10^5\] or even \[10^6\] . As we’ll see in future videos, this high (ideally infinite) gain is very important. Not because we often need to increase the amplitude of the signal by five or six orders of magnitude, but because the amplifier will be high Gain and differential input stages provide a convenient way to take advantage of the beneficial properties associated with negative feedback. Let’s look at some of the other features.
An op amp is a differential amplifier: it amplifies the difference between two input voltages.
According to the previous statement, op amps exhibit common mode rejection. The op amp will reject (ie ignore) any voltage components present in the two input signals, such as noise or DC offset.
An op amp has a differential input stage and a single-ended output; therefore, we can think of it as a differential-to-single-ended converter. However, it turns out that real-life op amp applications are more closely related to single-ended input signals. In fact, there is another name for a device: an instrumentation amplifier.
Second, the characteristics of the operational amplifier
One of the reasons the operational amplifier (op-amp) is so famous is because of its versatility. In this article, you’ll see that almost anything we can do with an op amp. It is also worth noting that this integrated circuit has many properties that are close to those considered ideal.
The integrated circuit has many properties that are close to ideal.
Ideal op amp
Because op amps have near-ideal characteristics, it is fairly easy to design and build circuits using IC op amps. It is also important that the operational amplifier circuit components operate at the predicted theoretical level. This article will cover analyzing the important nonideal properties of op amps.
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Practical considerations for op amps
Basic Amplifier Configuration
An operational amplifier has three terminals: two input terminals and one output terminal. The following diagram, Figure 1.1, illustrates the symbols used for the op amps discussed in this article. The two terminals 1 and 2 on the left side of the op amp are the two input terminals. And the terminal 3 on the right side is the output terminal. In order to operate the amplifier, we need to connect it to a DC power source. In general, most integrated circuit op amps do not need one, but two DC power supplies, as Figure 1.2 shows.
We connect two terminals 4 and 5 to the positive voltage source V cc and the negative voltage source V ee , respectively. Figure 1.2 (b) shows a DC power supply as a battery with a common ground source. The ground source is really just the common terminal of the two power supplies. Interestingly, this is because we did not physically connect one of the terminals on the op amp package to ground. In this article, for simplicity, we will not describe the op amp power supply.
Figure 1.1
Figure 1.2
In addition to the five terminals discussed so far, op amps may have other terminals for specific purposes. We may use these for frequency compensation and negative feedback or offset nulling to reduce small DC offsets that we can amplify.
Introduce the characteristics of an ideal op amp
The true function of the circuit inside the op amp is to determine the difference between the voltage signals applied directly to the two input terminals (difference of v2 – v1). Following the discovery of this quantity, the number A multiply it, yielding the voltage term A (v 2 -v 1). When we talk about voltage at a terminal, we’re talking about the voltage between that terminal and ground; for example, v 1 is the voltage applied between terminal 1 and ground.
An ideal op amp should not sink any current for the input; this means that both the current flowing into terminal 1 and the signal flowing into terminal 2 are zero. This means that the input impedance of an ideal op amp should be infinite.
Now focus on the output terminal, it should behave like the terminal of an ideal voltage source. In short, the voltage between terminal 3 and ground will always be equal to A (v 2 – v 1 ), regardless of the current that may or may not be drawn into the load impedance from the third terminal.
In summary, As Figure 1.3 shows, we can describe the model of the op amp. Looking at the model, you can see that the output terminal has the same sign as v 2 but the opposite sign from v 1 . With this in mind, we call this input the inverting input, a “-” sign denote it, while we call input 2 the non-inverting input, a “+” sign denote it.
As mentioned earlier, we designed op amps to detect differences between voltage signals and will ignore any given signal common to both inputs. This means that if v1 = v2 = 1 V, then the output will be correspondingly (ideally) zero. This phenomenon is common mode rejection. We can express this as zero common mode gain, or similarly, infinite common mode rejection. Now, we can say that the op amp is a differential input, single-ended output amplifier, which means that the output of the op amp is between ground and terminal 3.
Figure 1.3
The term A is differential gain. Another name we can associate with this term is open loop gain. We can achieve this gain when using no feedback in the IC op amp. Typically, open-loop gain tends to have extremely high values; an ideal op amp has virtually infinite open-loop gain.
A notable characteristic of an op amp is the DC amplifier or direct coupling, which stands for DC or DC as it amplifies signals with frequencies close to zero. Because op amps are far more adaptable, allowing us to use them in a wider range of applications. Direct coupling, on the other hand, can result in some major issues.
Moving to the bandwidth, the ideal op amp has a gain A that will remain constant to zero frequency, all the way to infinite frequency. In other words, an ideal amplifier can amplify signals of any frequency with the same gain, giving them infinite bandwidth. So far, we have discuss all properties and properties of an ideal op amp. Except for one: the gain A of an ideal op amp should have a large and infinite value, ideally. However, this brings up a good question: how can we use op amp for any application if there is an infinite gain? Here’s the answer: because we do not use op-amps alone in an open loop configuration in almost every conceivable application.
Summary
So far, we’ve discussed how op amps are so popular. Because of their versatility, and the characteristics and capabilities of an ideal op amp. In summary, the characteristics of an ideal op amp are as follows:
Ideal gain inside the op amp for unlimited bandwidth
Infinite open loop gain A
Zero or Infinite common mode gain
Infinite Input Impedance
The output impedance is zero
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