September 2022
Introduction to Op Amps
Table of Contents
04
CHAPTER - 1
Introduction
04
CHAPTER - 2
What is an Op Amp?
05
CHAPTER - 3
Ideal Op Amp Model
06
CHAPTER - 4
Example Op Amp Use Cases
09
CHAPTER - 5
Op Amp Characteristics
10
CHAPTER - 6
Conclusion
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Introduction to Op Amps
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Operational amplifiers are versatile components that can be used in a wide variety of applications,
including amplifiers, filters, and buffers. In this ebook, we will discuss what’s inside an op amp, how they
work, design considerations, example applications, and more.
element14 Community Team
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CHAPTER - 1
Introduction
The operational amplifier (op amp) is an integrated circuit in electronics that finds use in a wide range of
devices. With only a handful of external components, they can perform a wide variety of analog signal
processing tasks, such as signal conditioning and filtering. The learning module covers the fundamentals of
op amps. Overall, by the end of this tutorial, the reader should have an understanding of op amps and how
they operate, as well as where they fit into the larger world of electronics. This tutorial will cover the topics
listed below:
•
What is an Op Amp?
•
Ideal Op Amp Model
•
Example Op Amp Use Cases
•
Op Amp Characteristics
CHAPTER - 2
What is an Op Amp?
An op amp is a DC-coupled, differential amplifier
with a single output that comes in various forms,
shapes, and sizes. Packages can include larger
through-hole models, metal cans, and surface mount
devices. Additionally, they can include one device,
two devices, or four or more devices in a single
package. Typically, an op amp has two inputs, one
output, and two terminals for a positive and negative
supply. Some op amps feature additional input
pins that can be used to tune out the non-idealities
that may exist in the device. Furthermore, op amps
feature very large amounts of gain, so high that in
most applications, negative feedback is employed to
determine the circuit’s performance.
Figure 1 shows the common schematic symbol used
for op amps. The negative terminal is often called
the inverting input while the positive terminal is often
called the noninverting input. The general concept
behind the operation of the op amp is simple: the
voltages at the input are compared and the output
goes positive when the noninverting input is larger
than the inverting input. On the other hand, when the
inverting input is larger than the noninverting input, the
output goes negative. In summary, the voltage supplies
determine the output voltage limits.
Figure 1: General op amp schematic symbol
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Figure 2: Block diagram of an op amp
As illustrated in Figure 2, a typical op amp has four functional stages that include an input differential
amplifier, an intermediate gain stage, a level translator circuit, and an output stage. Most of the voltage gain
takes place at the input differential amplifier. The intermediate stage is typically another differential amplifier.
The next stage shifts the DC level to 0V with respect to ground. The output stage is a final gain stage with
low output resistance, that is capable of supplying the load current.
CHAPTER - 3
Ideal Op Amp Model
The previous section presented a high-level description of an op amp. What makes an op amp special and
highly versatile is its large amounts of open loop gain. Gain tells us what factor the input voltage is amplified
by; for example, with a gain of 10, the output voltage will be ten times greater than the input voltage. The
gain is so high that when used with feedback, the performance of the circuit is almost entirely determined
by the feedback network. A typical voltage gain for an op amp is approximately 106. Although an op amp
can be used in an open loop configuration, there are limited applications where this is beneficial. Most
applications utilize feedback, which leads to two golden rules when analyzing op amp circuits:
1. The voltage difference between the inputs is zero.
2. The inputs draw no current.
These two rules also make up the ideal op amp model. The first rule is an assumption based on the large
gain and feedback that greatly simplifies circuit analysis. The second rule is also an assumption used for
convenience. In reality, an op amp input may draw some small amount of current that might be in the range
of picoamps. To appreciate the simplicity of analysis these two rules provide us, we will look at the example
of a simple inverting amplifier:
Figure 3: Inverting amplifier configuration
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From the second rule, we know that no current flows
into either terminal. In addition, from the first rule,
we know that the inputs of the noninverting and
inverting terminals are equal. Thus, the inverting
node is now a virtual ground. We can also write a
Kirchoff’s Current Law (KCL) equation at this node,
as shown in Figure 3. Solving for the voltage gain
gives us the gain (-R2/R1) for the inverting amplifier.
CHAPTER - 4
The ideal op amp model provides us with a
convenient method for analyzing circuits. As can
be seen, it allows us to use Kirchhoff’s laws just as
we would with other linear circuits. In addition, it
proves to be very accurate for most situations. Most
amplifiers and filters using op amps are analyzed
using the two ideal op amp rules.
Example Op Amp Use Cases
Op amps find use in a wide variety of applications. Two of the most common uses of op amps are for
amplification and filtering. We have already visited the inverting op amp configuration. An additional
configuration for amplification is the noninverting op amp. A schematic completed in LTSpice for the
noninverting op amp is shown in Figure 4.
Figure 4: Noninverting op amp schematic
As can be seen, the input node is connected to the noninverting input. In addition, the output of the op amp
is connected to a voltage divider that feeds back to the inverting input. Using the ideal op amp rules and
some circuit analysis, we can find that the voltage gain is determined to be: G = 1+(R1/R2). With the 18k
resistor and 2k resistor in this configuration the gain comes out to be 10. Running the simulation in LTSpice,
we find that the result matches our expectations.
Figure 5: Plot of simulation run in LTSpice
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From the simulation plot, we can see the input waveform (red) is of an amplitude of 0.5V. The output
waveform (green) has a magnitude of 5V. Furthermore, we can see that the input and output waveforms are
both in phase. Hence, the noninverting characteristic of the configuration.
As for filtering, there are a variety of configurations available; however, one of the most commonly used
filters, due to its simplicity, is the Sallen-Key filter. The Sallen-Key filter is a circuit topology for low pass
and high pass filters. Many resources and calculators exist to help an engineer design a filter using the
Sallen-Key circuit. An example schematic of a low pass filter is shown in Figure 6, along with the frequency
response of the circuit.
Figure 6: Sallen-Key circuit and frequency response
As can be seen, the filter has a frequency response with a cutoff frequency around 12 kHz. This means any
frequency passing through the circuit higher than 12 kHz will be attenuated while frequencies below 12 kHz
can pass with no loss of signal level. Also note that by adding a resistive divider into the negative feedback
path, we can easily add gain to the circuit similar to what was shown with the noninverting amplifier. Low
pass, high pass, bandpass, and notch filters can be created using op amps.
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Another popular application for op amps (and arguably the simplest) is for buffering outputs. Many times, a
circuit such as a digital-to-analog converter (DAC) or IC can only source a very limited amount of current.
As a result, a buffer is used to isolate the IC from the load. The buffer can also provide a larger amount
of output power, providing the capability to drive loads of varying impedances. An example of an op amp
buffer is shown in Figure 7. Again, LTSpice was used to simulate the input and output of the buffer. Note
that the plot displays both the input and output waveforms; however, they overlap with one another because
the input of an ideal buffer is the same as the output.
Figure 7: Op amp buffer simulated in LTSpice
Although there are countless applications of op amps, the last one we will consider here is the summing
amplifier. This circuit can take in multiple inputs and sum them all into a single output. This provides us with
a means to apply mathematical functions to circuit design. Shown in the schematic below is an example of a
summing simplifier with two input signals. More than two signals can be added but, to simplify the example,
a 1 kHz signal is summed with a 100 Hz signal. This helps us clearly see the two signals in the output
waveform (red). Both inputs have an amplitude of 0.5V, as shown in the green and blue waveforms. Notice
that this configuration can also provide gain if desired.
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Figure 8: Summing amplifier schematic and plot in LTSpice
When working with op amp circuits, LTSpice provides a great way to simulate and prove out your circuit
before prototyping or building. Note that the op amp model used in the previous examples is an ideal model
that is already provided in LTSpice.
CHAPTER - 5
Op Amp Characteristics
Op amps have certain non-ideal characteristics that are important to understand. Some of these include
slew rate, input and output currents, input common mode range, and offset voltages, to name a few. These
characteristics can have an impact on the circuit topology, performance, and the components required.
For example, op amps do not have perfectly balanced inputs. Input-offset voltage is required at the input
terminals to obtain a zero volt output. This is due to an inherent mismatch that exists within the part. As an
example, the TL071 from STMicroelectronics, a commonly used op amp in audio applications, has a
specification that lists the input offset voltage as typically 3mV, with a worst case offset voltage of 10mV.
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However, the TL071 (and many others) offers pins for
nulling out the offset voltage using a potentiometer
or resistor. Furthermore, “precision” op amps exist
and have very small input offset voltages (on the
order of microvolts).
The input common mode range of an op amp is
important to consider. This is required for the op
amp to operate as expected. The input range of
most op amps have a limit that is less than the
input supply applied to them. The input common
mode range is expressed relative to the positive and
negative supply. Using the TL071 as an example,
the nominal supply voltage is +/-15V; however, the
typical input common mode range is -12V to +15V.
There are certain op amps that can operate with
an input very close to the supply voltage; these are
known as rail-to-rail input op amps.
Input and output currents are also of concern when
choosing an op amp for a given application. For
instance, if you need to connect to a load that
requires a lot of current, it is important to choose an
op amp that can supply the needed current. As for
the input current, this is a non-ideal characteristic.
As discussed previously, an ideal op amp draws
no current at its input. In reality, all op amps draw
some amount of current at their inputs that can be
anywhere from picoamps to nanoamps. BJT inputs
will draw slightly more input current than their FET
CHAPTER - 6
counterparts; however, FET op amps will generally
have a larger input offset voltage.
An additional parameter all op amps characterize is
the slew rate. To keep the op amp unconditionally
stable, some form of compensation exists internal to
the op amp. This is usually a capacitor that creates
a low pass pole in the op amp. As a result of this
capacitance, an op amp has a limitation on how
fast its output can change in response to its input.
The slew rate is generally specified in the unity gain
configuration and in units of V/µSec. Generally,
violating the slew rate specification will introduce
distortion into the output signal.
The last parameter worth mentioning here is the
bandwidth. All op amps have a finite bandwidth.
This is also limited, to some extent, by the internal
compensation circuitry. As a result, the higher the
operational frequency of the circuit, the lower the
open loop gain may be. This can have a drastic
effect on the performance, as well as the ideal
assumption that both inputs of the op amp are
always equal. The bandwidth of a typical op amp
is approximately 1 MHz.
Conclusion
We have covered an overview of the op amp and described some common applications. Furthermore, the
ideal characteristics of an op amp have been discussed, along with many of the real world limitations of an
op amp. There are a plethora of op amps available on the market, each with its own unique advantage for
a given application. There are also general-purpose op amps that fit the needs of many use cases. Overall,
the op amp is a very important component in the world of electronics. They find use in a wide variety of
applications and are an important tool for every designer to have in their toolbox.
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