Op-Amp: Summing Amplifier (Inverting and Non-Inverting Summing Amplifiers)

Summary

This video from the All About Electronics YouTube channel explains how to use an operational amplifier (op-amp) as a summing amplifier. It covers both inverting and non-inverting configurations, demonstrating how to add multiple input voltages and scale them using resistors. The video also explores practical applications like averaging input voltages, providing DC offsets, and mixing audio signals. It highlights the isolation of voltage sources in inverting amplifiers and the complexity involved with increasing inputs in non-inverting configurations, concluding with a preference for inverting summing amplifiers in practical scenarios.

Takeaways

• 😀 The video discusses the use of an op-amp as a summing amplifier, explaining how to add different input voltages using this configuration.
• 🔬 It covers both inverting and non-inverting op-amp configurations, detailing how multiple inputs can be applied to achieve summing.
• 🎚️ In the inverting summing amplifier, multiple inputs are applied to the inverting terminal, and the output voltage is expressed in terms of these inputs.
• ⚖️ The video assumes ideal op-amp conditions, where no current flows into the op-amp, and uses the concept of virtual ground to derive equations.
• 🔗 Kirchhoff's Current Law (KCL) is applied at node X to derive the relationship between input voltages and output voltage.
• 🔢 The output voltage for the inverting configuration is given by a formula that involves the feedback resistor and the input resistors.
• 📉 If all input resistors have the same value, the output voltage is the negative sum of the input voltages multiplied by the feedback resistor ratio.
• 🔄 The inverting summing amplifier can also perform scaling and averaging operations, adjusting the output based on the resistor ratios.
• 🎧 Practical applications of the inverting summing amplifier include adding, averaging, and scaling input voltages, as well as providing DC offsets and mixing audio signals.
• ⚠️ The number of inputs that can be practically added is limited by the power dissipation and the current supply capabilities of the op-amp.
• 🔀 The video highlights that in the inverting configuration, individual voltage sources are isolated due to the virtual ground concept, which is an advantage.

Q & A

What is the main topic of the video?

The main topic of the video is the use of an operational amplifier (op-amp) as a summing amplifier, explaining how to add different input voltages using this configuration.

What is a summing amplifier?

A summing amplifier is a type of electronic circuit that adds multiple input voltages together to produce a single output voltage.

How does the inverting summing amplifier configuration work?

In the inverting summing amplifier configuration, multiple input voltages are applied to the inverting input terminal of the op-amp. The output voltage is the algebraic sum of the individual responses, scaled by the resistor values.

What is the significance of the virtual ground concept in the inverting summing amplifier?

The virtual ground concept implies that the inverting input terminal of the op-amp is at ground potential due to negative feedback. This allows for the assumption that the voltage at this node is zero, simplifying the analysis of the circuit.

How does the non-inverting summing amplifier differ from the inverting one?

In the non-inverting summing amplifier, input voltages are applied to the non-inverting input terminal. The output voltage is a function of the voltage at the non-inverting terminal, which is influenced by the input voltages and the resistor values.

What is the expression for the output voltage in an inverting summing amplifier?

The output voltage in an inverting summing amplifier is given by Vout = -(Rf/R1 * V1 + Rf/R2 * V2 + Rf/R3 * V3), where Rf is the feedback resistor, and R1, R2, R3 are the input resistors.

What happens if all input resistors in an inverting summing amplifier have the same value?

If all input resistors in an inverting summing amplifier have the same value, the output voltage Vout will be equal to -Rf/R * (V1 + V2 + V3), effectively summing the input voltages without scaling.

Can an op-amp be used to perform operations other than addition in a summing amplifier configuration?

Yes, an op-amp can be used for scaling and averaging operations in addition to addition. It can also provide DC offsets to input signals, be used in digital to analog conversion, and mix different audio signals.

Why might the inverting summing amplifier be preferred over the non-inverting one in practical applications?

The inverting summing amplifier is often preferred because it isolates individual voltage sources from each other due to the virtual ground, whereas in the non-inverting configuration, the sources influence each other.

How can the output voltage of an inverting summing amplifier be made positive?

The output voltage of an inverting summing amplifier, which is inherently negative, can be made positive by connecting an additional inverting op-amp with unity gain.

What is the limitation of adding multiple input voltages in a summing amplifier?

Theoretically, an unlimited number of inputs can be added, but practically, the number of inputs is limited by the power dissipation and the total current that the op-amp can supply.

Outlines

00:00

🔌 Inverting Summing Amplifier Configuration

This paragraph introduces the concept of using an op-amp as a summing amplifier, specifically focusing on the inverting configuration. The video explains how multiple input voltages (V1, V2, V3) are applied to the inverting terminal through resistors (R1, R2, R3) and how the output voltage (Vout) is calculated based on these inputs. The principle of virtual ground in op-amps is discussed, which leads to the Kirchhoff's Current Law (KCL) application at node X, resulting in an equation relating the input voltages to the output voltage through the resistor values. The paragraph also explores the scenario where resistor values are equal, simplifying the output voltage calculation, and mentions the potential for scaling and averaging operations with different resistor ratios.

05:00

🔄 Applications and Advantages of Inverting Summing Amplifier

The second paragraph delves into the practical applications of the inverting summing amplifier, such as addition, averaging, scaling, and providing DC offsets to AC signals. It also touches on the use of this configuration in digital-to-analog conversion and audio signal mixing. The video points out the theoretical capability to add an unlimited number of input voltages, but practical limitations arise from power dissipation and op-amp current supply capabilities. A key advantage highlighted is the isolation of individual voltage sources due to the virtual ground concept, ensuring no interference between sources. The paragraph concludes with a discussion on how to obtain a positive output voltage by adding a unity gain inverting op-amp stage.

10:00

🔩 Non-Inverting Summing Amplifier Configuration

Paragraph three shifts focus to the non-inverting summing amplifier configuration, explaining how multiple input voltages are applied to the non-inverting terminal and how the output voltage is determined. The principle of superposition is used to calculate the voltage at the non-inverting terminal (Vplus) when multiple inputs are considered. The video simplifies the explanation by assuming equal resistor values (R1=R2=R) and shows how the output voltage can represent a simple addition of input voltages when certain conditions are met. The complexity of the configuration increases with the number of inputs, and the video provides a simplified scenario for three inputs, assuming equal resistor values.

15:04

📉 Comparison and Conclusion on Summing Amplifiers

The final paragraph compares the inverting and non-inverting summing amplifiers, noting that the non-inverting configuration becomes more complex with an increasing number of inputs and that individual voltage sources are not isolated, which can influence the input voltage at the non-inverting terminal. It concludes that the inverting summing amplifier is often preferred in practical applications due to its simplicity and isolation of sources. The video ends with an invitation for questions and suggestions in the comments and encourages viewers to like and subscribe for more content.

Keywords

💡Op-amp

An operational amplifier, or op-amp, is a type of high-gain electronic voltage amplifier with a differential input and a single-ended output. In the context of the video, the op-amp is used to construct a summing amplifier, which adds multiple input voltages together. The script describes how to configure the op-amp as an inverting or non-inverting summing amplifier, which is central to the video's theme of explaining how to use op-amps for summing different input voltages.

💡Summing Amplifier

A summing amplifier is a type of electronic circuit that adds multiple input signals together to produce a single output signal. The video script explains how to use an op-amp as a summing amplifier, either inverting or non-inverting, to add different input voltages. This concept is crucial to the video's educational goal of demonstrating how to perform addition and scaling of voltages using op-amps.

💡Inverting Summing Amplifier

An inverting summing amplifier is a configuration where multiple input signals are applied to the inverting input terminal of an op-amp. The video script provides a detailed explanation of how this configuration works, including the application of Kirchhoff's Current Law (KCL) at node X and the derivation of the output voltage expression in terms of the input voltages. This concept is key to understanding how the op-amp can be used to add and scale input voltages.

💡Non-inverting Summing Amplifier

A non-inverting summing amplifier is a configuration where multiple input signals are applied to the non-inverting input terminal of an op-amp. The script discusses how to find the output voltage in terms of the input voltages using the principle of superposition. This concept is important for understanding an alternative method of summing input voltages using an op-amp.

💡Virtual Ground

In the context of op-amp circuits, a virtual ground refers to a point in the circuit that acts as if it were at ground potential due to the high input impedance of the op-amp. The script explains how the concept of virtual ground is used in inverting op-amp configurations, where the inverting input terminal is effectively at ground potential, allowing for the summing of input voltages without interference.

💡KCL (Kirchhoff's Current Law)

Kirchhoff's Current Law states that the sum of currents entering a junction is equal to the sum of currents leaving the junction. In the video script, KCL is applied at node X to derive the relationship between the input voltages and the output voltage in an inverting summing amplifier. This law is fundamental to understanding how current flows are managed in the circuit to achieve the desired summing effect.

💡Feedback Resistor

In the context of op-amp circuits, a feedback resistor is used to provide negative feedback, which stabilizes the output and determines the gain of the amplifier. The script describes how the value of the feedback resistor (Rf) influences the output voltage in both inverting and non-inverting summing amplifier configurations, highlighting its importance in the operation of the circuit.

💡Scaling Operation

Scaling in the context of the video refers to the process of adjusting the magnitude of input signals before they are added together. The script explains how the ratio of the feedback resistor to the input resistors in an op-amp circuit can be used to scale the input voltages, allowing for both addition and scaling of input signals.

💡Averaging Operation

An averaging operation in the context of the video is a specific type of scaling where the output voltage is the mean of the input voltages. The script demonstrates how to configure an inverting summing amplifier to perform an averaging operation by setting the ratio of the feedback resistor to the input resistors equal to the reciprocal of the number of input voltages.

💡DC Offset

DC offset refers to a constant voltage added to an AC signal to shift it up or down. The script mentions that an inverting summing amplifier can be used to apply a DC offset to input signals, which is useful for adjusting the baseline of signals from sensors or other AC sources.

Highlights

Introduction to using an op-amp as a summing amplifier.

Explanation of how to add different input voltages using an op-amp configuration.

Inverting summing amplifier configuration and its application.

Derivation of the output voltage expression for the inverting summing amplifier.

Concept of virtual ground in op-amps and its significance.

KCL application at node X to determine input currents in the inverting summing amplifier.

Simplified expression for the output voltage in the inverting summing amplifier.

Output voltage calculation when all resistors have the same value in the inverting summing amplifier.

Scaling operation using the inverting summing amplifier with different resistor values.

Averaging operation using the inverting summing amplifier with equal resistor values.

Practical limitations on the number of input voltages in an op-amp configuration.

Isolation of individual voltage sources due to the virtual ground concept.

Method to obtain a positive output voltage from an inverting summing amplifier.

Introduction to the non-inverting summing amplifier and its configuration.

Derivation of the output voltage for the non-inverting summing amplifier using superposition.

Simplified output voltage equation for the non-inverting summing amplifier with equal resistors.

Comparison of inverting and non-inverting summing amplifiers in terms of complexity and isolation.

Practical applications of summing amplifiers including addition, averaging, scaling, and DC offset.

Closing remarks and invitation for questions and suggestions from the audience.

Transcripts

Hey friends, welcome to the YouTube channel ALL ABOUT ELECTRONICS.

So, in this video, we will see how to use this op-amp as summing amplifier and using

this op-amp configuration how we can add the different input voltages.

So, now in the earlier video of inverting and non-inverting op-amp configuration, we

have applied the single input either to the non-inverting or the inverting op-amp terminals.

Now, in this video, we will apply the multiple inputs either to the inverting or the non-inverting

op-amp terminal and we will see how this configuration can be used as summing amplifier.

So, first we will see the inverting summing amplifier and in this configuration, we will

apply the multiple inputs to the inverting input terminal.

And we will find the expression of the output voltage in terms of the different input voltages.

So, as you can see here, we have applied the input voltages V1, V2 and V3 to this inverting

terminal via resistors R1, R2, and R3.

And here let's assume that current I1, I2, and I3 are flowing through this resistor R1,

R2, and R3.

And let's say the current If is flowing through this resistor Rf.

And here, we are assuming the op-amp as the ideal op-amp.

So, no current is going inside this op-amp.

And let's consider this node as node X.

Now, in the earlier video of inverting op-amp configuration, we have seen the concept of

virtual ground.

And in that concept, we had seen that whenever we are applying the negative feedback to any

op-amp, then there is virtual short exist between the inverting and the non-inverting

terminals.

So, if one terminal is at ground potential, then another terminal will also be at ground

potential.

Or we can say that it will act as a virtual ground.

So, this node will have zero potential.

So, now let's apply KCL at this node X.

So, applying KCL we can write I1 +I2 +I3, that is equal to this current If.

Now, as this terminal is at ground potential, so the current I1 will be equal to V1 minus

zero divides by R1.

Likewise, this current I2 will be equal to V2 minus zero divides by R2.

And likewise, current I3 will be equal to V3 minus zero divides by R3.

Now, this current If will be equal to zero minus Vout divide by this feedback resistor

Rf.

That is zero minus Vout, divide by this feedback resistor Rf.

So, if we simplify this expression then we can write it as

V1 divide by R1, plus V2 divide by R2, plus V3 divide by R3 that is equal to minus Vout

divide by Rf.

And if we further simplify it then we can write it as,

Vout that is equal to minus, Rf divide by R1 times V1, plus Rf divide by R2 times V2,

plus Rf divide by R3 times V3.

Now, we know that if we apply the single input to this non-inverting op-amp configuration,

then the output voltage Vout can be given as -minus Rf divide by R1 times the input

voltage.

Now, here instead of single input voltage we have applied the multiple input voltages.

So, the output voltage will be equal to the algebraic sum of the individual responses.

So, this is the expression of the output voltage in case of the inverting summing amplifier.

Now, in this inverting summing amplifier configuration, let's assume that the value of R1, R2, and

R3 is same.

So, in that case, the output voltage Vout will be equal to -Rf divide by R times, V1

plus V2 plus V3.

So, now here if the value of Rf is equal to R, then, in that case, our output voltage

vout will be equal to -(V1 plus V2 plus V3).

So, in this way, we can use this op-amp as an adder and we can add the multiple input

voltages.

So, now suppose in this configuration, if R1, R2, and R3 are different then, in that

case, the ratio of this feedback resistor over this resistors R1, R2 and R3 will also

be different.

So, in that case along with the addition, we can also perform the scaling operation.

So, in that case, our output voltage Vout will be equal to -(A*V1 +B*V2 +C*V3).

Where A, B, and C represents the ratio of this Rf divide by R1, Rf divide by R2 and

Rf divide by R3.

So, along with addition, we can also perform the scaling operation.

So, along with this scaling and addition operation, we can also perform the averaging operation

using this configuration.

So, now let's see how we can perform the averaging of the different input voltages using this

configuration.

So, let's once again assume that the R1, R2, and R3 are having the same value.

And let's say the ration of this Rf divide by R is equal to 1 divide by n.

Where n represents the number of input voltages that are being applied to this inverting terminal.

So, in that case, our output voltage Vout will be equal to, minus Rf divide by R times,

V1 plus V2 plus V3.

And we put the value of this Rf by R as 1divide by n, that is 1 divided by 3 in this case,

then our output voltage will be equal to minus V1 plus V2 plus V3, divide by 3.

So, in this way, our output voltage will be the average of the three different input voltages.

So, we can also perform the averaging operation using this configuration.

So, now let's see some more applications in which this inverting summing amplifier can

be used.

So, as I already told you, this summing amplifier can be used for the operation of addition,

averaging as well as the scaling.

Apart from that, it can also be used to provide the DC offset to the input signals.

So, suppose if your input voltages are AC signals or let's say it is coming from some

sensor, and if we want to apply some DC offset, then using this configuration we can also

apply some DC offset to the incoming signals.

Likewise, this inverting summing amplifier can also be used for the digital to analog

conversion.

And apart from that, it can also be used for the mixing the different audio signals.

So, these are the different applications in which this summing amplifier can be used.

Now, theoretically, we can add the n number of input voltages to this op-amp configuration,

but practically if you see the number of input voltages depends on the power dissipation

as well as the total current that can be supplied by the op-amp.

Because here if you see, the total current If is the summation of the individual currents

I1, I2, I3 upto In.

So, this value should not exceed the maximum value that is supported by the op-amp.

Now, one more interesting fact about this inverting op-amp configuration is that all

the individual voltage sources are isolated with respect to each other.

And that is because of the concept of virtual ground.

Because if you consider the individual voltage sources at a time, then at that time, the

remaining voltage sources will act as a short circuit.

So, if we consider this voltage source V1, that is acting alone, then at that time, this

voltage source V2 and V3 will act as a short circuit.

Or we can say that they are at ground potential.

Now, because of the virtual ground, this node is also at the ground potential.

So, we can say that effectively this R2 and R3 does not exist in the circuit.

So, the effective impedance that is seen by the voltage source V1 is the series resistance

R1 of that voltage source.

So, we can say that there is no interference between the different voltage sources.

And that is the biggest advantage of this inverting summing amplifier.

Now, the output of this inverting summing amplifier is the negative voltage.

But suppose if we want the positive voltage then what we can do, we can connect one more

inverting op-amp which is having unity gain.

So, as you can see here, suppose if we connect one more inverting op-amp, which is having

unity gain at this point then the output voltage will be positive.

So, by using one more inerting op-amp, we can get the positive output voltage.

So, this is all about the inverting summing amplifier.

Now, let's see what happens when we apply the multiple input voltages at the non-inverting

input terminal.

So, now in this non-inverting summing amplifier, we have applied the two input voltages at

this non-inverting end.

And we will find the output voltage in terms of the input voltages V1 and V2.

Now, we know that in case of the non-inverting configuration, the output voltage is equal

to one plus Rf divide by Ra times the voltage at this node.

Let's say that is equal to Vplus.

So, as we have applied the multiple inputs at this end, so first of all, we need to find

this voltage Vplus.

And we can do so by applying the principle of superposition.

So, what we will do, we will consider the one voltage source at a time and we will find

the voltage at this node.

And later on, we will combine the individual responses to get the final response.

So, first of all, let's assume that this voltage source V1 is acting alone and we have removed

this voltage source V2.

That means V2 is equal to zero.

So, in that case, let's say the voltage at this point is equal to V1 plus.

So, V1plus will be equal to R2 divide by (R1 plus R2) times this voltage V1.

Likewise, when we consider this voltage source V2 is acting alone, and V1 is equal to zero.

In that case, the voltage at this point is let's say V2plus.

So, V2plus will be equal to R1 divide by (R1 plus R2) times this voltage V2.

So, in this way, when this voltage source V1 and V2 are acting alone, then, in that

case, the V1plus will be equal to this value and V2 plus will be equal to this value.

So, the overall voltage Vplu will be equal to the summation of the individual voltages.

that is (V1plus) plus (V2 plus) That is equal to R2 divide by (R1 plus R2)

time V1, plus R1 divide by (R1 plus R2 ) times V2.

So, this will be voltage at this because of the voltage V1 and V2.

Now, we know that the output voltage is equal to one plus Rf divide by Ra times the voltage

at this end.

That is equal to Vplus.

So, our final output voltage will be equal to one plus Rf divide by Ra times, R2 divide

by (R1 plus R2 ) times V1, plus R1 divide by (R1 plus R2 ) times V2.

So, this will be the final voltage that we get at the output of this non-inverting summing

amplifier.

So, now let's simplify this equation a bit and let's assume that this R1 is equal to

R2 is equal to R. So, when R1=R2=R, then, in that case, the

output voltage vout will be equal to one plus Rf divide by Ra times V1 plus V2, divide by

2.

Now, if we consider Rf = Ra, in that case, the output voltage Vout will be equal to V1

+V2.

That is the addition of the voltage V1 and V2.

So, in this way, using this non-inverting summing amplifier also, we can perform the

addition.

Now, let's take the case when we have three different input voltages that are connected

to this non-inverting terminal.

So, as you can see here, we have three different input voltages V1, V2, and V3, which are connected

to this non-inverting terminal via resistor R1, R2, and R3.

So, here also, we will consider the one voltage source at a time and we will find the voltage

at this non-inverting terminal.

And later on, we will add the individual responses to get the voltage at this non-inverting terminal.

So, first of all, let's assume that this voltage source V1 is acting alone and V2 and V3 are

equal to zero.

So, in that case, the circuit will look like this.

So, the voltage at this point let's say is equal to V1 plus.

So, V1 plus will be equal to (R2 in parallel R3) divide by (R1 plus + (R2 in parallel R3

) ) times this voltage V1.

So, this is the voltage that you get at this point when this voltage source V1 is acting

alone.

likewise, we can have voltage V2 plus that is equal to,

(R1 parallel R3) divide by this R2 plus (R1 parallel R3) times this voltage V2.

this is the expression when the voltage source V2 is acting alone.

And likewise, we can have V3 plus that is equal to (R2 parallel R1) divide by R3 plus

(R2 parallel R1) times this voltage V3.

So, the total voltage that appears at this point will be the summation of this individual

voltages.

So, as you can see here, in this non-inverting configuration, as the number of input voltages

is increasing, the complexity is also increasing.

And to here reduce the complexity, let's assume that R1, R2, and R3 are equal.

So, when all the three values are same, then the value of V1plus will be equal to V1/3.

Likewise, the value of V2plus will be equal to V2/3.

And likewise, V3 plus will be equal to V3/3.

So, in short, when R1, R2, and R3 are equal in that case, Vplus will be equal to (V1 +V2

+V3 )/3 And the output voltage Vout will be equal

to (1+ (Rf/Ra)) times Vplus.

Or we can say that (1+(Rf/Ra)) (V1 +V2+V3)/3 So, now suppose if the value of 1+ (Rf/Ra)

that is equal to 3, in that case, Vout will be equal to V1+V2+V3.

So, as you can see here, in this configuration as the number of input voltage increases,

the complexity is also increasing.

And here, the individual input voltage that appears at this point does not only depend

on the value of this V1 and R1, but it also depends upon the different series resistors

of the individual voltage sources.

Like, here, in this case, it also depends upon the value of this R2 and R3.

So, in this configuration, we can say that the individual voltage sources are not isolated

with respect to each other.

And they have their own influence on the input voltage that appears at this non-inverting

terminal.

So, that is the reason, in most of the practical applications, this inverting summing amplifier

is more preferred over this non-inverting summing amplifier.

So, that is all about the inverting and the non-inverting summing amplifier.

So, I hope in this video, you understood about this inverting and the non-inverting summing

amplifiers.

So, if you have any question or suggestion, do let me know in the comment section below.

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