Operational Amplifiers - Inverting & Non Inverting Op-Amps

Summary

This video delves into operational amplifiers (op-amps), explaining their function as high-gain differential amplifiers. It outlines the basics of the 741 op-amp, detailing its pin configuration and the significance of input and output impedances. The video explores inverting and non-inverting amplifier circuits, illustrating how to calculate closed-loop voltage gain and emphasizing the role of feedback resistors. It also touches on the importance of slew rate in high-frequency applications, providing an example problem to demonstrate these concepts.

Takeaways

• 🔬 An op-amp (operational amplifier) is a high-gain differential amplifier that amplifies the difference between two input voltages, V1 and V2.
• 🔌 Op-amps have a very high input impedance and a very low output impedance, which is important for circuit design.
• 🔄 The negative terminal of an op-amp is the inverting input, and applying a signal here results in an output signal that is 180 degrees out of phase.
• 🔀 The positive terminal is the non-inverting input, and signals applied here produce an output signal in phase with the input.
• 💡 The 741 is a common type of operational amplifier, with pin 2 as the inverting input, pin 3 as the non-inverting input, and pin 6 as the output.
• 🔋 Pins 4 and 7 of the 741 op-amp are used for the power supply, connecting to batteries or other voltage sources.
• 🔩 The offset null pins (1 and 5) can be used to adjust the output offset voltage to zero, compensating for any inherent bias in the op-amp.
• ⚙️ In an inverting amplifier circuit, the feedback resistor (Rf) is used to create a closed-loop voltage gain, calculated as Rf divided by the input resistor (Rn).
• 🔄 Removing the feedback resistor puts the op-amp in open-loop mode, which has a much higher voltage gain, typically up to 200,000.
• 🔌 Connecting batteries to an op-amp involves setting the ground between the positive and negative terminals of the batteries, with pin 7 connected to the negative terminal and pin 4 to the positive.
• 📈 The slew rate of an op-amp, which is its ability to respond to rapid changes in input signal, is crucial for high-frequency applications and affects the maximum operating frequency.

Q & A

What does 'op amp' stand for and what is its primary function?

An 'op amp' stands for operational amplifier. Its primary function is to amplify the difference between two input voltages, v1 and v2, with a high gain.

What are the characteristics of an op amp's input and output impedance?

An op amp has a very high input impedance and a very low output impedance.

How does applying a signal to the inverting input affect the output signal of an op amp?

Applying a signal to the inverting input of an op amp will result in an output signal that is 180 degrees out of phase with the input signal.

What is the purpose of the feedback resistor (rf) in an op amp circuit?

The feedback resistor (rf) in an op amp circuit takes a portion of the output signal and feeds it back to the input, which helps to reduce the voltage gain.

How is the closed-loop voltage gain calculated in an inverting amplifier circuit?

The closed-loop voltage gain in an inverting amplifier circuit is calculated as the feedback resistor (rf) divided by the input resistor (rn), with a negative sign indicating inversion.

What is the significance of the output offset voltage in an op amp?

The output offset voltage is the voltage that an operational amplifier can generate at its output even when there is no input signal. It can be adjusted to zero using the offset null pins.

What is the typical open-loop voltage gain of an op amp?

The typical open-loop voltage gain of an op amp, represented by g_sub_v, could be as high as 200,000.

How should batteries be connected to an op amp for power supply?

Batteries should be connected in series with the positive terminal of one battery connected to pin 4 of the op amp and the negative terminal of the other battery connected to pin 7, with the ground in between.

What is the difference between an inverting and a non-inverting amplifier circuit?

In an inverting amplifier, the input signal is applied to the inverting input, and the output signal is out of phase with the input. In a non-inverting amplifier, the input signal is applied to the non-inverting input, and the output signal is in phase with the input.

How is the closed-loop voltage gain calculated for a non-inverting amplifier circuit?

For a non-inverting amplifier circuit, the closed-loop voltage gain is calculated as the feedback resistor (rf) divided by the input resistor (rn), but with an additional '1' added to the result.

What is the slew rate and why is it important in op amp circuits?

The slew rate is a measure of how quickly the output of an op amp can change in response to a change in input voltage. It's important because it affects the maximum operating frequency of the op amp and the ability to accurately amplify high-frequency signals.

Outlines

00:00

🔌 Introduction to Operational Amplifiers (Op Amps)

This paragraph introduces operational amplifiers (op amps) as high-gain differential amplifiers that amplify the difference between input voltages V1 and V2. It explains the characteristics of op amps, including high input impedance and low output impedance. The video script describes the inverting and non-inverting inputs and how they affect the phase of the output signal. It also details the 741 op amp's pin configuration, including the inverted input (pin 2), non-inverting input (pin 3), output pin (pin 6), power supply pins (pins 4 and 7), and unused pins (pins 1 and 8). The concept of output offset voltage and its adjustment using offset null pins is briefly mentioned. The paragraph concludes with a basic circuit diagram of an inverting amplifier, explaining the role of the feedback resistor (Rf) and how it affects the closed-loop voltage gain, which is calculated as Rf divided by the input resistance (Rn).

05:01

🔋 Connecting Op Amps to Power Supplies and Understanding Slew Rate

This paragraph discusses how to connect a 741 op amp to a power supply, typically using two batteries with their positive and negative terminals connected to pins 4 and 7, respectively, and the ground in the middle. It then transitions to explaining the non-inverting amplifier circuit, where the input signal is applied to the non-inverting input, and the output signal is in phase with the input. The paragraph highlights the importance of the feedback resistor in determining the closed-loop voltage gain, which is calculated as the feedback resistor divided by the input resistor plus one. The concept of slew rate (Sluvate) is introduced, explaining its significance in determining the maximum operating frequency of an op amp. The paragraph concludes with an example of how the slew rate affects the gain at high frequencies, noting that some op amps are designed for higher frequency operations while others are not.

10:03

⚙️ Calculating Closed-Loop Voltage Gain and Output Voltage

In this paragraph, an example problem is presented to demonstrate how to calculate the closed-loop voltage gain and output voltage of an operational amplifier circuit. The problem involves identifying the type of amplifier (in this case, an inverted amplifier) and using the given feedback and input resistor values to determine the gain. The calculation is straightforward: the gain is the negative of the feedback resistor divided by the input resistor. The negative sign indicates a phase reversal. The paragraph then calculates the output voltage based on the gain and input signal voltage, demonstrating how to apply the gain to the input signal to find the output signal's amplitude. The example serves to clarify the practical application of the concepts discussed in the previous paragraphs.

Keywords

💡Operational Amplifier (Op-Amp)

An operational amplifier, or op-amp, is a high-gain, differential amplifier used in various applications to process analog signals. It is characterized by a very high input impedance and a low output impedance. In the video, op-amps are introduced as the main subject, with a focus on their ability to amplify the difference between two input voltages, V1 and V2. The video explains the basic layout of a 741 op-amp, which is a widely used type, and how it functions with its inverted and non-inverted inputs.

💡Inverting Input

The inverting input of an op-amp is the negative terminal where a signal is applied. When a signal is input here, the output signal will be 180 degrees out of phase with the input signal. This concept is crucial for understanding how op-amps process signals and is demonstrated in the video through the explanation of an inverting amplifier circuit where the signal is applied to pin 2 of the 741 op-amp.

💡Non-Inverting Input

The non-inverting input is the positive terminal of an op-amp. Applying a signal to this input results in an output signal that is in phase with the input. This is in contrast to the inverting input and is important for designing circuits where phase preservation is necessary. The video script describes how the output signal at V2 will be in phase with the input signal when applied to the non-inverting input.

💡Input Impedance

Input impedance refers to the measure of opposition to current flow into an input of a device. In the context of op-amps, they are designed to have a very high input impedance, which means they draw very little current from the input signal source. This is beneficial for maintaining the integrity of the input signal and is highlighted in the video as a key characteristic of op-amps.

💡Output Impedance

Output impedance is the measure of the opposition to current flow at the output of a device. Op-amps typically have a very low output impedance, which allows them to drive loads effectively without significant voltage drop. The video emphasizes this feature as it enables the op-amp to be connected to various output devices without affecting the signal quality.

💡Feedback Resistor (Rf)

In the context of op-amp circuits, the feedback resistor is used to create a closed-loop system where a portion of the output signal is fed back to the input. This is done to control the gain of the circuit. The video explains that the feedback resistor, denoted as Rf, is connected to the output and the inverting input, and it plays a critical role in determining the closed-loop voltage gain of the circuit.

💡Closed-Loop Voltage Gain

The closed-loop voltage gain of an op-amp circuit is the ratio of the output voltage to the input voltage when feedback is present. It is calculated as the feedback resistor (Rf) divided by the input resistor (Rn). The video script provides an example of calculating this gain for an inverting amplifier circuit, emphasizing the importance of understanding this concept for circuit design.

💡Open-Loop Voltage Gain

Open-loop voltage gain refers to the gain of an op-amp when no feedback is present. This gain is typically very high, often in the range of 200,000 as mentioned in the video. The script explains that without a feedback resistor, the op-amp operates in open-loop mode, and the gain is referred to as the open-loop voltage gain, which is different from the closed-loop gain.

💡Offset Null Pins

Offset null pins on an op-amp are used to adjust the output offset voltage to zero. The video script mentions that even if there's no signal applied at the input, an op-amp can still generate an output voltage known as the output offset voltage. The offset null pins, typically pins 1 and 5 on a 741 op-amp, can be used to nullify this offset voltage, ensuring accurate circuit operation.

💡Slew Rate

Slew rate is a measure of how quickly an op-amp can respond to changes in its input signal, especially at high frequencies. The video script explains that the slew rate is important for determining the maximum operating frequency of an op-amp. It is defined as the rate of change of the output voltage per unit time and is crucial for high-frequency signal processing. The script provides an example of calculating the maximum frequency based on the slew rate.

💡Non-Inverting Amplifier Circuit

A non-inverting amplifier circuit is a configuration where the input signal is applied to the non-inverting input of the op-amp, resulting in an output signal that is in phase with the input. The video script describes how to set up such a circuit using the 741 op-amp, including the placement of resistors and the calculation of the closed-loop voltage gain, which is the feedback resistor divided by the input resistor plus one.

Highlights

An op amp is a high gain, differential amplifier that amplifies the difference between input voltages V1 and V2.

Op amps have a very high input impedance and a very low output impedance.

The negative terminal of an op amp is the inverted input, causing a 180-degree phase shift in the output signal.

The positive terminal is the non-inverting input, maintaining the phase of the input signal in the output.

The 741 operational amplifier is introduced with pin 2 as the inverted input and pin 3 as the non-inverting input.

Op amps can generate an output voltage even without an input signal, known as output offset voltage.

The basic layout for an inverting amplifier is explained, with the signal applied to the inverting input.

The feedback resistor (Rf) in an inverting amplifier reduces the voltage gain by feeding back part of the output signal to the input.

The closed-loop voltage gain for an inverting amplifier is calculated as Rf divided by Rn with a negative sign.

Removing the feedback resistor puts the op amp in open-loop mode, changing the gain calculation.

The typical open-loop voltage gain (Gv) of an op amp can be as high as 200,000.

The output voltage of an op amp is limited by the supply voltages at pins 4 and 7.

Connecting batteries to an op amp involves placing the ground between the batteries' positive and negative terminals.

The non-inverting amplifier circuit is explained with the input signal applied to the positive terminal.

The closed-loop voltage gain for a non-inverting amplifier is Rf divided by Rn plus one.

Slew rate is a critical factor for op amps at high frequencies, affecting their switching capabilities.

As the input signal frequency increases, the op amp's gain decreases, impacting high-frequency performance.

An example problem demonstrates calculating the closed-loop voltage gain and output voltage for an operational amplifier circuit.

Transcripts

in this video we're going to talk about

op amps

an op amp is short for an operational

amplifier

op amps are basically high gain

differential amplifiers

these devices they amplify the

difference between the input voltages v1

and v2

now the op amp has a very high input

impedance but a very low output

impedance

the negative terminal here is known as

the inverted input

so if you apply a signal to the

inverting input

the output signal will be 180 degrees

out of phase with that

the positive terminal

is the non-inverting input

so if you apply a signal there

the output signal will be in phase

with

the input signal at v2

so here's the basic layout for the 741

operational amplifier

pin 2 represents the inverted input

and pin three represents the

non-inverting input

six

is the output pin

pins four and seven

is used for

the power supply that's where you'll be

connecting the batteries to this op amp

number eight is unused

and one in five is the offset no pins

even if there's no signal applied at the

input

the operational amplifier can still

generate

an output voltage and this is known as

the output offset voltage

and so you could use these pins

to set the output the output offset

voltage to zero

but that's another topic for discussion

so here is a basic circuit diagram of an

inverting amplifier

the signal is applied to the inverting

input

which is represented by pin two

and so the output will have the inverted

signal

rf is known as the feedback resistor

this resistor takes some of the output

signal and feeds it back to the input

so as a result it reduces the voltage

gain

now when you have a feedback resistor

the voltage gain is said to be

a closed loop voltage gain

and that gain

is equal to

rf divided by

rn or the input resistance

now of course because the signal has

been inverted we need to put a negative

sign here so that's how you can

calculate the gain

for this particular circuit

now

r

should be set equal to the parallel

combination of these two

resistors so r is going to be the

product

of the input resistance times the

feedback resistor

divided by the sum

of those two resistors

now what happens if we get rid of the

feedback resistor

and let's get rid of r as well

in this case the op amp will no longer

be in closed loop mode but rather it's

going to be in the open loop mode

so the gain will no longer be called the

closed loop voltage gain

instead it's going to be called the open

loop of voltage gain and so that

equation won't apply if we don't have a

feedback resistor

now

the typical open loop voltage gain

represented by g sub v

could be as high as 200 000.

now the output voltage will be limited

based on the supply voltages at pins

four and seven so keep that in mind

now let's talk about how we can connect

a battery or a series of batteries

to the op amp

so this is how you want to connect the

supply voltages

so let's say if we have two

nine volt batteries

so here's the positive terminal of one

of the batteries here's the negative

terminal of the other one

the ground

will be at the middle between those two

batteries

and this will be connected to

basically pin seven of the op amp and

this part will be connected to pin four

of the op amp

so let's put this all together

so here is the 741 op amp

with its two input

voltages

here is pin seven and this is pin four

and here's pin six the output pin

and here we're gonna put

the batteries

so this is negative v

and over here this is positive v

so this is v out

this is v1 v2

and then

we also need to connect the ground to it

but let's add some resistors to this

circuit

let's turn this into an inverted op amp

so this is the input resistance

now we do need our feedback resistor

this is going to be r

and we need to connect this to the

ground

and the ground connection is here as

well

so if you want to you can just

draw a line that

connects those two parts of the circuit

so that's how you can connect the 741 op

amp to

a series of batteries

so you need basically two batteries

so this is nine volts each it could be

more

but in the middle that's where you're

going to put the ground

now let's go over the next type of

circuit

which is

the non-inverting

amplifier circuit

so we're going to use the 741 op amp one

more time

so this will be the positive terminal

and this will be the negative terminal

so notice that i flipped it

now we're going to apply the input

signal to the positive terminal

so because it's a non-inverting

amplifier

the output signal

will be in the same phase as the input

signal it's not going to be inverted

now we're going to have a resistor

between

pin 2

that is the inverted input

and the ground

so let's call that the input resistance

and we're still going to have our

feedback resistor

between inputs 2 and 6.

so notice that if you compare this

circuit with the inverted amplifier

circuit

the feedback resistor

was still between

pins 6 and 2.

so that hasn't changed

now let's not forget to put

the supply voltages

so positive v

and negative v

for pins four

and seven

now the closed loop voltage gain

for this particular

non-inverting amplifier

is going to be

it's still the feedback resistor divided

by the input resistance but with

an addition of one

so it's the same gain as the inverted

amplifier but

adding one to it as well

so that's how you can calculate the gain

for this particular circuit

now there's another term that you need

to be familiar with when dealing with op

amps

and it's something called sluvate

the transistors inside the integrated

circuit of an op amp

has

limited switching capabilities

at high frequencies they may not be able

to switch

on and off as quickly

based on how fast the signal is

alternating from its positive to

negative cycles

so

this value becomes very important when

calculating the maximum operating

frequency of a particular op amp

it's the slew rate

divided by two pi

times the peak voltage

this is the peak output voltage

of

the signal

at pin six

now a typical slew rate value would be

like 0.5 volts

per microsecond

so that's 0.5 divided by 1 times 10 to

the minus 6

seconds

so that will correspond to a frequency

of 500 kilohertz

what you need to understand is this

as the frequency of the input signal

increases

the gain

of the op amp decreases

and so certain op amps don't work at

very high frequencies

some are designed to handle higher

frequencies

but

others

just don't work well at high frequencies

so keep that in mind

let's work on an example problem

so here we have an operational amplifier

circuit

and we're given

the voltage of the input signal

and we want to calculate two things

what is the closed loop voltage gain of

this particular circuit and also what is

the output voltage

let's focus on the gain

now we need to know what type of

amplifier we have is it an inverted

amplifier or a non-inverting amplifier

and we can see that the signal is

applied to the negative

inverting input of the op amp so it's an

inverted amplifier which means that the

gain is negative

rf

over the input resistance

now the feedback resistor is 40 kilo

ohms

the input resistance is 2 kilo ohms

so negative 40 divided by 2 the closed

loop voltage gain

is negative 20. now keep in mind the

negative sign

simply tells us that

the polarity has been reversed or

that the output signal is 180 degrees

out of phase with the input signal

now what is the output voltage

if the gain is 20 it's going to be 20

times more than 10 millivolts

so it's 10 millivolts times 20

10 times 2 is 20 so 10 times 20 is 200

so thus

the output signal

will have a voltage

that will vary between positive and

negative

200

millivolts