We are Apogeeweb Semiconductor Electronic

WELCOME TO OUR BLOG

Home arrow Amplifiers arrow What is the Use of Op Amp Voltage Follower?

arrow left

arrow right

What is the Use of Op Amp Voltage Follower?

Author: Apogeeweb Date: 24 Jun 2021  245

op amp follower

Introduction

Do you know buffer amplifier or isolation amplifier? The operational amplifier is an extremely efficient and versatile device. As we all known, the op amp is a component that amplifies the weak signal, which can be made into different forms according to the circuit requirements, and the voltage follower is one of them. Most voltage follower circuit will use an Op-amp. A follower is specifically an op amp wired to have a gain of +1. IE, the output is the same polarity and voltage as the input. That is, the output signal is exactly the same as the input signal. Here op amp voltage follower is used to isolate the signal and enhance load capacity. 

Op-amp as Voltage Follower

Introduction

Ⅰ Voltage Follower OP Amplifier

Ⅱ Voltage Follower Characteristics

2.1 Op Amp Impedance Matching

2.2 Buffer Amplifier & Isolation Amplifier

Ⅲ Op Amp Follower Circuit Analysis

3.1 Op Amp Voltage and Load

3.2 Op Amp Voltage Follower Stability

3.3 Op Amp Phase Difference Problem

3.4 Adding Feedback Resistance

Ⅳ Op Amp Voltage Follower Application


Ⅰ Voltage Follower OP Amplifier

The op amp follower sacrifices the voltage amplification factor in exchange for the performance of increasing the input impedance and reducing the output impedance. Because the gain of the op amp is extremely high, the input impedance of the op amp follower tends to be infinite, and the output impedance tends to zero. Within the rated output current range, the feedback voltage is equal to the output voltage, the output voltage is in phase with the input voltage, and the output voltage is slightly smaller than the input voltage. It should be noted that voltage follower is a special case of negative feedback amplifier (voltage series).
Op amp voltage follower is actually a simple circuit structure which play a role in impedance matching. When a weaker signal is used to drive a relatively high current, voltage follower is often added in the middle, so that it can make weak signal stronger. It improves the load capacity to a considerable extent, while ensuring that the waveform and amplitude of the signal remain unchanged.
For example, a single-chip microcomputer outputs a PWM signal to control LED lights. One LED does not require much current, so there is generally no big problem, however, when multiple LEDs need to light, current may definitely not large enough. If the current output is not enough, which may affect the signal output by the single-chip microcomputer, in this way, the voltage follower comes in handy.

voltage follower

Ⅱ Voltage Follower Characteristics

2.1 Op Amp Impedance Matching

When the op amp gain is approximately 1, that is, the magnification is approximately 1. The "follow" in the follower means that the voltage remains unchanged before and after, and the output waveform is almost not lost. It can be composed of transistors or operational amplifiers (best). Because the op amp input impedance is large and the output impedance is small, voltage follower can reduce the impact on the signal and improve the load capacity.

2.2 Buffer Amplifier & Isolation Amplifier

Here is a question, how to understand the buffering effect? Is the voltage of the former having a small impact on the back circuit? No, it is equivalent to a constant voltage source. Within the design requirement, no matter how the circuit connected to the subsequent stage changes, the output voltage is constant and does not change. In this way, the magnification or other performance of the previous stage can be kept unchanged. Otherwise, if the previous-stage input impedance is large, and the latter stage is small, the signal will definitely be distorted. For example, if a sinusoidal voltage waveform with a peak value of 10V, the sinusoidal peak value loaded to the latter stage may only be 8V. After adding a voltage follower, the waveform loaded on the input of the voltage follower will basically not change, and the input-output stage voltage ratio is very close to unity. So there will be no distortion.
Since the output impedance of the voltage amplifier is generally relatively high, usually in the range of several kiloohms to tens of kiloohms. If the input impedance of the subsequent stage is relatively small, part of the signal will be lost in the output resistance of the previous stage. At this time, a voltage follower is needed to buffer from it. Another advantage of applying a voltage follower is that the op amp input impedance is increased, so that the capacity of the input capacitance can be greatly reduced, which provides a prerequisite guarantee for the application of high-quality capacitors.
Another question, what about isolation? Because the op amp input impedance of the voltage follower is very large, it can be approximated as an open circuit. Of course, this open circuit is for the previous circuit. In this case, the previous circuit will not affect the subsequent circuit. However, "open circuit" means what, is it really open? No, the previous voltage is transmitted, but the corresponding current is not transmitted. This is the isolation effect.
For example, if the MCU outputs a PWM waveform, you want to use it to control the brightness of a small light bulb. However, the output capacity of the IO port of the general MCU is limited. You can directly use the PWM output from the IO port to drive one light bulb. More than one doesn't work. In this case, you can add a voltage follower, so that the voltage is still the original voltage, but the driving ability has improved. Of course, the output capacity is not increased out of thin air, but comes from the input power of the op amp. In electronics, the diode has current amplification capability, and its source of amplification capability also comes from the power supply.
In Hi-Fi circuits, the controversy about negative feedback has been around for a long time. In fact, if there is no negative feedback, most amplifying circuits will not work well. However, due to the introduction of a large loop negative feedback circuit, the back EMF of the speaker will pass through the feedback circuit and be superimposed with the input signal. The sound quality is blurred and the clarity is reduced. Therefore, some of the final stages of the power amplifier adopt a circuit without large loop negative feedback, trying to eliminate the disadvantages by disconnecting the negative feedback loop. However, since the operating current of the final stage of the amplifier varies greatly, its distortion is difficult to control. Here, the function of the voltage follower is just for the application. Putting the circuit between the front stage and the power amplifier can cut off the interference effect of the back electromotive force of the speaker on the front stage, so that the clarity of the sound quality is greatly improved.

 

Ⅲ Op Amp Follower Circuit Analysis

3.1 Op Amp Voltage and Load

Op Amp Voltage Follower Schematic

Figure 1. Op Amp Voltage Follower Schematic

The output and the inverting input terminal are connected in series with a 10k resistor to ensure excellent characteristics. An ac signal is input at the non-inverting input terminal. Of course, dc and ac are all okay, so you will get a very high voltage at the output terminal. AC voltage that is similar and has excellent load capacity, with buffering and isolation effects.

3.2 Op Amp Voltage Follower Stability

The problem of using a voltage follower to keep the operational amplifier stable, that is, how to reduce the oscillation in the amplifier circuit using negative feedback to maintain stability, there is still no final conclusion. The ideal operating state of the op amp is that the output voltage and the input voltage are in phase, that is, when the applied voltage at the negative input causes the output to increase, the op amp can reduce the increased voltage accordingly. However, there is always a difference in phase between the input and output in reality. When the phase difference between the output and the output is 180°, the negative input and the positive input are exactly the same, but the output that should have been reduced is enhanced. It becomes a state of positive and negative collapse. If it falls into this state in a specific frequency band and still maintains the original amplitude, then the output frequency and oscillation state will continue.

feedback loop

Figure 2. Feedback Loop

3.3 Op Amp Phase Difference Problem

The main reason for the phase difference between the input and the output:
1) Due to the inherent characteristics of op amps.
2) Due to the characteristics of the other feedback loop in circuit.

gain-phase-frequency curve

Figure 3. Gain-frequency, Phase-frequency Curve

Fig 3(a), Fig 3(b)and Fig 3(c) respectively represent the voltage gain-frequency characteristic and phase-frequency characteristic of the operational amplifier. As shown in the figure, the voltage gain and phase vary with frequency. The difference between the op amp gain and the gain after feedback (0dB when using a voltage follower) is the gain (feedback gain) of the feedback loop. If the feedback gain is less than 1 time (0dB), then, the phase changes by 180° and returns to the positive feedback state, the negative gain will gradually attenuate in the circuit and theoretically will not cause oscillation.
On the contrary, when the phase changes by 180°, if the loop gain corresponding to the frequency is 1 time, the original amplitude will be maintained. If the loop gain corresponding to the frequency is greater than 1, the amplitude will gradually diverge. In most cases, in the process of amplitude divergence, the amplitude is limited due to the influence of nonlinear elements such as the maximum output voltage, and the oscillation state will be maintained.
Therefore, the difference between the phase corresponding to the frequency when the loop gain is 0dB, 180° is an important factor for judging the stability of the negative feedback loop, and this parameter is called the phase margin. Unless otherwise specified, when a single amplifier is used as a voltage follower, sufficient phase margin must be maintained (Fig 3b.).

3.4 Adding Feedback Resistance

When the operational amplifier is used as a follower, when the internal resistance of the signal source is large, adding a feedback resistor with the same resistance as the internal resistance of the signal source can reduce the output offset voltage and improve the follow accuracy. The follower with feedback resistance has a certain current limiting protection effect on the circuit when the circuit is "blocked", which is its advantage.
The voltage follower is originally a non-inverting operational amplifier. One of the common features of it is that a common-mode voltage is added to the non-inverting terminal and the inverting terminal.
Once this common-mode voltage exceeds the allowable common-mode input voltage range, for example, if the inverting terminal signal is too large, it will cause the input stage transistor to saturate. The inverting terminal signal will be directly added to the second stage of the op amp, making the inverting input becomes non-inverting input, that is, negative feedback becomes positive feedback, and the output signal passes through the feedback loop to further saturate the input stage transistor. As a result of this, the amplifier is of course no longer in normal working condition. Even if the input signal is canceled, it will not immediately return to the normal state. This phenomenon is called blocking.
When it occurs, if the feedback loop resistance is not large enough, the current in the feedback loop may burn the input stage transistors and even harm the second stage. In order to avoid blocking, in addition to choosing an op amp with a large common-mode input voltage range, a clamp circuit is often added to the input of the amplifier to ensure that the common-mode voltage at the input does not exceed the allowable range.
Of course, in a small-signal inverting operational amplifier, especially in circuits with capacitive elements such as integrating operational amplifiers, blocking may also occur. The processing method is the same as that of the non-inverting amplifier.

 

Ⅳ Op Amp Voltage Follower Application

In many typical circuit designs, there will be an op amp follower before the AD converter. Whether this follower is necessary or not depends on the requirements of the circuit based on the understanding of the function of the follower. First analyze the role of the voltage follower here:
The function of the voltage follower here is impedance transformation.
Impact 1: The input impedance becomes very high, so that the impact on the input signal can be small.
Impact 2: The output impedance becomes very low, and the impact of AD input impedance on the input signal can be very small.
It can be seen that the follower is very meaningful. Secondly, analyze your own circuit and the signal under test to make a decision whether to use a follower. Here are some rules to confirm:
1) If the output impedance of the signal is very small, then the Impact 1 can be ignored.
2) If the input impedance of AD converter is very large, then two impacts can be ignored.
3) If both impacts can be ignored, no voltage follower is necessary.
4) If there is an impact, a voltage follower is needed.

 

Frequently Asked Questions about Op Amp Voltage Follower

1. Which amplifier is called as voltage follower Why?
This means that the op amp does not provide any amplification to the signal. The reason it is called a voltage follower is because the output voltage directly follows the input voltage, meaning the output voltage is the same as the input voltage.

 

2. What is the use of voltage follower?
A voltage follower can be used as a buffer because it draws very little current due to the high input impedance of the amplifier, thus eliminating loading effects while still maintaining the same voltage at the output.

 

3. What do you mean by voltage follower circuit?
A voltage follower is also known as a unity gain amplifier, a voltage buffer, or an isolation amplifier. In a voltage follower circuit, the output voltage is equal to the input voltage; thus, it has a gain of one (unity) and does not amplify the incoming signal.

 

4. What is an op amp buffer?
An op-amp voltage buffer mirrors a voltage from a high-impedance input to a low-impedance output. 8 min read. A voltage buffer, also known as a voltage follower, or a unity gain amplifier, is an amplifier with a gain of 1. It's one of the simplest possible op-amp circuits with closed-loop feedback.

 

5. What is an op amp buffer circuit used for?
A buffer is a unity gain amplifier packaged in an integrated circuit. Its function is to provide sufficient drive capability to pass signals or data bits along to a succeeding stage. Voltage buffers increase available current for low impedance inputs while retaining the voltage level.

Ordering & Quality

Photo Mfr. Part # Company Description Package PDF Qty Pricing
(USD)
INA129P INA129P Company:Texas Instruments Remark:Instrumentation Amplifier 1 Circuit 8-PDIP Package:8-DIP (0.300"", 7.62mm)
DataSheet
In Stock:On Order
Inquiry
Price:
1+: $9.06000
10+: $8.18100
25+: $7.80040
100+: $6.46850
250+: $5.89776
500+: $5.51726
1000+: $4.94650
2500+: $4.75625
5000+: $4.68015
Inquiry
MCP6054T-E-SL MCP6054T-E-SL Company:Microchip Technology Remark:IC OPAMP GP 4 CIRCUIT 14SOIC Package:14-SOIC (0.154", 3.90mm Width)
DataSheet
In Stock:On Order
Inquiry
Price:
2600+: $1.28000
Inquiry
TLV2474IDR TLV2474IDR Company:Texas Instruments Remark:CMOS Amplifier 4 Circuit Rail-to-Rail 14-SOIC Package:14-SOIC (0.154"", 3.90mm Width)
DataSheet
In Stock:On Order
Inquiry
Price:
1+: $3.01000
10+: $2.69900
25+: $2.55160
100+: $2.09790
250+: $1.88244
500+: $1.81440
1000+: $1.50822
2500+: $1.45152
5000+: $1.40616
10000+: $1.36080
Inquiry
MC33178PG MC33178PG Company:ON Semiconductor Remark:IC OPAMP GP 2 CIRCUIT 8DIP Package:8-DIP (0.300", 7.62mm)
DataSheet
In Stock:On Order
Inquiry
Price:
Call
Inquiry
OPA192QDGKRQ1 OPA192QDGKRQ1 Company:Texas Instruments Remark:General Purpose Amplifier 1 Circuit Rail-to-Rail 8-VSSOP Package:8-TSSOP, 8-MSOP (0.118"", 3.00mm Width)
DataSheet
In Stock:On Order
Inquiry
Price:
1+: $3.16000
10+: $2.84200
25+: $2.68640
100+: $2.20890
250+: $1.98204
500+: $1.91040
1000+: $1.58802
2500+: $1.52832
5000+: $1.48056
10000+: $1.43280
Inquiry
TLE2082IDRG4 TLE2082IDRG4 Company:Texas Instruments Remark:J-FET Amplifier 2 Circuit 8-SOIC Package:8-SOIC (0.154"", 3.90mm Width)
DataSheet
In Stock:On Order
Inquiry
Price:
2500+: $1.56672
Inquiry
OPA4172AQPWRQ1 OPA4172AQPWRQ1 Company:Texas Instruments Remark:IC OPAMP GP 4 CIRCUIT 14TSSOP Package:14-TSSOP (0.173", 4.40mm Width)
DataSheet
In Stock:2000
Inquiry
Price:
2000+: $1.50272
6000+: $1.45576
10000+: $1.40880
Inquiry
MCP6S28T-I-SL MCP6S28T-I-SL Company:Microchip Technology Remark:IC OPAMP PGA 8 CIRCUIT 16SOIC Package:16-SOIC (0.154", 3.90mm Width)
DataSheet
In Stock:On Order
Inquiry
Price:
2600+: $1.69000
1+: $2.24000
25+: $1.86000
100+: $1.69000
Inquiry

Related Articles

Definition of Power Amplifier and Its Classification

Apogeeweb 20 Mar 2018  3498

Warm hints: The word in this article is about 3000 words and the reading time is about 15 minutes. 1 What is Power AmplifierIn this article, we will mainly talks about how power amplifier works and i...

Continue reading »

Using Op Amps as Comparators Characteristics Overview

Apogeeweb 18 Aug 2020  2857

IntroductionIn electronics, operational amplifiers are generally dual/quadruple configurations. So users can consider using the extra amplifier as a comparator. Electrical symbols of the comparator an...

Continue reading »

Typical Parameters of Op Amp and Common Types Explained

Apogeeweb 15 Jan 2021  243

IntroductionAn operational amplifier, or op amp is used in a wide variety of applications in electronics. It generally comprises a differential-input stage with high input impedance, an intermediate-g...

Continue reading »

Non-Inverting and Inverting Amplifiers Basic Analysis

Apogeeweb 13 Nov 2019  3346

Ⅰ. IntroductionIn electronics, an operational amplifier is a circuit unit with a very high amplification factor. In the actual circuit, usually combined with the feedback network to form a certai...

Continue reading »

Operational Amplifier Principle and Circuit

Apogeeweb 15 Jan 2019  4903

Warm hints: This article contains about 6000 words and reading time is about 22 mins. Introduction The operational amplifier is referred to as an op amp. It was named "Operational Amplifier" because...

Continue reading »

Ideal Op-Amp Circuit Characteristics Update

Apogeeweb 6 Nov 2020  1650

IntroductionOperational amplifier (op amp for short) is basically a voltage amplifying device designed to be used with components like capacitors and resistors, between its in/out terminals, or i...

Continue reading »

pinglun 0 comment

Leave a Reply

Your email address will not be published.

 
 
   
 
code image
Rating: poor fair good very good excellent

# 0 1 2 3 4 5 6 7 8 9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z