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Amplifier Tutorial: Amplifier Basic and Amplifier Circuit

Author: Apogeeweb
Date: 3 Jun 2019
operational amplifier diagram

An amplifier is a device that amplifies the voltage or power of an input signal, consisting of tubes or transistors, power transformers, and other electrical components. Used in communications, broadcasting, radar, television, automatic control and other devices.

Op-amps are the backbone of analog circuit design, and it has two differential inputs and very high gain. This video describes the symbol and properties of an op-amp.



Ⅰ Introduction

Ⅱ History

Ⅲ Application

Ⅳ Op-Amp Design

4.1 Summary

4.2 Basic Structure

4.3 Classifications

Ⅴ Amplifier Circuits

5.1 Small-signal Common-emitter Amplifier Circuit

5.2 Small-signal Common-collector Amplifier Circuit

5.3 Small-signal Common-base Amplifier Circuit

Ⅵ Question Related to Op Amp and Going Further

Ⅰ Introduction

A device that increases signal amplitude or power, which is an important component of processing signals in automated technology tools. The amplification of the amplifier is realized by the input signal to control the energy, and the power consumption required for amplification is provided by the energy. For linear amplifiers, the output is the reproduction and enhancement of the input signal; for a non-linear amplifier, the output has a certain functional relationship with the input signal. The amplifier is divided into mechanical amplifiers, electromechanical amplifiers, electronic amplifiers, hydraulic amplifiers and pneumatic amplifiers according to the physical quantities of the processed signals.

The most widely used ones are electronic amplifiers. With the popularization of jet technology, the application of hydraulic or pneumatic amplifiers has gradually increased. The electronic amplifiers are further classified into vacuum tube amplifiers, transistor amplifiers, solid-state amplifiers, and magnetic amplifiers according to the active devices used, and the transistor amplifiers are the most widely used. In automation instruments, transistor amplifiers are commonly used for voltage amplification and current amplification of signals. The main forms are single-ended amplification and push-pull amplification. In addition, it is often used for impedance matching, isolation, current-to-voltage conversion, charge-to-voltage conversion (such as charge amplifiers), and the use of amplifiers to achieve a certain functional relationship between the output and the input (such as an operational amplifier).

high power digital amplifier

Ⅱ History

The first practical device that could amplifier was the triode vacuum tube invented by Lee De Forest in 1906, which produced the first amplifiers around 1912. Until the 1960s and 1970s, vacuum tubes were used almost exclusively for all amplifiers, and transistors invented in 1947 replaced vacuum tubes. Today, most amplifiers use transistors, but vacuum tubes are still used in some applications.

The development of audio communication technology in the form of telephone, first patented in 1876, created the need to increase the amplitude of electrical signals to extend the longer distances of signal transmission. In telegraph, this problem has been solved by intermediate devices at the site that supplement the energy consumed by back-to-back operation of the signal recorder and transmitter to form a relay that enables the local energy of each intermediate station to power the next transmission segment. For the duplex transmission of two-way transmission and reception, starting from the telegraph transmission work of C.F.Varley, a two-way relay repeater was developed. Duplex transmission is essential for the telephone, and it was not solved until 1904. When H. E. Shreeve of the American Telephone and Telegraph Company improved the existing back-to-back carbon-granule emitter and electrodynamic receiver pair, and the attempt to build a telephone repeater. The Shreeve repeater was first tested on the line between Boston and Amesbury, MA., and the more sophisticated equipment was still in use for a while. After the turn of the century, it was discovered that negative resistance mercury lamps can be amplified or used in repeaters, but with little success.

Since 1902, the development of thermionic valve has provided a fully electronic way of amplifying signals. The first practical version of this device was the Audion triode invented by Lee De Forest in 1906, which produced the first amplifier around 1912. Since the only device previously used to enhance the signal is the relay used in the telegraph system, the amplifying vacuum tube was originally called an electron relay. The terms "amplifier" and "amplification" are derived from the Latin "amplificare" (for magnification or enlargement), which was first used for this new capability around 1915, when the trio became popular.

The amplification of the vacuum tube revolutionized electrical technology and created a new field of electronics, namely active electronic device technology. It made long-distance telephone lines, public address systems, radio broadcasting, talking motion pictures, practical audio recording, radar, television and the first computer possible. For almost 50 years, vacuum tubes have been used in almost all consumer electronic devices. Early tube amplifiers often had positive feedback (regeneration), which increased gain, but also made the amplifier unstable and prone to oscillation. From the 1920s to the 1940s, Bell Telephone Laboratories developed mathematical theories for many amplifiers. Until Harold Black developed negative feedback in 1934, this caused the distortion level to be greatly reduced, but at the expense of reduced gain, at the status of the distortion level of early amplifiers was always high, usually around 5%. In addition, Harry Nyquist and Hendrick Wade Bode made other advances in the theory of magnification.

Vacuum tubes are actually the only amplification devices for 40 years, not specialized power devices such as magnetic amplifiers and amplidyne. Power control circuits used magnetic amplifiers, and until the second half of the twentieth century, power semiconductor devices became more economical and had higher operating speeds. The old Shreve electroacoustic carbon repeater was used in an adjustable amplifier in the telephone subscriber sets for the purpose of hearing impaired until the 1950s transistor provided a smaller, higher quality amplifier.

In the 1960s and 1970s, the replacement of bulky tubes with transistors created a revolution in electronics that made a wide range of portable electronic devices possible, such as the transistor radio developed in 1954. Today, for some high power applications, such as radio transmitters, use of vacuum tubes is limited.

Since the 1970s, more and more transistors have been connected to a chip, resulting in higher integration (small, medium, and large scale, etc.) in integrated circuits. At present, many of the amplifiers available on the market are based on integrated circuits.

For special purposes, other active elements have been used. For example, in the early days of satellite communication, parametric amplifiers were used. The core circuit is a diode whose capacitance was changed by the local RF signal. In some cases, this RF signal provides energy and is modulated by extremely weak satellite signals received by earth stations.

Since the later 20th century, the development of digital electronics has provided new alternatives to traditional linear gain amplifiers, which use digital switches to change the pulse shape of a fixed amplitude signal to produce devices such as Class D amplifiers.


Ⅲ Application

Principle: The high-frequency power amplifier is used in the final stage of the transmitter. The function is to amplify the high-frequency modulated signal to meet the requirements of the transmission power, and then radiate it to the space through the antenna to ensure reception in a certain area, therefore, the machine can receive a satisfactory signal level in a certain region and does not interfere with the communication of adjacent channels.

High frequency power amplifiers are an important component of transmitting devices in communication systems. It is divided into narrow-band high-frequency power amplifier and wide-band high-frequency power amplifier according to the width of its working frequency band. The narrow-band high-frequency power amplifier usually takes the frequency selection circuit with the function of frequency selection filter as the output circuit, so it is also called a tuned power amplifier or resonant power amplifier; the output circuit of the wideband high-frequency power amplifier is a transmission line transformer or other broadband matching circuit, so it is also called a non-tuned power amplifier. A high-frequency power amplifier is an energy conversion device that converts the DC energy supplied by the power supply into a high-frequency AC output.

The amplifier can be divided into three types according to the different conduction angles of the current. Class A, Class B and Class C. Class A amplifier currents have a flow angle of 360o and are suitable for small signal low power amplification. The flow angle of the Class B amplifier current is approximately equal to 180o; the flow angle of the Class C amplifier current is less than 180o. Both Class B and Class C are suitable for high-power work. The output power and efficiency of the Class C operating state are the highest of the three operating states. Most of the high frequency power amplifiers are Class C. However, the current waveform distortion of the class C amplifier is too large to be used for low frequency power amplification, and can only be used for resonant power amplification using a tuning loop as a load. Since the tuning loop has filtering capability, the loop current and voltage are still very close to a sinusoidal waveform with little distortion, and the distortion is very small.

op amp symbol

Op Amp Symbol

Ⅳ Op-Amp Design

4.1 Summary

An operational amplifier is one of the most common and important units in an analog-to-digital conversion circuit. Fully differential op amps mean that the input and output are differential signals. Compared with ordinary single-ended output op amps, they have the following advantages: The output voltage swing is large, the common-mode noise is better suppressed, the noise is lower, and the output voltage swing is large, even order terms that suppress harmonic distortion are better. Therefore, high performance op amps are often used in fully differential form. In recent years, the higher unity gain bandwidth of the fully differential op amp and the larger output swing make it more widely used in high speed and low voltage circuits. With the increasing data conversion rate, high-speed analog-to-digital converters are becoming more and more demanding, and high-speed analog-to-digital converters require high gain and high unity gain bandwidth op amps to meet system accuracy and rapid setup requirements. Speed and accuracy are the two most important performance indicators of analog circuits. However, the requirements of the two are mutually restricted and contradictory. Therefore, it is difficult to meet both requirements at the same time. However, the The folded common gate technique can solve this problem more successfully. The op amp of this structure has a high open loop gain and a high unity gain bandwidth. The disadvantage of the fully differential op amp is that the common mode loop gain of its external feedback loop is small, and the output common mode level cannot be accurately determined. Therefore, a common mode feedback circuit is generally required.

4.2 Basic Structure

There are three important types of op amps:

(a) simple two-stage op amps

(b) folded common gate op amps

(c) common gate op amps

The design specifications of the op amp designed this time require a differential output amplitude of ±4V, that is, the sum of VDSAT and N of all NMOS transistors at the output is less than 0.5V, and the sum of VDSAT and P of all PMOS transistors at the output must also be less than 0.5V.

(1) main structure

A. Cascode  B. DC gain and common source

(2) Common mode negative feedback

For fully differential op amps, a common mode negative feedback circuit should be added to stabilize the output common-mode voltage. When designing a fully differential operational amplifier with balanced output, the following must be considered: the open-loop DC gain requirement for common-mode negative feedback is large enough, preferably equal to the difference-divided loop DC gain; unity gain for common-mode negative feedback, the bandwidth is also required to be large enough, preferably close to the differential unity gain bandwidth. In order to ensure the stability of the common mode negative feedback: common mode loop compensation is generally required; the common mode signal monitor requires good linearity; the common mode negative feedback is independent of the difference mode signal, even if the difference mode signal path is turn it off.

In addition, the op amps take a continuous time approach to realize common mode negative feedback.

This structure shares the current mirror and output load in the input stage of the common mode amplifier and the differential mode amplifier. In this way, on the one hand, the power consumption is reduced; on the other hand, the AC characteristics of the common mode amplifier and the differential mode amplifier are kept consistent. Because the output stage of the common mode amplifier and the output stage of the differential mode amplifier can be completely shared, so can the capacitance compensation circuit. As long as the frequency characteristic of the differential mode amplifier is stable, the common mode negative feedback is also stable. This common-mode negative feedback circuit allows a fully differential op amp to be designed like a single-ended output op amp, regardless of the effect of the common-mode negative feedback circuit on the fully differential amplifier.

(3) Voltage bias circuit: wide swing current

To maximize the dynamic range of the input stage, a wide swing current source is used to generate the three required bias voltages which required in the common source and common gate input stage.


4.3 Classification

1. Main categories and introduction of integrated operational amplifiers

(1) general integrated operational amplifier

The general integrated operational amplifier means that its technical parameters are moderate and can meet the requirements of most cases. The general-purpose integrated operational amplifier is further divided into type I, type II and type III, wherein type I is a low gain operational amplifier, type II is a medium gain operational amplifier, and type III is a high gain operational amplifier. Types I and II are basically early products with an input offset voltage of around 2mV and an open loop gain of typically greater than 80 dB.

(2) High-precision integrated operational amplifier

High-precision integrated op amps are those that have small offset voltages, small temperature drift, and high gain and common-mode rejection ratios, and the noise of such op amps is also relatively small. The offset voltage of a single high-precision integrated operational amplifier can be as small as a few microvolts, and the temperature drift is as small as tens of microvolts per degree Celsius.

(3) High-speed integrated operational amplifier

High-speed integrated op amps have high output voltage conversion rates, some up to 2~3kV/μS.

(4) High input impedance integrated operational amplifier

The input impedance of the high input impedance integrated op amp is very large and the input current is very small, and the input stage of such an operational amplifier often uses a MOS transistor.

(5) Low power consumption integrated operational amplifier

Low power consumption integrated op amps operate at very low currents and supply voltages, for example, the entire op amp may consume only a few tens of microwatts. And these integrated operational amplifiers are mostly used in portable electronics.

(6) Broadband integrated operational amplifier

Broadband integrated op amps have a wide frequency band and unity gain bandwidths up to gigahertz, often used in broadband amplifier circuits.

(7) High voltage integrated operational amplifier

Ordinary integrated op amps have supply voltages below 15V, while high-voltage integrated op amps have supply voltages of tens of volts.

(8) Power integrated operational amplifier

The output stage of the power integrated operational amplifier can provide large power output to the load.

2. Optical fiber amplifier

Optical fiber amplifier can not only amplify the optical signal directly, but also has the functions of real-time, high gain, broadband, online, low noise and low loss, so it is an indispensable key device in the new generation optical fiber communication system. Because this technology not only solves the limitation of attenuation on the transmission rate and distance of optical network, but also creates the wavelength division multiplexing of 1550nm frequency band, which will make ultra-high speed, super large capacity and super long distance wavelength division multiplexing (WDM), dense wavelength division multiplexing (DWDM), all-optical transmission and optical soliton transmission have become a reality, which is an era-making mileage in the development history of optical fiber communication. In the present practical optical fiber amplifier, erbium-doped fiber amplifiers (EDFA), semiconductor optical amplifiers (SOA), and Raman fiber amplifier (FRA) are mainly used. Because the outstanding features of EDFA, it is widely used in high-rate optical fiber communication system, access network, optical fiber CATV network, military system (radar multi-path data multiplexing, data transmission, guidance, etc.) and so on, as power amplifier, relay amplifier and pre-amplifier.

Optical fiber amplifiers are typically composed of a gain medium, pump light, and an input-output coupling structure. According to their applications in fiber-optic networks, optical fiber amplifiers mainly have three different uses: as a power amplifier on the transmitter to improve transmission; the optical pre-amplifier is used before the receiver to greatly improve the sensitivity of the optical receiver; used as a relay amplifier in the optical fiber transmission line to compensate for the transmission loss of the optical fiber and prolong the transmission distance.


Ⅴ Amplifier Circuit

How to determine whether a basic amplifier circuit is common-base, common-emitter or common collector? Remove the input and output, and the rest is common terminal, as shown in the following figure: ui is connected to the base, uo is connected to the emitter, the rest is a basic common collector circuit.

Basic Amplifier Circuit 

Figure 1. Basic Amplifier Circuit

5.1 Small-signal Common-emitter Amplifier

Small-signal common-emitter amplifier circuit

Figure 2. Small-signal Common-emitter Amplifier Circuit


Amplify small amplitude voltage signal

Operational principle:

Q1 is connected to a common-emitter configuration single transistor amplifier, R1 and R2 provide base bias voltage, R4 is a stable operating point, R3 is a collector load resistor. R5 is load, R6 is signal-source internal resistor. The change of signal-source voltage causes the change of iB of Q1 and the change of iC. After R3 is converted into vCE, the voltage of C2 through AC coupling change is output to R5.

Normal working condition: Q1 is in the amplification region, and its collector voltage is about 6 V.

Adjusting method: 

Adjust the resistance of resistor R1 and measure the collector voltage of Q1 to 6 V.

Measurement parameters

1) Measure the terminal voltage of the signal source Vs, AC voltage Vi of Q1, AC voltage Vo of resistor R5, and calculate voltage gain AV and source voltage gain AVS.

2) Measure the phase difference between the input signal and the output signal.

3) When the amplitude of the output signal decreases to 0.7 times of the low frequency, the frequency of the input signal is recorded, and the passband BW is obtained by increasing the frequency of the input signal and keeping the input voltage unchanged.

4) Increase the amplitude of the input signal, measure the output signal, and record the amplitude of the input signal and the output signal when the output signal begins to clip.

5) Measure the input current of the power supply and calculate the efficiency of the amplifier.


5.2 Small-signal Common Collector Amplifier

Small-signal common collector amplifier  Circuit

Figure 3. Small-signal Common Collector Amplifier Circuit


Voltage signal impedance transformation

Operational principle:

Q1 is connected to a common-collector single-tube amplifier. R1 and R2 provide the base bias voltage, R4 is a stable operating point, R5 is the load, and R6 is the signal source internal resistance. The change of the signal source voltage causes the iB of Q1 to change, causing the IE to change. Since IE is much larger than ib, current amplification (impedance conversion function) can be realized.

Normal working condition: Q1 is in the amplification area, and the emitter voltage of Q1 should be measured to be about 6V.

Adjusting method: 

Adjust the resistance of resistor R1 and measure the emitter voltage of Q1 to 6V.

Measurement parameters

1) Measure the terminal voltage Vs of the signal source, the AC voltage Vi of the base of Q1, the AC voltage Vo of the resistor R5, and calculate the voltage gain AV and the source voltage gain AVS.

2) The phase difference between the input signal and the output signal is measured.

3) Increasing the frequency of the input signal, the input voltage is constant, and the input signal frequency is recorded when the output signal amplitude is reduced to 0.7 times the low frequency, thereby obtaining the passband BW.

4) Increase the input signal amplitude, measure the output signal, and record the input signal and output signal amplitude when the output signal begins to clip.

5) Measure the power input current and calculate the efficiency of the amplifier.


5.3 Small-signal Common-base Amplifier


Small-signal common-base amplifier Circuit

Figure 4. Small-signal Common-base Amplifier Circuit


Small signal wideband voltage amplification

Operational principle:

Q1 is connected to a common-base configuration single-tube amplifier, R1, R2 provide the base bias voltage, R4 stabilizes the operating point, and R3 is the collector load resistor. R5 is the load and R6 is the internal resistance of the signal source. The change of the signal source voltage causes the change of Q1, causing the iC to change. After R3 is converted into vCE, the voltage converted by C2 AC coupling is output to R5.

Normal working condition:

Q1 is in the amplification area, and the collector voltage of Q1 is measured to be about 6V.

Adjusting method:

Adjust the resistance of resistor R1 and measure the collector voltage of Q1 to 6V.

Measurement parameters

1) Measure the terminal voltage Vs of the signal source, the AC voltage Vi of the emitter of Q1, the AC voltage Vo of the resistor R5, and calculate the voltage gain AV and the source voltage gain AVS.

2) Measure the phase difference between the input signal and the output signal.

3) Increasing the frequency of the input signal, the input voltage is constant, and the input signal frequency is recorded when the output signal amplitude is reduced to 0.7 times the low frequency, thereby obtaining the passband BW.

4) Increase the input signal amplitude, measure the output signal, and record the input signal and output signal amplitude when the output signal begins to clip.

5) Measure the power input current and calculate the efficiency of the amplifier.


6.1 Question

What is the ideal op amp?

6.2 Answer

The ideal op amp is an amplifier with infinite input impedance, infinite open-loop gain, zero output impedance, infinite bandwidth, and zero noise. It has positive and negative inputs which allow circuits that use feedback to achieve a wide range of functions.


Frequently Asked Questions about Amplifier Tutorial

1. What is amplifier and its types?
Voltage Amplifier: An amplifier that amplifies given voltage for a larger voltage output. ... It is characterized by a high input impedance and low output impedance. Transconductance Amplifier: An amplifier that changes output current according to changing input voltage.


2. What is the amplifier used for?
An electronic amplifier is a device that is used to increase the power, current, or voltage of a signal. Amplifiers are used in music equipment, electronic devices such as television and radio receivers, audio equipment, and computers to increase the amplitude of a signal.


3. How do you design an amplifier?
The most basic form of common emitter amplifier design is the simple logic buffer / output, consisting of a transistor and a couple of resistors. This can have a few extra components added to enable it to become an AC coupled amplifier with DC biasing and emitter bypass resistor.


4. Which class amplifier is best?
Class “A” amplifiers are considered the best class of amplifier design due mainly to their excellent linearity, high gain and low signal distortion levels when designed correctly.


5. What is the current amplifier?
A Current amplifier is an electronic circuit that increases the magnitude of current of an input signal by a fixed multiple, and feeds it to the succeeding circuit/device.


6. Is Class D amplifier better?
So the more often the swings take place, the more opportunity for waste. Even so, the efficiency of a practical Class-D amplifier can be better than 90 percent, which is significantly better than a Class-AB design (78.5 percent at best and typically closer to 50 percent).

7. How do I choose an amplifier?
Generally you should pick an amplifier that can deliver power equal to twice the speaker's program/continuous power rating. This means that a speaker with a “nominal impedance” of 8 ohms and a program rating of 350 watts will require an amplifier that can produce 700 watts into an 8 ohm load.

8. Which amplifier has highest efficiency?
Class D amplifier
Class D amplifier is the highest power efficient amplifier class in the A, B, AB, and C and D segment. It has smaller heat dissipation, so small heatsink is needed. The circuit requires various switching components like MOSFETs which has low on resistance.

9. How does an amplifier amplify sound?
Some types of audio equipment use two separate amplifiers—a pre-amplifier ("pre-amp") and a main amplifier. The pre-amplifier takes the original signal and boosts it to the minimum input level that the main amplifier can handle. The main amplifier then boosts the signal enough to power loudspeakers.

10. How many types of audio amplifiers are there?
In electronics, Amplifier is the most commonly used circuit device with huge application possibilities. In Audio related electronics pre-amplifier and power amplifiers are two different types of amplifier systems which are used for sound amplification related purposes.


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