Amplifier

An amplifier is a two-port electronic device used to amplify the signal or increase the power of a signal with the help of a power supply. The power is supplied through the input terminal of the amplifier. The output of the amplifier can be the increased amplitude, etc.

The gain of the amplifier determines its amplification. It is the major factor that determines the output of a device. Amplifiers are used in almost every type of electronic component. The gain is calculated as the ratio of the output parameter (power, current, or voltage) to the input parameter.

Amplifiers are used in various applications, such as automation, marine, sensors, etc. The power gain of an amplifier is generally greater than one. Let's understand some basic characteristics of an ideal amplifier.

Here, we will discuss an ideal amplifier, types of amplifiers, properties, functions, and applications of amplifiers.

Let's start.

Ideal Amplifier

Let's consider the characteristics of an ideal amplifier, which are listed below:

• Input impedance: Infinite
• Output impedance: Zero
• Gain at different frequencies: Fixed

The input port of an amplifier can be the voltage source or the current source. The voltage source only depends on the input voltage and accepts no current. Similarly, the current source accepts the current and no voltage. The output will be proportional to the voltage or current throughout the port.

The output of an ideal amplifier can either be a dependent current source or a dependent voltage source. The source resistance of the dependent voltage source is zero, while that of the dependent current source is infinite.

The voltage or current of the dependent source only depends on the input voltage or current. It means that output voltage will depend on the input voltage, and output current will depends on the input current independent voltage source and current source, respectively.

The ideal amplifiers are further categorized as CCCS (Current Control Current Source), CCVS (Current Control Voltage Source), VCVS (Voltage Control Voltage Source), and VCCS (Voltage Control Current Source).

The input impedance of the CCVS and CCCS is zero, while VCCS and VCVS are infinite. Similarly, the output impedance of the CCCS and VCCS is infinite, while that of CCVS and VCVS is zero.

Types of Amplifier

Let's discuss the different types of amplifiers.

Operational Amplifiers

Operational Amplifiers or Op-amps are high-gain direct coupled (DC) amplifiers that perform various mathematical operations, such as addition, differentiation, subtraction, integration, etc.

It has two input terminals and one output terminal. The input terminals are called inverting and non-inverting terminals. The signal applied to the inverting terminal will appear as phase-inverted, and the signal applied to the non-inverting terminal appears without any phase inversion at the output terminal.

The voltage applied at the inverting input is represented as V- and voltage at the non-inverting input is represented as V+.

Note: The output Impedance and drift of an ideal op-amp are 0. The voltage gain, input impedance, and bandwidth of an ideal op-amp are infinity.

The operational amplifiers are further categorized as Inverting and Non-Inverting Amplifiers. Let's discuss the above two types of operational amplifiers in detail.

Applications

Op-amps are used in various applications in electronics. For example,

• Filters
• Voltage comparator
• Integrator
• Current to voltage converter
• Summer amplifier
• Phase shifter

The inverting and non-inverting input of an amplifier is shown below:

Inverting Amplifier

The inverting amplifier is shown below:

It is the voltage shunt feedback configuration of the op-amp. A signal voltage applied to the inverting input of the op-amp results in the flow of current I1 into the op-amp. We know that the input impedance of the op-amp is infinity. It will not allow the current to flow into the amplifier. The current will flow through the output loop (through resistance R2) to the op-amp's output terminal.

The voltage gain at the output terminal of the inverting amplifier is calculated as:

A =Vo/Vs = -R2/R1

Where,

Vo and Vs are the output and signal voltage.

The negative sign depicts that the output of the amplifier is 180 degrees out-of-phase with the input.

Inverting amplifier is one of the most used op-amps. It has very low input and output impedances.

Non-Inverting Amplifier

The non-inverting amplifier is shown below:

The above configuration is the voltage-series feedback connection. A signal voltage applied to the op-amp's non-inverting input results in the flow of current I1 into the op-amp and current I2 out of the op-amp.

According to the concept of a virtual short circuit, I1 = I2 and Vx =Vs.

The voltage gain of the non-inverting amplifier can be calculated as:

A = A + (R2/R1)

Non-inverting amplifiers have high input and low output impedances. It is also regarded as the voltage amplifier.

DC Amplifiers

DC or Direct Coupled Amplifiers are used for amplifying low-frequency and direct-coupled signals. The two stages of a DC amplifier can be interconnected by using a direct coupling between these stages.

Direct coupling is a simple and easy type of connection. It can be calculated by directly connecting the first-stage transistor's collector to the second-stage transistor base, mentioned as T1 and T2.

But, DC amplifiers causes two problems called drift shifting and level shifting. The design of the Differential Amplifier removed such problems. Let's discuss the differential amplifier.

Differential Amplifiers

The structure of the differential amplifier solved the problem of drift and level shifting. The structure comprises two BJT (Bipolar Junction Transistor) amplifiers connected only through the power supply lines. It is named as a differential amplifier because the output of the amplifier is the difference between the individual inputs, as represented below:

Vo = A (Vi1 - Vi2)

Where,

Vo is the output, and Vi1 and Vi2 are the two inputs.

A is the gain of the differential amplifier.

Now, if

Vi1 = -Vi2

Vo = 2AVi1 = 2AVi

The above operation is called a differential mode operation. Here, the input signals are out of phase with each other. Such out-of-phase signals are known as difference-mode (DM) signals.

If,

Vi1 = Vi2

Vo = A (Vi1 - Vi1)

Vo = 0

This operation is known as common-mode (CM) because the input signals are in phase with each other. The zero output of such signals depicts that there will be no drift in the amplifier.

Power Amplifiers

Power amplifiers are also called current amplifiers. These amplifiers are required to raise the current level of an incoming signal to easily drive the loads. The types of power amplifiers include audio power amplifiers, radio frequency power amplifiers, etc.

Power amplifiers are classified as Class A, Class AB, Class B, and Class C amplifiers. We will discuss the power amplifier classes later in this topic.

Switch mode Amplifiers

Switch-mode amplifiers are a type of non-linear amplifier with high efficiency.

A common example of such type of amplifiers is class D amplifiers.

Instrumental Amplifier

The instrumental amplifier is used in analog sensing and measuring instruments. Let's consider an example.

A voltmeter used to measure very low voltages requires an Instrumental amplifier for its proper functioning. It has various features, such as very high voltage gain, good isolation, very low noise, low power consumption, large bandwidth, etc.

Negative feedback

Negative feedback is one of the essential features to control the distortion and bandwidth in amplifiers. The primary purpose of negative feedback is to reduce the gain of the system. The part of the output in the opposite phase fed back into the input. The value is further subtracted from the input. In the distorted output signal, the output with distortion is fed back in the opposite phase. It is subtracted from the input; we can say that negative feedback in amplifiers reduces the non-linearities and unwanted signals.

The below image represents negative feedback:

With the help of negative feedback, crossover distortion and other physical errors can also be eliminated. The other advantages of using negative feedback are bandwidth extension, rectifying temperature changes, etc.

The negative feedback can be a voltage negative feedback or current negative feedback. In both cases, the voltage or current feedback is proportional to the output.

We should not get confused between positive and negative feedback. Positive feedback tends to amplify the change, while negatives feedback tends to reduce the change. Another difference is that the input and output signals in positive feedback are in phase and get added. In the case of negative feedback, the input and output signals are out of phase and subtracted.

Active devices in amplifier

The amplifier consists of some active devices that are responsible for the amplification process. It can be a single transistor, vacuum tube, a solid-state component, or any part of the Integrated circuits.

Let's discuss the active devices and their role in the amplification process.

BJT

BJT is commonly known as a current-controlled device. Bipolar Junction Transistors are used as switches to amplify the current in amplifiers.

MOSFET

MOSFET or Metal Oxide Semiconductor Field Effect Transistors are commonly used in the amplification of electronic signals. MOSFETs can be used to change the conductivity by controlling the gate voltage. MOSFET can also enhance the strength of the weak signal. Hence, MOSFETs can be used as an amplifier.

Vacuum tubes amplifiers

The vacuum tube amplifier uses vacuum tubes as the source device. It is used to increase the amplitude of the signal. Below the microwave frequencies, tube amplifiers were replaced by solid-state amplifiers around the late 19^th century.

Microwave amplifiers

Microwave amplifiers are commonly used in microwave systems. It is used to raise the level of the input signal with very little distortion. It can also switch or raise electric power. It provides better single device output as compared to solid-state devices at microwave frequencies.

Magnetic amplifiers

Magnetic amplifiers were developed in the 20^th century to overcome the drawbacks (high current capacity and strength) of the vacuum tube amplifiers. Magnetic amplifiers are similar to transistors. It controls the magnetic strength of the core by energizing the control coil (another winding coil).

Integrated Circuits

Integrated circuits can hold several electronic devices, such as capacitors and transistors. The popularity of IC has also spread electronic devices all over the world.

Power Amplifier Classes

Power amplifier classes are classified as class A, class B, class AB, and class C. Let's discuss a short description of the power amplifier classes.

Class A power amplifiers

The input of the class A amplifier is small, due to which the output is also small. Hence, it does not produce much power amplification. With transistors, it can be used as voltage amplifiers. Class A amplifiers with vacuum pentodes can also provide a single power amplification stage to drive loads, such as loudspeakers.

Class B power amplifiers

BJTs generally require Class B power amplifiers to drive loads, such as loudspeakers. The input of class B amplifiers is large, due to which the output is also very large. Thus, it produces a large amplification. But, in the case of a single transistor, only half of the input signal is amplified.

Class AB power amplifiers

The configuration of AB power amplifiers lies between class A and class B amplifiers. Class AB amplifiers are produced by combining the high output of class B power amplifiers with the low distortion of class A power amplifiers.

In the case of small outputs, the class AB power amplifier can behave as class A. It can behave as a class B power amplifier in the case of very large outputs.

Class C power amplifiers

The conduction element of class C type power amplifiers is transistors. It has better efficiency, but due to the conduction of less than the half cycle, it causes large distortion. Hence, class C power amplifiers are not preferred in audio applications. The common applications of such amplifiers include radio frequency circuits.

Properties of Amplifier

Amplifiers are defined as per their input and output properties. The gain of the amplifier determines its amplification. Hence, gain and multiplication factors are the two essential properties of the amplifiers.

Let's discuss the properties that are defined by different parameters, which are listed below:

• Gain
  An amplifier's gain is calculated as the ratio of output (power, current, or voltage) to input. It determines the amplification of the amplifier. For example, a signal with an input of 10 volts and an output of 60 volts will have a gain of 6.
  Gain = Output/Input
  Gain = 60/10
  Gain = 6
  The gain is expressed in the unit dB (decibels). Passive components generally have gain less than one, while active components have gain greater than 1.
• Bandwidth
  Bandwidth is defined as the width measured in Hertz of the useful frequency range.
  Frequency range- The frequency range is generally specified in terms of frequency response or bandwidth.
• Noise
  Noise is defined as any unwanted signal that acts as a disturbance in the system.
• Efficiency
  The higher efficiency of an amplifier would result in less heat generation and more output power. It is calculated as the ratio between the output power and the utilization of total power.
• Slew rate
  The slew rate is measured in volts per microsecond. It is defined as the maximum rate of change of output. A slew rate above the audible range of an amplifier would result in less distortion and errors.
• Linearity
  It is defined as the amplifier's ability to produce precise copies of the input signal.
• Stability
  The amplifier circuits require being stable at all available frequencies. It is defined as the ability to avoid unwanted oscillations in an electronic device.

Functions of different amplifiers

Other types of amplifiers have different characteristics. Let' discuss the function of various types of amplifiers in use today.

• The linear amplifiers do not provide perfect linear capability because no amplifier is perfect. It is because of the use of amplifying devices, such as transistors, which are non-linear in nature. These devices can produce some non-linearity. The linear amplifiers are less prone to distortion. It means that linear amplifiers generate less distortion.
• Specially designed audio amplifiers can amplify the audio frequency.
• The narrowband amplifier amplifies over the narrow band of frequencies, while wideband amplifiers amplify over a wide range of frequencies.
• The non-linear amplifiers produce distortion compared to linear devices. But, non-linear devices are still in use today. Examples of non-linear amplifiers are RF (Radiofrequency) amplifiers, etc.
• The structure of the logarithmic amplifier produces an output proportional to the logarithmic of its input. The circuit comprises two diodes and two op-amps (operational amplifier).
• CMOS (Complementary Metal Oxide Semiconductors) can also be used as an amplifier if its operating point is fixed in the active region. It can be constructed by using constant current sources or current mirrors.

Applications of Amplifier

The amplifiers are used in different applications. Let's discuss it in detail.

• Voltage follower
  Voltage follower is also known as unity gain amplifier. It has a very large input impedance and very low output impedance, which is the basic principle of buffering action. The inverting terminal of the operational amplifier is short with the output terminal.
  It means that the output is equal to the input. It is called the voltage follower because the output of the amplifier is following the input.
  The voltage follower provides no loading effects, no power and current gain, which are its advantages.
• Current to Voltage converter
  The construction of a current to voltage converter is shown below:
  Where,
  RT: Thermistor or light-dependent resistor.
  IT: Current
  RF: Feedback resistor
  IF: Feedback current
  VO: Output voltage
  The thermistor drives the op-amp in its inverting mode. The change in temperature results in the variation of thermistor resistance. It further varies the current passing through it. The current flows into the output through the feedback resistor as feedback current developing the output voltage. Since the thermistor current is equal to the feedback current, we can say that the output voltage is proportional to the thermistor current.
  Thus, an input current is converted into an output voltage.
• Microwave amplifiers
  TWTA and Klystrons are the common devices used as microwave amplifiers. Traveling Wave Tube Amplifier (TWTA) provides good amplification even at low microwave frequencies. It means that TWTA is preferred for high power amplification. But, klystrons are better tunable as compared to TWTA.
  Klystrons are also used at microwave frequencies for high power applications. But, it provides wide tunable amplification as compared to TWTA. It also has a narrow bandwidth compared to TWTA.
  Solid-state devices, such as MOSFET, diodes, semiconductor materials (Silicon, Gallium, etc.), are used at low power and microwave frequencies in various applications. For example, mobile phones, portable radio frequency terminals, etc. In such applications, size and efficiency are the major factors that determine its capability and usage. The use of solid-state devices in microwave amplifiers also provides wide bandwidth.
• Musical Instruments
  The amplifiers are used in various musical instruments, such as guitars and drum machine, to convert the signal from different sources (strings in guitar, etc.) into the powerful electronic signal (power amplifier) that produces sound. The sound is audible enough to the audience or nearby people. The output of some musical instruments is connected to the speakers for louder sound.
  Instrument amplifiers in musical instruments also have the signal tuning function that allows the performer to change the signal's tone.
• Oscillators
  The oscillator circuits are used to generate electrical waveforms of any desired frequency, shape, and power. The use of amplifiers in oscillators provides the constant output amplitude and amplifies the feedback frequency.
• Video amplifiers
  The amplifier present in the video amplifier amplifies the signal comprising of high-frequency components. It also prevents it from any distortion. The video amplifiers have different bandwidths according to the video signal quality, such as SDTV, HDTV, 1080pi, etc.