Classification Of Amplifiers Based On Q-Point | Power Amplifiers

 Another article on the classification of amplifiers. This type of classification is based on the position of the Q point on the load line.

Learning Objectives:

• Learn about the difference between classes of power amplifiers

• FAQs

• Why is power dissipation considered a big issue in power amplifiers?

• Is a large-signal operation different from a small-signal operation?

• How to eliminate crossover distortion?

Introduction 

Large signal amplifiers or power amplifiers provide a large amount of power to the load. This amplifier category has different classes. Every class has a different region of operation on the load line. The types of amplifiers are usually at the output stage of the system to drive or power the device connected as a load. Some important features are 

• Power efficiency

• Maximum power 

• The power dissipated should be as low as possible

• Impedance matching to the output device. Low output resistance so that the power loss is minimum.

This classification can also be viewed as the output stage of an amplifier. It means different classes of amplifiers can be used as an output stage of a multistage amplifier system. Each class has a different shape of the output waveform. The following figure compares the output waveforms obtained from each class.

How does a power amplifier look like?

A power amplifier can either be a simple power transistor (not a common transistor but designed with some extra thermal stability properties) or an integrated circuit along with special thermal properties. Power transistors have a larger surface area for efficient heat sinking.

Power amplifiers process large signals, and a larger amount of power is required. So, all of the power devices must dissipate the internally generated heat. For power, the BJT collector is a critical junction. a heat sink is necessary to attach with it. Also, the case of power transistors has a large contact area. Internally generated heat flows from the case then heat sinks and then dissipates in the surrounding air.

Why is power dissipation considered a big issue in power amplifiers?

Power is defined as the rate at which energy is consumed or dissipated in a device. Power generated must be dissipated fast enough to avoid temperature buildup. The heat generated is equal to heat dissipated or consumed. 

Large signal operation is also applied to digital switching circuits. As you know switching devices operate either in cut-off or saturation modes. But its operation is limited to either of the two states only. So, heat dissipation is not an issue in a digital switching system.

Is large-signal operation different from small-signal operation:

For signals that have a power greater than 1W are considered large signals. In large signal operations, approximation and models are not applicable. In a small signal, it is assumed that the incoming signal is small enough and can not be able to change device parameters that are beta and re`.

A small-signal swings over a small part of the AC load line. While a large signal approaches the limits of the AC load line.

Class A Amplifiers - A brief overview:

It necessarily operates in a linear region for 360 degrees of the input cycle and cut off at 0 degrees. Following is the simple BJT class A amplifier circuit and its load line.

The maximum possible signal swing is achieved when the Q point is centred at the AC load line. With the help of a load line, we can conclude the following points.

• Collector current is maximum (peak value of current or peak of the sinusoid) at IC(sat) and minimum at its cut off value.

• Similarly, VCE collector to emitter voltage is maximum (peak voltage or peak of the sinusoid) is at Vce(sat) and minimum at its cut off value.

Class B Amplifiers - A brief overview:

Class B amplifiers are biased such that they operate in linear region for 180 degrees of the input signal half cycle of the sinusoid) and then cut off for the next 180 degrees. Q point is at cut off or it is biased at cut off. 

Since half of the input cycle is clipped, the output of the class B amplifier is highly distorted. As I discussed earlier, they are linear amplifiers, the shape of the output waveform is the same as that of the input wave. To get the same output waveform similar to the input waveform (similar in shape) there are two complementary transistors. This class of amplifiers use two complementary transistors. One of the transistors amplifies the positive part of the signal and the other transistor amplifies the negative part of the signal.

A simple class B amplifier with the help of two complementary BJTs (one is NPN and the other is PNP) and the load line is given below.

During the positive half cycle, Q1 is ON Vce varies from its Q point value (that is 0V) to its maximum value (that is Vcc). During the negative half-cycle, Q1 remains turned off. Similarly, during the positive half cycle, Q2 remains turned off. During the negative half-cycle, it produces the negative half of the Vce.

During the positive half cycle, Q1 is ON Ic varies from its Q point value (that is 0V) to its maximum value (that is IC(sat)). During the negative half-cycle, Q1 remains turned off. Similarly, during the positive half cycle, Q2 remains turned off. During the negative half-cycle, it produces the negative half of the Vce.

Why does cross over distortion occur?

One of the most simple classes B circuits is shown in the figure above.

To turn on the transistor VBE ≥ 0.7V. When the base voltage is 0 (VBE = 0 or VBE < 0.7V) neither of the transistors conducts. So there is no waveform at the output. Because of this output waveform is not the same in shape as that of an input waveform. This type of distortion is known as cross over distortion.

Class AB Amplifiers - A brief overview:

This class of amplifiers are biased such that they operate in a linear region and conduct slightly more than 180 degrees. The amplifier is then cut off for the rest of the cycle. The Q point is slightly above the cut off point.

How to eliminate crossover distortion?

This class is specifically designed to eliminate crossover distortion. Slight changes made in the class B circuit will eliminate the distortion.

Class C Amplifiers - A brief overview:

The class C amplifiers are biased such that they operate less than 180 degrees of the input signal. The output is highly distorted. They are also known as tuned amplifiers and are used at radio frequencies.

Class C Amplifier

The figure above shows a simple CE amplifier. The idea of biasing is noticeable. The base is negatively biased with the voltage source VBB. The signal applied must have an amplitude (peak value) greater than VBB+VBE. The transistor is ON for a very short interval of time and produces a pulse (pulse of Ic) at the output. The output is highly distorted. To get the output voltage similar to the input voltage in shape and tank circuit is used.

Classification Of Amplifiers (Based On Input & Output Parameters)

 Topological Classification of Amplifiers:

• Discuss types of amplifiers based on input and output parameters or topological classification of amplifiers

• Voltage amplifiers

• Current amplifiers

• Transresistance amplifiers

• Transconductance amplifiers

• How do we select input and output impedance of various amplifier topologies

• Comparison of ideal and real amplifiers

All amplifiers can be categorised according to the input and output parameters. This is the basic classification. This classification is based on the users’ requirements. The output signal can be voltage or current and similarly, a voltage signal can either be current or voltage. 

• Voltage amplifier

• Current amplifier

• Transconductance amplifier

•  Transconductance amplifier

Voltage Amplifier:

The voltage signal is applied at the input and gets the amplified voltage signal at the output. This is voltage amplification. The ratio of output voltage to the input voltage is termed voltage gain Av. It is the internal gain of the amplifier. At any instant, Vout is proportional to Vin. 

\[A_V = \frac {V_{out}}{V_{in}}\]

Design Guide: The input impedance of voltage amplifiers should be high (ideally infinite) and output impedance should be very low (ideally zero). 

Let me explain why?? 

Consider an ideal system. Av is independent of source and load resistor. 

A voltage source with zero internal resistance. And input impedance is set to infinite. At this condition, the whole voltage signal without any loss appears on the output circuit.

\[R_i = \infty\]

\[R_S = 0 \]

\[V_i = V_S\]

At the output circuit, output resistance Ro, should be zero, so there's no voltage drop across it. The voltage signal is amplified by the factor Av appears at the load.

\[R_O = 0\]

\[R_L = \infty\]

\[V_O = A_VV_S\]

Now consider a real system. Source resistance can not be zero, and input impedance can not be infinite. To minimize signal loss, input impedance should be high as compared to source resistance.

\[R_i \rightarrow \text{high as compared to } R_S\]

\[R_S = low\]

\[V_i = \frac {V_SR_i}{R_i+R_S}\]

At the output circuit, output resistance Ro, can not be zero. To minimize signal loss, Ro should be very less compared to load resistance. 

\[R_O = low\]

\[R_L \rightarrow \text {high as compared to } R_O\]

\[V_O = \frac {A_V*V_i*R_L}{R_L+R_O}\]

Current Amplifier:

The current signal is applied at the input and the amplified current is the signal at the output. This is current amplification. The ratio of output current to the input current is termed as current gain AI. AI is the internal current gain. At any instant Iout is proportional to Iin. 

\[A_i = \frac {I_{out}}{I_{in}}\]

Design Guide: The input impedance of current amplifiers should be low (ideally zero) and output impedance should be very high (ideally infinite).

Let me explain why.

Consider an ideal system. Ai should be independent of load resistor and source resistor.

A current source with infinite internal resistance. And input impedance is set to zero. At this condition, the whole current signal without any loss appears on the output circuit. See figure below. 

\[R_i=0\]

\[R_S = \infty \]

\[I_i = I_S\]

At the output circuit, output resistance Ro, should be infinite, so there's no current flowing across it. The current signal is amplified by the factor Ai appearing at the load.

\[R_L = 0\]

\[R_O = \infty\]

\[I_L = A_i*I_S\]

Now consider a real system. Source resistance can not be infinite, and input impedance can not be zero. To minimize signal loss, input impedance should be very low as compared to source resistance.

\[R_i = low\]

\[R_S \rightarrow \text {high as compared to } R_i\]

\[I_i = I_S(\frac{R_S}{R_S+R_i})\]

At the output circuit, output resistance Ro, can not be infinite. To minimize signal loss, Ro should be very high compared to the load resistance. 

\[R_L = low \]

\[R_O \rightarrow \text {high as compared to }R_L\]

\[I_L = A_i*I_i(\frac {R_O}{R_O+R_L})\]

Transresistance Amplifier:

The current signal is applied at the input and gets the amplified voltage signal at the output. The ratio of output voltage to the input current is termed transfer gain. At any instant, Vout is proportional to Iin. 

\[R_M = \frac {V_{out}}{I_{in}}\]

Design Guide: The input and output impedance of transresistance amplifiers should be low (ideally zero).

Consider an ideal system. RM should be independent of source and load resistor. In transresistance amplifiers input is a current source. The internal resistance of the source is infinite, and the input impedance is zero. There is no signal drop across the zero impedance path.

\[R_i = 0\]

\[R_S = \infty\]

\[I_i = I_S\]

At the output side, we get voltage amplified by factor RM. The internal resistance of the voltage source is zero. So we get a rated output signal, that is voltage.

\[R_O = 0\]

\[R_L = \infty\]

\[V_O = R_M*I_S\]

Consider the real system. The internal resistance of the current source can not be infinite and input resistance can be zero. So, choose your current source such that it has very high internal resistance as compared to the input impedance.

\[R_i = low\]

\[R_S \rightarrow \text {high as compared to } R_i\]

\[I_i = I_S(\frac {R_S}{R_S+R_i})\]

At the output circuit, the internal resistance of the voltage source can not be zero. Choose a load which has high resistance as compared to the output impedance.

\[R_O = low\]

\[R_L\rightarrow \text {high as compared to }R_O \]

\[V_O = R_M*I_i(\frac {R_L}{R_L+R_O})\]

Transconductance Amplifier:

The voltage signal is applied at the input and get the amplified current signal at the output. The ratio of output current to the input voltage is termed transfer gain. At any instant Iout is proportional to Vin. 

\[G_M = \frac{I_{out}}{V_{in}}\]

Design Guide: The input and output impedance of transconductance amplifiers should be high (ideally infinite).

Consider an ideal system. GM should be independent of source and load resistor.

In transconductance amplifiers input is a voltage source. The internal resistance of the source is zero, and the input impedance is infinite. There is no voltage drop, the whole signal appears at the output circuit.

\[R_i = \infty \]

\[R_S = 0\]

\[V_i = V_S\]

At the output side, we get current amplified by factor GM. The output resistance Ro is infinite. Ro is in parallel with the load resistor, the small load resistance allows maximum current to flow through it. The whole current signal appears on the load resistor.

\[R_L = 0\]

\[R_O = \infty\]

\[I_L = G_M*V_S\]

Consider the real system. In transconductance amplifiers input is a voltage source. The voltage source has some internal resistance, and the input impedance can't be infinite. Input impedance should be higher than source resistance. At this condition, you can get the maximum possible voltage at the output circuit.

\[R_i \rightarrow \text {high as compared to }R_S\]

\[R_S = low\]

\[V_i = V_S(\frac{R_i}{R_i+R_S})\]

At the output side, we get the current signal amplified by factor GM . Output resistance can't be infinite. Select output impedance such that it is higher enough than the load resistor. Since Ro is in parallel with the load resistor, small load resistance allows maximum current to flow through it.

\[R_L = low\]

\[R_O \rightarrow \text {high as compared to } R_L\]

\[I_L = G_M*V_i (\frac {R_O}{R_O+R_L})\]

At the end:

This is not the end, it is just an introductory article on amplifiers. There is a wide range of amplifiers available, there are so many classifications. Like operational amplifiers, low noise amplifiers, feedback amplifiers, audio, video and radio amplifiers etc. 

Reference:

1. Classification of amplifiers and their applications

2. ANALOG ELECTRONICS
   By A. KANDASWAMY, ANDRÉ PITTET

3. Understand Amplifiers by Owen Bishop

4. Electronic Devices and Circuits
   By Cheruku, Cheruku Dharma Raj

Amplifier Fundamentals

 Introduction To Amplifiers:

Amplify or Amplifiers, what comes to your mind when you hear these words? I think it will increase the sound levels. This is the first impression of the word amplifier in a common man’s mind. But from an engineer's perspective amplifiers are not only the sound levels, but it is also much more. Radio, television and computers all use amplifiers. In this post, I will explain amplifiers, their types, and important parameters. The article provides knowledge of amplifiers on the system level. It means I suppose an amplifier is a block, I am not interested in its internal circuitry or component. At the system level, you can think of an amplifier as a two-port network. It has two input terminals and two output terminals.

Outline:

• Amplification and amplifiers

• Ideal amplifier characteristics

• Need for linearity

• Imperfections in practical amplifier

• Distortion

• Noise

• Loading effect

• Slew rate

• Design parameters

• Bandwidth

• Frequency response

• Gain or transfer ratio

• Explanation Of Power Gain In Amplifiers: (How does an amplifier increase the power gain?)

• Efficiency

• Input impedance

• Output impedance

Amplification and Amplifiers:

Amplification is the process of boosting a signal or strengthening a signal. For example, increasing the volume of sound etc. 

The amplifier is an electronic device that is used to enlarge the input signal. 

There is a small signal at the input of the amplifier, it processes the signal and produces a larger signal at the output. A transistor is the simplest amplifying device.

Ideal amplifier characteristics:

An ideal amplifier is perfectly linear. At any instant output current, voltage or power is constantly proportional to input current, voltage or power. There are several characteristics of an ideal amplifier

• The gain (Av, Ai) of an ideal amplifier remains the same irrespective of the applied input signal amplitude (vin, vout)

• Gain must be independent of the frequency of the signal. The signal of all frequencies must be amplified by the same amount

• The ideal amplifier is free from noise

• Temperature stability for an amplifier should be very important. It works well in high temperatures as well

• Time stability, another performance parameter of an ideal amplifier. It should be stable over a period of time

Need For Linearity:

During the amplification process, it is desirable to process a signal so that the information contained in it must remain unchanged. The output of an amplifier should be a replica of the input signal, except the magnitude. Or in other words, the output and input waveforms are exactly similar in shape but different in magnitudes.

Now come to the imperfections. Almost all the amplifiers are linear up to some part of their operating range. However, some of them deviate from linear behaviour at higher frequencies. No real amplifier is an ideal amplifier. A real system must have imperfections, we engineers are here to minimize them. 

Imperfections in a practical amplifier:

Distortion

The purpose of an amplifier is to reproduce the exact signal with some gain. Any change in signal shape is referred to as Distortion. It is an undesirable effect. Practically it won't reproduce the exact signal. The output signal might be distorted in three ways: amplitude, phase and frequency.

Amplitude distortion:

This type of distortion is linear and is due to the bandwidth of the amplifier. The input of an amplifier is alternating and of course the output as well. Signals of smaller amplitude amplify properly because they can swing freely on either side. But signals of larger amplitudes drive the amplifier (transistor-based amplifiers) into a non-linear region and hence the edges of the signal attenuate. This type of distortion not only affects the gain but also results in distorted amplitude. 

Harmonic distortion

It is nonlinear distortion caused by device characteristics. In this type of distortion, the output signal contains harmonic components as well, which are not part of the input signal. Lower harmonics are more dominant These harmonics appear at the output and give rise to distorted output. 

Phase distortion

This type of distortion is also non-linear. When a signal flows through inductors and capacitors, there is a phase shift. The output signal appears after a delay as compared to the input signal. The amount of phase shift is related to frequency.

Noise

An unwanted signal, superimposed on an electrical signal.  Noise may be added from the outside of the circuits or it may be added by the circuit components. In amplifiers, transistors are responsible for producing noise. Low noise transistors are available. The amount of noise produced is somewhat related to the amount of current. So, keep the current low in your design.

Loading effect:

When you are understanding the concepts of amplifiers you must come across the term “loading effect”. What is the loading effect and how to minimize it? Consider a situation, your amplifier output resistance is 1k and you have attached a load of resistance 100 ohms in parallel with output. What is going to happen? The effective output resistance decreases and hence gain decreases. To avoid this there's a technique known as impedance matching or impedance transformation is used. 

Slew rate

The ability of an amplifier to change the output voltage with respect to rapid change in input. It is measured in V/s. 

Slew rate = ∆V/∆t

Design Parameters:

Each amplifier has three important design parameters. Gain, input impedance and output impedance.

Bandwidth:

It is a range of frequencies over which an amplifier gives its best results. An amplifier is designed to operate over a selected frequency range. This range is called bandwidth.

Frequency response:

It is an actuarial representation of output voltage versus the frequency of the amplifier. It shows the effect of frequency on the output of the amplifier. Look at the graph, at lower frequencies, the output decreases because of capacitive reactances of coupling and bypass capacitors. At higher frequencies, the output voltage drops because of transistor Internal capacitances and stray wiring capacitances. The middle range of frequencies is called the mid-band. Over this frequency range, amplifiers produce maximum output. An amplifier with a larger bandwidth has a smaller gain.

To design an amplifier we must select a suitable range of frequencies according to the design requirements.

Critical Frequency:

The frequency at which output voltage reaches 0.707 of Vmax. There are usually two critical frequencies in an amplifier frequency response. These are lower critical frequency f1 and upper critical frequency f2. The middle part of the graph shows the range of frequencies where the amplifier produces maximum output. Usually, amplifiers operate in this range of frequencies.

Gain or Transfer ratio

The ability of an amplifier to enhance the input signal. It is the ratio of output signal divided by input signal. It is expressed in decibel dB. The three gains in an amplifier system are given below.

• Voltage gain

• Current gain

• Power gain

Voltage Gain:

It is the voltage amplification factor and denoted by AV. If the output voltage (vo) of an amplifier is greater than the input voltage (vi) then it is a voltage gain. Therefore, it is the ratio of output voltage to the input voltage.

\[A_v = \frac {v_o}{v_i}\]

It is important to note that voltage gain is the ratio, not the difference. Some amplifiers produce the inverted output and it is denoted by a 'minus sign. The voltage gain as a function of current gain can be expressed as,

\[A_v = \frac {i_o*R_L}{v_i*r_i}\]

\[A_v =A_i \frac {R_L}{r_i}  ..equation 1\]

Where, 

ri = input resistance seen from the signal

RL = output resistance

Equation 1 holds for any type of structure of an amplifier. An important consideration in this context is that input and output currents are entering the positive terminals.

\[A_v = \frac {-i_o*R_L}{v_i*r_i}\]

-io is the output current leaving the positive terminals.

Current Gain:

Like the voltage gain, it is the current amplification factor and denoted by Ai. If the output current is greater than the input current then it is a current gain. Therefore, it is the ratio of the output current to the input current.

\[A_i = \frac {i_o}{i_i}\]

Power Gain:

It is the power amplification factor and denoted by AP. If the output power is greater than the input power then it is the power gain. Therefore it is the ratio of the output power to the input power. 

\[A_P = \frac {p_o}{p_i}\]

Where, 

\[p_o = v_o*i_o\]

\[p_i = v_i*i_i\]

\[A_P = \frac {v_o*i_o}{v_i*i_i} = A_vA_i\]

Explanation Of Power Gain In Amplifiers: (How does an amplifier increase the power gain?)

Look at the schematic and connections. There are two DC voltage sources V+ and V- to drive the transistor. vi is the input signal to be amplified. 

It is important to note that an amplifier increases the signal power. In comparison with the transformers, the applied power (input power) is almost equal to or less than the power delivered (output power).

The reason behind this additional power is the external DC supplies. To operate an amplifier external DC source is required. Additional power delivered and heat dissipated (due to internal circuit) is provided from the power supply. 

\[P_{DC} = P_1+P_2\]

\[P_{DC} +P_I = P_L+P_{dissipated} ..equation 2\]

V1  and V2 are necessary for operating an amplifier. P1 and P2 are the power obtained from voltage sources V1  and V2. PDC is the total DC power input to the circuit. Equation 2 is the power balance equation.

Efficiency:

Efficiency is always an important performance parameter for the system. In power amplifiers (that enhance voltage and current of the signal) efficiency is important.

\[\eta = \frac { P_L}{P_DC}\]

The real-life signal is very small (signals obtained from the transducers are usually in millivolts), so power drawn from the signal source is not considered while calculating the efficiency.

Input impedance Zin

It is a design parameter of the amplifier. It is the impedance offered by input terminals. There is no single element that creates input impedance. Input impedance is the combined effect of source internal resistance and biasing resistors and any other resistance in the path of the signal. 

Output impedance

It is also a design parameter of the amplifier. Same as the input impedance, there is no single component that creates output impedance. It is a little bit difficult to explain where the output impedance comes from. It is the equivalent impedance of signal source at the amplifier output, load resistor and any other resistance in the path of output signal flow.