Introduction to the Amplifier

Introduction to the Amplifier Tutorial

In "Electronics", signal amplifiers are widely used devices as they have the ability to amplify a relatively small input signal, for example from a Sensor such as a microphone, into a much larger output signal to drive a Relay, lamp or loudspeaker for example. There are many forms of electronic circuits classed as amplifiers, from Operational Amplifiers and Small Signal Amplifiers up to Large Signal and Power Amplifiers. Amplifiers can be thought of as a simple box or block containing the amplifying device, such as a Transistor, Field Effect Transistor or Op-amp, which has two input terminals and two output terminals (ground being common) with the output signal being much greater than that of the input signal as it has been "Amplified".

An ideal amplifier has three main properties, Input Resistance or ( Rin ), Output Resistance or ( Rout ) and of course amplification known commonly as Gain or ( A ). No matter how complicated an amplifier circuit is, a general amplifier model can be used to show the relationship of these three properties.

Ideal Amplifier Model

The difference between the input and output signals is known as the Gain of the amplifier and is basically a measure of how much an amplifier "amplifies" the input signal. For example, if we have an input signal of 1 and an output of 50, then the gain of the amplifier would be 50. Gain is a ratio, it has no units but is given the symbol "A", which can be simply calculated as the output signal divided by the input signal.

Amplifier Gain

Then the gain of an amplifier can be said to be the relationship that exists between the signal measured at the output with the signal measured at the input. There are three different kinds of Amplifier Gain, Voltage Gain, ( Av ), Current Gain ( Ai ) and Power Gain ( Ap ) and examples of these are given below.

Amplifier Gain of the Input Signal

Voltage Amplifier Gain

Current Amplifier Gain

Power Amplifier Gain

Note that for the Power Gain you can also divide the power obtained at the output with the power obtained at the input. Also when calculating the gain of an amplifier, the subscripts v, i and p are used to denote the type of signal gain being used.

The power Gain or power level of the amplifier can also be expressed in Decibels, (dB). The Bel is a logarithmic unit (base 10) of measurement that has no units. Since the Bel is too large a unit of measure, it is prefixed with deci making it Decibels instead with one decibel being one tenth (1/10th) of a Bel. To calculate the gain of the amplifier in Decibels or dB, we can use the following expressions.

•   Voltage Gain in dB:    a_v  =  20 log Av
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•   Current Gain in dB:    a_i  =  20 log Ai
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•   Power Gain in dB:    a_p  =  10 log Ap

Note that the DC power gain of an amplifier is equal to ten times the common log of the output to input ratio, where as voltage and current gains are 20 times the common log of the ratio. Note however, that 20dB is not twice as much power as 10dB because of the log scale. Also, a positive value of dB represents a Gain and a negative value of dB represents a Loss within the amplifier. For example, an amplifier gain of +3dB indicates that the amplifiers output signal has "doubled", (x2) while an amplifier gain of -3dB indicates that the signal has "halved", (x0.5) or in other words a loss.

The -3dB point of an amplifier is called the half-power point which is -3dB down from maximum, taking 0dB as the maximum output value.

Example No1

Determine the Voltage, Current and Power Gain of an amplifier that has an input signal of 1mA at 10mV and a corresponding output signal of 10mA at 1V. Also, express all three gains in decibels, (dB).

Amplifier Gain.

in Decibels (dB).

Then the amplifier has a Voltage Gain of 100, a Current Gain of 10 and a Power Gain of 1,000.

Generally, amplifiers can be divide into two distinct types depending upon their power or voltage gain, Small Signal Amplifiers such as pre-amplifiers, instrumentation amplifiers etc, which are designed to amplify very small signal voltage levels of only a few micro-volts (μV) from sensors or audio signals and Large Signal Amplifiers such as audio power amplifiers or switching amplifiers, which are designed to amplify large input voltage signals or switch heavy load currents.

Power Amplifiers

Small signal amplifiers are generally referred to as "Voltage" amplifiers as they convert a small input voltage into a much larger output voltage. Sometimes an amplifier is required to drive a motor or feed a loudspeaker and for these types of applications where high switching currents are needed Power Amplifiers are required.

The main function of Power amplifiers (also known as large signal amplifiers) is to deliver power, which as we know from above, is the product of the voltage and current applied to the load with the output signal power being greater than the input signal power. In other words, a power amplifier amplifies the power of the input signal which is why they are used in amplifier output stages.

The power amplifier works on the basic principle of converting the DC power drawn from the power supply into an AC voltage signal delivered to the load. Although the amplification is high the efficiency of the conversion from the DC power supply input to the AC voltage signal output is usually poor. The perfect or ideal amplifier would give us an efficiency rating of 100% or at least the power IN would be equal to the power OUT. However, this can never happen as some of its power is lost in the form of heat and also, the amplifier itself consumes power during the amplification process. Then the efficiency of an amplifier is given as:

Amplifier Efficiency

Ideal Amplifier

We can know specify the characteristics for an ideal amplifier from our discussion above with regards to its Gain, meaning voltage gain:

• The amplifiers gain, ( A ) should remain constant for varying values of input signal.
• Gain is not be affected by frequency. Signals of all frequencies must be amplified by exactly the same amount.
• The amplifiers gain must not add noise to the output signal. It should remove any noise that is already exists in the input signal.
• The amplifiers gain should not be affected by changes in temperature giving good temperature stability.
• The gain of the amplifier must remain stable over long periods of time.

Amplifier Classes

The classification of an amplifier as either a voltage or a power amplifier is made by comparing the characteristics of the input and output signals by measuring the amount of time in relation to the input signal that the current flows in the output circuit. We saw in the Common Emitter transistor tutorial that for the transistor to operate within its "Active Region" some form of "Base Biasing" was required. This small Base Bias voltage added to the input signal allowed the transistor to reproduce the full input waveform at its output with no loss of signal. However, by altering the position of this Base bias voltage, it is possible to operate an amplifier in an amplification mode other than that for full waveform reproduction. By changing the amplifiers Base bias voltage different ranges or modes of operation can be obtained and these are categorized according to their Classification better known as Amplifier Class.

Audio power amplifiers are classified in an alphabetical order according to their circuit configurations and mode of operation. Amplifiers are designated by different classes of operation such as class "A", class "B", class "C", class "AB", etc. These different classes of operation range from a near linear output but with low efficiency to a non-linear output but with a high efficiency. No one class of operation is "better" or "worse" than any other class with the type of operation being determined by the use of the amplifying circuit. There are typical maximum efficiencies for the various types or class of amplifier, with the most commonly used being:

• Class A   -   has low efficiency of less than 40% but good signal reproduction and linearity.
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• Class B   -   is twice as efficient as class A amplifiers with a maximum theoretical efficiency of about 70% because the amplifying device only conducts (and uses power) for half of the input signal.
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• Class AB   -   has an efficiency rating between that of Class A and Class B but poorer signal reproduction than class A amplifiers.
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• Class C   -   is the most efficient amplifier class as only a very small portion of the input signal is amplified therefore the output signal bears very little resemblance to the input signal. Class C amplifiers have the worst signal reproduction.

Class A Operation

Class A Amplifier operation is were the entire input signal waveform is faithfully reproduced at the amplifiers output as the transistor is perfectly biased within its active region, thereby never reaching either of its Cut-off or Saturation regions. This then results in the AC input signal being perfectly "centred" between the amplifiers upper and lower signal limits as shown below.

Class A Output Waveform

In this configuration, the Class A amplifier uses the same transistor for both halves of the output waveform and due to its biasing arrangement the output transistor always has current flowing through it, even if there is no input signal. In other words the output transistors never turns "OFF". This results in the class A type of operation being very inefficient as its conversion of the DC supply power to the AC signal power delivered to the load is usually very low. Generally, the output transistor of a Class A amplifier gets very hot even when there is no input signal present so some form of heat sinking is required. The DC current flowing through the output transistor (Ic) when there is no output signal will be equal to the current flowing through the load. then a pure Class A amplifier is very inefficient.

Class B Operation

Unlike the Class A amplifier above that uses a single transistor for its output stage, the Class B Amplifier uses two complimentary transistors (an NPN and a PNP) for each half of the output waveform. One transistor conducts for the positive half of the waveform and another conducts for the negative half of the waveform. This means that each transistor spends half of its time in the active region and half its time in the Cut-off region thereby amplifying only 50% of the input signal. Class B operation has no DC bias voltage instead the transistor only conducts when the input signal is greater than the base-emitter voltage and for silicon devices is about 0.7v. Therefore, at zero input there is zero output. This then results in only half the input signal being presented at the amplifiers output giving a greater efficiency as shown below.

Class B Output Waveform

In a class B amplifier, no DC current is used to bias the transistors, so for the output transistors to start to conduct each half of the waveform, both positive and negative, they need the base-emitter voltage Vbe to be greater than the 0.7v required for a bipolar transistor to start conducting. Then the lower part of the output waveform which is below this 0.7v window will not be reproduced accurately resulting in a distorted area of the output waveform as one transistor turns "OFF" waiting for the other to turn back "ON". The result is that there is a small part of the output waveform at the zero voltage cross over point which will be distorted. This type of distortion is called Crossover Distortion and is looked at later on in this section.

Class AB Operation

The Class AB Amplifier is a compromise between the Class A and the Class B configurations above. While Class AB operation still uses two complementary transistors in its output stage a very small biasing voltage is applied to the Base of the transistor to bias it close to the Cut-off region when no input signal is present. An input signal will cause the transistor to operate as normal in its Active region thereby eliminating any crossover distortion which is present in class B configurations. A small Collector current will flow when there is no input signal but it is much less than that for the Class A amplifier configuration. This means then that the transistor will be "ON" for more than half a cycle of the waveform. This type of amplifier configuration improves both the efficiency and linearity of the amplifier circuit compared to a pure Class A configuration.

Class AB Output Waveform

The class of operation for an amplifier is very important and is based on the amount of transistor bias required for operation as well as the amplitude required for the input signal. Amplifier classification takes into account the portion of the input signal in which the transistor conducts as well as determining both the efficiency and the amount of power that the switching transistor both consumes and dissipates in the form of wasted heat. Then we can make a comparison between the the most common types of amplifier classifications in the following table.

Power Amplifier Classes

Badly designed amplifiers especially the Class "A" types may also require larger power transistors, more expensive heat sinks, cooling fans, or even an increase in the size of the power supply required to deliver the extra power required by the amplifier. Power converted into heat from transistors, resistors or any other component for that matter, makes any electronic circuit inefficient and will result in the premature failure of the device. So why use a Class A amplifier if its efficiency is less than 40% compared to a Class B amplifier that has a higher efficiency rating of over 70%. Basically, a Class A amplifier gives a much more linear output meaning that it has, Linearity over a larger frequency response even if it does consume large amounts of DC power.

In the next tutorial about Amplifiers we will look at the most commonly connected type of transistor amplifier circuit, the Common Emitter Amplifier. Most transistor amplifiers are of the Common Emitter or CE type circuit due to their large gains in voltage, current and power as well as their excellent input/output characteristics.