Introduction to the Amplifier
Introduction to the Amplifier Tutorial
Not all amplifiers are the same and are therfore classified according to their circuit configurations and
methods of operation. In "Electronics", small signal amplifiers are commonly used devices as they have the ability to amplify
a relatively small input signal, for example from a Sensor
such as a photo-device, 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 with the classification of an amplifier depending upon the size of
the signal, its configuration, class and application as shown in the following table.
Classification of Amplifiers
|Type of Signal||Type of|
|Small Signal||Common Emitter||Class A Amplifier||Direct Current (DC)|
|Large Signal||Common Base||Class B Amplifier||Audio Frequencies (AF)|
| ||Common Collector||Class AB Amplifier||Radio Frequencies (RF)|
| || ||Class C Amplifier||VHF, UHF and SHF|
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
An ideal signal 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 still
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 volt and an output of 50 volts, then the gain of the amplifier would be "50". In other words, the input
signal has been increased by a factor of 50. This increase is called Gain. Gain is the ratio of output÷input, it
has no units but in Electronics it is commonly given the symbol "A", for Amplification. Then the
gain of an amplifier is simply calculated as the "output signal divided by the input signal".
The introduction to the amplifier gain 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
which can be measured and these are: Voltage Gain ( Av ), Current Gain
( Ai ) and Power Gain ( Ap ) depending
upon the quantity being measured with examples of these different types of gains 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: av = 20 log Av
- Current Gain in dB: ai = 20 log Ai
- Power Gain in dB: ap = 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.
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).
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 sub-divided into two distinct types depending upon their power or voltage
gain. One type is called the Small Signal Amplifier which include pre-amplifiers, instrumentation amplifiers etc.
Small signal amplifies are designed to amplify very small signal voltage levels of only a few micro-volts (μV) from
sensors or audio signals.
The other type are called Large Signal Amplifiers such as audio power amplifiers or power switching
amplifiers. Large signal amplifiers are designed to amplify large input voltage signals or switch heavy load currents as you
would find driving loudspeakers.
The Small Signal Amplifier is generally referred to as a "Voltage" amplifier because they
usually convert a small input voltage into a much larger output voltage. Sometimes an amplifier circuit is required to drive
a motor or feed a loudspeaker and for these types of applications where high switching currents are needed Power Amplifiers
As their name suggests, the main job of a "Power Amplifier"
(also known as a large signal amplifier), is to deliver power to the load, and 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 these types of amplifier circuits are used
in audio amplifier output stages to drive loudspeakers.
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, in reality this can never happen as some of the 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:
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.
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
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. With the introduction to the amplifier of a Base bias voltage,
different operating ranges and modes of operation can be obtained which are categorized according to their classification. These
various mode of operation are 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 Amplifier Classes
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 Amplifier - has low efficiency of less than 40% but good signal reproduction and linearity.
- Class B Amplifier - 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.
- Class AB Amplifier - has an efficiency rating between that of Class A and Class B
but poorer signal reproduction than class A amplifiers.
- Class C Amplifier - 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 Amplifier Operation
Class A Amplifier operation is where 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
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 as most of the DC power is converted to heat.
Class B Amplifier Operation
Unlike the Class A amplifier mode of operation above that uses a single transistor for its output
power 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 one-half of the signal waveform while the other conducts for the other
or opposite half of the signal 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 direct DC bias voltage like the class A amplifier, but 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 amount of amplifier 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 Amplifier 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
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 most common types of amplifier classifications in the following table.
Power Amplifier Classes
||Less than 90o
||180 to 360o|
|Centre Point of
the Load Line
|Exactly on the
|In between the|
X-axis and the
Centre Load Line
|Poor, 25 to 30%
||Better, 70 to 80%
||Higher than 80%
||Better than A|
but less than B
50 to 70%
|None if Correctly
|At the X-axis
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 this Introduction to the Amplifier tutorial, we have seen that there are
different types of amplifier circuit each with its own advantages and disadvantages. 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.