AMPLIFIER:
In popular use, the term usually describes an electronic amplifier, in which the input
"signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systemsto make the human
voice louder or play recorded music. Amplifiers may be classified according to
the input (source) they are designed to amplify (such as a guitar amplifier, to perform with an electric guitar), the device they
are intended to drive (such as a headphone amplifier), the frequency range of the signals
(Audio, IF, RF, and VHF amplifiers, for example), whether they invert
the signal (inverting amplifiers and non-inverting amplifiers), or the type of
device used in the amplification (valve or tube
amplifiers, FET amplifiers, etc.).
The
quality of an amplifier can be characterized by a number of specifications,
listed below.
·
Gain
·
Bandwidth
·
Efficiency
·
Linearity
·
Noise
Gain:
The gain of
an amplifier is the ratio of output to input power or amplitude,
and is usually measured in decibels. (When measured in decibels it is logarithmically related to the power ratio: G(dB)=10
log(Pout /(Pin)). RF amplifiers
are often specified in terms of the maximum power gain obtainable,
while the voltage gain of audio amplifiers and instrumentation
amplifiers will
be more often specified (since the amplifier's input impedance will often be much higher than the
source impedance, and the load impedance higher than the amplifier's output
impedance).
§ Example: an audio amplifier with a gain given
as 20 dB will have a voltage gain of ten (but a power
gain of 100 would only occur in the unlikely event the input and output
impedances were identical).
If
two equivalent amplifiers are being compared, the amplifier with higher gain
settings would be more sensitive as it would take less input signal to produce
a given amount of power.
Bandwidth
The bandwidth of an amplifier is the range of
frequencies for which the amplifier gives "satisfactory performance".
The definition of "satisfactory performance" may be different for
different applications. However, a common and well-accepted metric is the half power points (i.e. frequency where the power goes
down by half its peak value) on the output vs. frequency curve. Therefore
bandwidth can be defined as the difference between the lower and upper half
power points. This is therefore also known as the −3 dB bandwidth.
Bandwidths (otherwise called "frequency responses") for other
response tolerances are sometimes quoted (−1 dB, −6 dB etc.) or
"plus or minus 1dB" (roughly the sound level difference people
usually can detect).
The
gain of a good quality full-range audio amplifier will be essentially flat
between 20 Hz to about 20 kHz (the range of normal human hearing). In
ultra high fidelity amplifier design, the amp's frequency response should
extend considerably beyond this (one or more octaves either side) and might
have −3 dB points < 10 and > 65 kHz. Professional
touring amplifiers often have input and/or output filtering to sharply limit
frequency response beyond 20 Hz-20 kHz; too much of the
amplifier's potential output power would otherwise be wasted on infrasonic and ultrasonic frequencies, and the danger of AM radio
interference would
increase. Modern switching
amplifiers need
steep low pass filtering at the output to get rid of high frequency switching
noise and harmonics.
Efficiency
Efficiency
is a measure of how much of the power source is usefully applied to the
amplifier's output. Class
A amplifiers are very
inefficient, in the range of 10–20% with a max efficiency of 25% fordirect coupling of the output. Inductive
coupling of
the output can raise their efficiency to a maximum of 50%.
Drain
efficiency is the ratio of output RF power to input DC power when primary input
DC power has been fed to the drain of an FET. Based on this definition, the
drain efficiency cannot exceed 25% for a class A amplifier that is supplied
drain bias current through resistors (because RF signal has its zero level at
about 50% of the input DC). Manufacturers specify much higher drain
efficiencies, and designers are able to obtain higher efficiencies by providing
current to the drain of the transistor through an inductor or a transformer
winding. In this case the RF zero level is near the DC rail and will swing both
above and below the rail during operation. While the voltage level is above the
DC rail current is supplied by the inductor.
Class
B amplifiers have a very high efficiency but are impractical for audio work
because of high levels of distortion (See: Crossover
distortion).
In practical design, the result of a tradeoff is the class AB design. Modern
Class AB amplifiers commonly have peak efficienies between 30–55% in audio
systems and 50-70% in radio frequency systems with a theoretical maximum of
78.5%.
Commercially
available Class D switching
amplifiers have
reported efficiencies as high as 90%. Amplifiers of Class C-F are usually known
to be very high efficiency amplifiers. RCA manufactured an AM broadcast
transmitter employing a single class-C low mu triode with an RF efficiency in
the 90% range.
More
efficient amplifiers run cooler, and often do not need any cooling fans even in
multi-kilowatt designs. The reason for this is that the loss of efficiency
produces heat as a by-product of the energy lost during the conversion of
power. In more efficient amplifiers there is less loss of energy so in turn
less heat.
In
RF linear Power Amplifiers, such as cellular base stations and broadcast
transmitters, special design techniques can be used to improve efficiency.
Doherty designs, which use a second output stage as a "peak"
amplifier, can lift efficiency from the typical 15% up to 30-35% in a narrow
bandwidth. Envelope Tracking designs are able to achieve efficiencies of up to
60%, by modulating the supply voltage to the amplifier in line with the
envelope of the signal.
Linearity
An
ideal amplifier would be a totally linear device, but real amplifiers are only
linear within limits.
When
the signal drive to the amplifier is increased, the output also increases until
a point is reached where some part of the amplifier becomes saturated and
cannot produce any more output; this is called clipping, and results in distortion.
In
most amplifiers a reduction in gain takes place before hard clipping occurs;
the result is a compression effect, which (if the amplifier is
an audio amplifier) sounds much less unpleasant to the ear. For these
amplifiers, the 1 dB compression point is defined as the input power
(or output power) where the gain is 1 dB less than the small signal
gain. Sometimes this nonlinearity is deliberately designed in to reduce the
audible unpleasantness of hard clipping under overload.
Ill
effects of nonlinearity can be reduced with negative feedback.
Linearization is an emergent field, and there are
many techniques, such as feedforward, predistortion, postdistortion, in order to avoid the
undesired effects of the non-linearities.
Noise
This
is a measure of how much noise is
introduced in the amplification process. Noise is an undesirable but inevitable
product of the electronic devices and components; also, much noise results from
intentional economies of manufacture and design time. The metric for noise
performance of a circuit is noise figure or noise factor. Noise figure is a
comparison between the output signal to noise ratio and the thermal noise of
the input signal.
BLOCK DIAGRAM:
Power Amplifier:
Common emitter amplifiers are the most
commonly used type of amplifier as they have a large voltage gain. They are
designed to produce a large output voltage swing from a relatively small input
signal voltage of only a few millivolt's and are used mainly as "small
signal amplifiers" as we saw in the previous tutorials. However, sometimes
an amplifier is required to drive large resistive loads such as a loudspeaker
or to drive a motor in a robot and for these types of applications where high
switching currents are needed Power Amplifiers are required.
The main function of the power amplifier,
which are also known as a "large signal amplifier" is to deliver
power, which is the product of voltage and current to the load. Basically a
power amplifier is also a voltage amplifier the difference being that the load
resistance connected to the output is relatively low, for example a loudspeaker
of 4 or 8Ωs resulting in high currents flowing through the collector of the
transistor. Because of these high load currents the output transistor(s) used
for power amplifier output stages such as the 2N3055 need to have higher
voltage and power ratings than the general ones used for small signal
amplifiers such as the BC107.
Since we are interested in delivering maximum
AC power to the load, while consuming the minimum DC power possible from the
supply we are mostly concerned with the "conversion efficiency" of the
amplifier. However, one of the main disadvantage of power amplifiers and
especially the Class A amplifier is that their overall conversion efficiency is
very low as large currents mean that a considerable amount of power is lost in
the form of heat. Percentage efficiency in amplifiers is defined as the r.m.s.
output power dissipated in the load divided by the total DC power taken from
the supply source as shown below.
Power Amplifier
Efficiency
 |
 |
Where:
·
η% - is
the efficiency of the amplifier.
·
·
Pout - is
the amplifiers output power delivered to the load.
·
·
Pdc - is
the DC power taken from the supply.
For a power amplifier it is very important
that the amplifiers power supply is well designed to provide the maximum
available continuous power to the output signal.
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