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CLASSIFICATION OF AMPLIFIERS

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CLASSIFICATION OF AMPLIFIERS Powered By Docstoc
					ECE, ME – I SEM


   BASIC
ELECTRONICS
Amplifiers
   Its Classification
Based on Transistor Configuration
A) Common Base Amplifier
B) Common Emitter Amplifier
c) Common collector Amplifier
Based on Active device used
 BJT Amplifier
 FET Amplifier
On the basis Of Output
 Voltage Amplifier:
  It raises the voltage level of the signal. It is designed to
  achieve the largest possible gain. Only very little power can
  be drawn from its output. Here the Load current is less to
  draw the Less Power.
 Power Amplifier:

 The power amplifiers are large signal amplifiers which raise
  the power level of signals. The power amplifier may also be
  defined as a device which converts dc power to ac power
  and whose action is controlled by the input signal. In fact,
  the power amplifier draws power from dc power supply
  connected to the output circuit and converts it into useful
  ac signal power. Here the Load current is high.
Important Points for Voltage & Power
Amplifier

   The voltage gain of an amplifier is given by the expression AV = ß Rc/Rin
    . So in order to achieve high voltage amplification
(i) transistors having thin base i.e. the transistors with high ß (exceeding
    100) are used
(ii) input resistance RIN is kept quite low in comparison to RC and
(iii) to permit high collector load, which is necessary to give high voltage
    gain, the voltage amplifiers are always operated at low collector current
    (of the order of 1mA).
 A power amplifier is required to deliver a large amount of power and such
    it has to handle large currents. So in order to achieve high power
    amplification
    (i) large sized power transistors are used in order to dissipate the heat
    produced in transistor during operation
    (ii) transistors having thick base (i.e. transistors with comparatively
    smaller ß ) are used in order to handle large currents
    (iii) transformer coupling is used for impedance matching
    (iv) collector resistance is made low.
On the basis of frequency range of
Operation
 DC Amplifier( 0- 10 Hz)
 Audio frequency Amplifier( 20 Hz- 20 kHz):
 It is an electronic amplifier that amplifies low-power audio
  signals (signals composed primarily of frequencies between
  20 hertz to 20,000 hertz, the human range of hearing) to a
  level suitable for driving loudspeakers and is the final
  stage in a typical audio playback chain. While the input
  signal to an audio amplifier may measure only a few
  hundred microwatts, its output may be tens, hundreds, or
  thousands of watts.
 Radio frequency Amplifier( few kHz to MHz)
  An RF amplifier is distinguished by its ability to tune over
  the desired range of input frequencies. A tuned amplifier
  that amplifies the high-frequency signals commonly used in
  radio communications. The frequency at which maximum
  gain occurs in a radio-frequency (RF) amplifier is made
  variable by changing either the capacitance or the
  inductance of the tuned circuit.
On the basis of input
 Small Signal Amplifiers:
  Small signal amplifiers are generally referred
  to as "Voltage" amplifiers as they convert a small
  input voltage into a much larger output voltage
 Large Signal Amplifiers:

  Large signal amplifiers are generally referred
  to as “Power" amplifiers. They deals with large
  signals.
On the basis of number of stages
   Single Stage
 Multi Stage

 further multi stage is divided on the basis of
    coupling:
A) R-C coupled

B) Transformer coupled

C) Direct Coupled
On the basis of operation
A)   Class A:
     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
B) Class B Amplifier
   Class B Amplifier uses two complimentary transistors
    (an NPN and a PNP) for each half of the output waveform.
    One transistors for the positive half of the waveform and
    another 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. 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
   As the output transistors for each half of the
    waveform, both positive and negative, requires a
    base-emitter voltage greater than the 0.7v
    required for the bipolar transistor to start
    conducting, 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". This type of distortion is called
Class AB Amplifier
   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. 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 Class A.
Class AB Output Waveform
Class AB Amplifier (Cont’d)
   The class of operation for an amplifier is very important as
    it determines both the efficiency and the amount of power
    that the amplifier consumes and dissipates in the form of
    wasted heat, which 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 makes any electronic circuit inefficient and will
    result in 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
    nearly 70%. Basically, a Class A amplifier gives a more
    linear output meaning that it has, Linearity over a larger
    frequency response.
Class C Amplifier
    In class C operation, collector current flows for less
     than one half cycle of the input signal,.
    Less than 50% of the input signal is used (conduction
     angle Θ < 180°). The advantage is potentially high
     efficiency, but a disadvantage is high distortion.
    The class C operation is achieved by reverse biasing
     the emitter-base junction, which sets the dc operating
     point below cutoff and allows only the portion of the
     input signal that overcomes the reverse bias to cause
     collector current flow
    The class C operated amplifier is used as a radio-
     frequency amplifier in transmitters.
Class D Amplifier
     A Class D amplifier or switching amplifier is an
     electronic amplifier where all power devices (usually
     MOSFETs) are operated as binary switches. They are
     either fully on or fully off. Ideally, zero time is spent
     transitioning between those two states. Theoretical
     power efficiency of class D amplifiers is 100%. That is
     all of the power supplied to it is delivered to the load,
     none is turned to heat. This is because a switch in its
     on state will conduct all current but has no voltage
     across it, hence no heat is dissipated. And when it is
     off, it will have the full supply voltage standing
     across it, but no current flows through it. Again, no
     heat is dissipated. Real life power MOSFETs are not
     ideal switches, but practical efficiencies well over
     90% are common
On the basis Of Bandwidth
A)   Narrow Band Amplifier( RF or Tuned Amplifier)
     An amplifier which increases the magnitude of signals over a
     band of frequencies whose bandwidth is small compared to the
     average frequency of the band
A)   Wide band Amplifier (Video Amplifier)
     These deal with video signals and have varying bandwidths
     depending on whether the video signal is for SDTV, EDTV,
     HDTV 720p or 1080i/p etc..

				
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posted:10/27/2012
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