AMPLIFIER
AMPLIFIER
 The amplifier is an electronic circuit capable of increasing
current, voltage and power. Its main component is a
transistor.
SMALL- SIGNAL AMPLIFIER
Small - Signal Amplifier
 A small- signal amplifier makes use of any of
these three circuit arrangements:
common base , common - emitter or
common – collector circuit whose names
are also explicit.For example in a commonbase circuit, the base terminal is common to
both the input and output circuits, hence
the term common-base. The word common
also denotes ac ground, so for a commonbase circuit, a capacitor bypasses the base
terminal.
common-base circuit
VOLTAGE GAIN
 Voltage gain describes how much amplification is done to the
input ac voltage. It is the ratio of the output ac voltage to the
input ac voltage.
Av = Vo / Vi
where, Av = Voltage gain
Vo = Output Voltage
Vi = Input Voltage
Biasing of Transistor
 In order for a transistor to amplify ac signals, it must operate
in the active condition. This is done by forward biasing the
base - emitter junction and by reverse biasing the base
collector junction. So, before applying any ac signal , the dc
potentials in the biasing circuit must be correct, otherwise ,
there will be no amplification.
 Other operating conditions of transistor include the cut-off
where it behaves as an open switch and the saturation where
it acts as a close switch . Both junctions have reverse bias in
cut-off while both have forward bias in saturation condition.
CASCADED AMPLIFIER
 An amplifier introduces gain in the circuit. It
increases the amplitude of input signal. However,
in a case where the input signal is too small , like
if the signal is coming from a microphone or from
an antenna, the gain of a single - stage small signal amplifier is not enough. This is when
cascading several amplifiers becomes necessary.
CASCADED AMPLIFIER
 A cascaded amplifier has several stages of amplifiers. The
output of one amplifier drives the input of the next amplifier.
Vi 1
Vo 1
Av1
Vo2
Vi2
Av 2
CASCADED AMPLIFIER
 A cascaded or multistage amplifier improves the overall gain
of the circuit. Each stage enhances the signal, thus , the signal
becomes larger and larger as it goes thru several stages of a
cascaded amplifier.
 The gain of a cascaded amplifier is equivalent to the product
of individual gains of each amplifier stage.
Avc = Av1 . Av2
Where, Avc = overall voltage gain of cascaded amplifier
Av1 = Voltage gain of first amplifier , Av1= Vo1 /Vi 1
Av2 = Voltage gain of 2nd amplifier , Av2 = Vo2 / Vi2
Seatwork:
 1. Given : 1st Stage amplifier :
Input Voltage – 5V
Output Volatge -10V
2nd Stage amplifier :
Input Voltage - 10V
Output Voltage -15 V
3rd Stage amplifier :
Input Voltage - 15 V
Output Voltage -20V
Solve for the following : Voltage gain of ampliers #1, #2 and #3 and
Total Voltage gain of the cascaded amplifiers
Solution:
 Av #1 = 10/5 = 2
 Av #2 = 15/10 = 1.5
 Av#3 = 20/15 = 1.33
 Total Av = 2 x 1.5 x 1.33
= 3.99
Coupling Techniques
 The three possible methods of linking the ac signals between
2 amplifiers in cascade are Direct Coupling, RC
Coupling and Transformer Coupling .
Direct Coupling - the ac signal runs through a wire. The
output immediately connects to the input of the next stage
without any component, hence the process is called direct
coupling.
RC Coupling – capacitors allow the flow of ac signal between
stages.
Direct Coupled Amplifier Circuit
RC Coupled Amplifier Circuit
 Transformer Coupling - the ac signal in the primary
winding induces a voltage across the secondary winding. The
primary and secondary windings are connected to the output
of the first stage and input of the second stage respectively.
NOTE : The gain of an amplifier differs when it has no load,
when it has a load and when it has bypass to ground
Transformer – Coupled Amplifier Circuit
How amplifiers work
POWER AMPLIFIERS
 Large Signal Amplifier
As its name implies, a large-signal amplifier handles large
signals. It serves as a driver in output stages of cascaded
amplifiers. Large-signal amplifier is the same as power
amplifier.
Power amplifiers raise the power level of a signal. The output
signal builds up more power across the load than the input
signal power to the amplifier. The enhancement in output
power generally results from an increase in the output
current.
Classifications of Power Amplifiers
 Power amplifiers are classified as Class A, B, AB or C.
Class A - the output current flows continuously ; thus it
is the least efficient class of amplifier. However , class A
produces the least distorted output signal.
Class B - it has a conduction of 180º. This means that
Class B allows current only during the half cycle of input
signal
 Class AB – it has characteristics in between class A and
class B amplifiers.
 Class C – It has the least period of conduction usually
from 150º to 120 º. It has the most distorted output
signal but has the highest efficiency .
Power Gain
 Power gain is the ratio of the output power to the input
power. It is also equivalent to the product of voltage and
current gain.
Av = Vo / Vi
where, Av = Voltage gain
Vo = Output voltage
Vi = Input voltage
Ai = Io / Ii
where , Ai = Current gain
Io = Output current
Ii = Input current
 Ap = Po/Pi
 Ap = Vo Io / Vi Ii
 Ap = Av Ai
where, Ap = Power gain
Po = Output power
Pi = Input Power
Voltages Vo and Vi should be expressed in root mean square
values.
Vrms = 0.707 (Vpk-pk / 2) where, Vrms = rms voltage
V pk-pk = peak-to-peak
voltage
Efficiency of an Amplifier
 The efficiency of an amplifier is the ac power of the load
divided by the dc power of the supply.
 n = P ac / Pdc
where, n = efficiency
Pac = output of ac power
Pdc = power of dc supply
The Class B Amplifier
 To improve the full power efficiency of
the previous Class A amplifier by reducing
the wasted power in the form of heat, it is
possible to design the power amplifier
circuit with two transistors in its output
stage producing what is commonly
termed as a Class B Amplifier also known
as a push-pull amplifier configuration.
 Push-pull amplifiers use two “complementary” or
matching transistors, one being an NPN-type and the
other being a PNP-type with both power transistors
receiving the same input signal together that is equal in
magnitude, but in opposite phase to each other. This
results in one transistor only amplifying one half or
180o of the input waveform cycle while the other
transistor amplifies the other half or remaining 180o of
the input waveform cycle with the resulting “two-halves”
being put back together again at the output terminal.
 Then the conduction angle for this type of
amplifier circuit is only 180o or 50% of the input
signal. This pushing and pulling effect of the
alternating half cycles by the transistors gives this
type of circuit its amusing “push-pull” name, but
are more generally known as the Class B
Amplifier.
Class B Push-pull Transformer Amplifier Circuit
Class B Push-pull Amplifier Circuit
VOLTAGE AMPLIFIER
 An amplifier whose function is to increase the voltage of the
applied signal, without necessarily increasing its power. The
output impedance must therefore be high.
 An electronic circuit whose function is to accept an input
voltage and produce a magnified, accurate replica of this
voltage as an output voltage.
 The voltage gain of the amplifier is the amplitude ratio of
the output voltage to the input voltage.
JUNCTION FIELD EFFECT TRANSISTOR
2 BASIC TYPES OF TRANSISTORS :
 The current – controlled bipolar junction transistors (BJT)
 The voltage – controlled unipolar field effect transistors
(FET)
BJT - Examples are NPN and PNP Transistors
FET - Examples are the junction field effect transistors
(JFET) and the metal oxide semiconductor transistors
(MOSFET)
Structure and symbol for JFET
FET
FET has three terminals : source , gate and drain. The source
corresponds to the emitter, the gate to the base, and the drain
to the collector in BJT. Normally , the gate is used as the
input while the drain is used as the output terminal.
FET – is a voltage- controlled transistor. The gate voltage
controls the output drain current . The controlled drain
current flows from drain to source, and the controlling gate
voltage is applied between the gate and source.
 FET – is a unipolar device. Its current
consists of electrons only if it uses an Nchannel and holes only if it uses a P-channel.
N-channel is more common than P-channel
FET. The channel is the semiconductor
connecting the drain and the source. The
gate layer uses the opposite semiconductor
type of the channel.
Some of the FET devices are depletion - mode while some are
enhancement - mode transistors.
Depletion mode transistors - are normally conducting. It
readily produces output drain current even without control
gate voltage. To turn it off, apply an appropriate gate bias
equivalent to the negative of the so called pinch - off voltage.
The pinch - off voltage produces saturation or constantcurrent region in FET, and its negative value applied across
gate and source terminals turns off the FET.
 Enhancement mode - devices are
normally non- conducting. The output drain
current is zero when there is no control
gate voltage. To turn it on , connect an
appropriate gate bias equivalent to the so
called threshold voltage.
JFET Amplifier
Amplifiers are circuits that increase the level of an input signal
without distortion. The amount of amplification can be
measured by its equivalent voltage, current or power gain.
Junction field effect transistors can be implemented as
amplifiers.
The gate terminal of JFET is normally where the input signal is
connected , and its drain terminal is where the output signal
is taken. Such connection is called a common source
amplifier. This connection is the counterpart of common
emitter amplifier in bipolar transistors.
 Other configurations of JFET are the common drain and
common gate amplifiers that correspond to common
collector and common base respectively.
 Amplifiers require good dc biasing circuit. For JFET, its gate
is normally reverse biased.
Amplifier’s Frequency Response
Frequency Response
Amplifiers have capacitances, like the external
coupling or bypass capacitors and the internal
parasitic capacitances of transistors . Frequency
affects the reactance of these capacitances and in
turn varies the gain of an amplifier. The frequency
response of an amplifier is the change in gain
when an input signal frequency varies.
 Decibel Gain
The voltage gain of an amplifier is the ratio of output voltage
to input voltage . It indicates how much the amplifier
increases the input signal.
Av = V load / V source
where,
Av = Voltage gain
V load = Load voltage
V source= Source Voltage
 Gain also uses a unit called decibel ( dB) . A decibel is the
logarithmic measurement of one voltage gain to another.
Av ( dB) = 20 log Av / Av (Ref)
where ,
Av (dB) = voltage gain in decibel
Av
= measured gain at certain frequency
Av (Ref) = reference voltage gain
The reference voltage gain Av (Ref) is usually the maximum
gain of amplifier and is evaluated as 0 dB.
 BANDWIDTH
An amplifier has an operating bandwidth and exhibits
maximum gain within this bandwidth. This means that an
amplifier has reliable gain only at a certain range of
frequencies - from the so called lower and upper cut-off
frequencies . A decrease or increase in signal frequency
beyond the operating bandwidth causes severe attenuation
making the amplifier almost useless.
Note: Attenuation is a general term that refers to any
reduction in the strength of a signal.
 BW = F U - FL
where ,
BW
FL
FU
= Bandwidth
= Lower cut-off frequency
= Upper cut-off frequency
The cut- off frequency is the frequency where the output
power drops to one-half its maximum value or where the
output voltage decreases to 70.7 percent of its maximum
value.This correspond to a 3 decibel reduction in gain
- 3 dB = 20 log 0.707
 Roll – Off
Roll -off is the attenuation in gain of an amplifier below
or above its cut-off frequencies. There is a certain
attenuation for every octave or decade . An octave is two
-times increase or decrease in frequency while a decade
is a multiple of ten.
FEEDBACK AMPLIFIER
Feedback means a portion of output signal is returned to
the input.
There are two basic categories of feedback depending on
the relative phase angle of returning feedback signal.
Positive or regenerative feedback works for oscillator
circuits. However for amplifiers, negative or
degenerative feedback is done to improve its overall
electrical characteristics.
 Benefits of a feedback system include
the ability to precisely control gain
(e.g., amplification of a signal in an op
amp), improve linear response, reduce
signal distortion, and to control signal
fluctuations. Feedback is sometimes
referred to as a “closed loop” system.
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