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RCCOUPLED AMPLIFIER
Object-To study
the frequency response
characteristics of^ an^
RC coupled
amplifier.
Apparatus
required-RC
coupled amplifier (single^
stage),
multimeter, signal
generator.
Principle-An
amplifier
is a device^
which amplifies
or increases magnitude
of the^
applied
input voltage,^
current or (^) power.
If a small^ signal^ (^
weak alternating^
voltage
or
current ) is
applied
between base and^ emitter of^ a^ transistor, it
gets
amplified.
Various parts
of the
amplifier
are described^ below-
() Input^
signal
is connected to^
the amplifier
between base^ and^
emitter.
Resistance Rs is
the internal^
resistance of the^ signal
source. The^ input
signal
is in series with capacitor
Cp.
It blocks^ the direct^
current of^ the collector circuit from (^) reaching
the input stage
of the
amplifier.
R
and Rs is^
an arrangement^
which is^
used for^
self biasing
of the^
transistor (^) by voltage
divider method.
(3) RE^
is a^
resistance
connected in^ the emitter circuit for
Q point
stabilization. CE is the
bypass
capacitor
which blocks
undesired alternating^
component
of the^ amplified
current.
(4) Rc^
is the^
resistance across which output
amplified
current flows in^ the^
collector
circuit.
Cc is called the^ coupling
capacitor
which
blocks direct^
current of^ the amplifier^
in
reaching
across the^
load
resistance Ri single^
stage
amplifier
and hence to^ the second unit
of similar amplifier^
in case^ of cascade^
amplifier.
Required
reverse bias to^ the
collector emitter
circuit is^ provided^
by
means of^ 6 volt
battery
to the^
NPN
transistor.
Frequency
Response
curve-
Voltage
gain
of the^ amplifier
is defined as:
Output voltage
of the^ amplifier
Voul
Voltage gain= Vin
Input
voltage
of the^ amplifier
Voltage
gain
of the^ single^
stage stage
common emitter amplifier^
in terms^
ofh-parameters
is given by
Av (^) Zin(hoet 1RL)
Negative
sign
indicates that^ output
and input
voltage
differ in phase by^
where Ay
voltage gain ofthe^
amplifier
hpeforward
current (^) gain of
amplifier
in common emitter
configuration
Zininput impedance(= hie)
hoe
output
admittance
which
includes various
condensers formed
between
different
elements of transistor.
R load resistance
Hence voltage
gain
of the^ amplifier
depends
on the
reactance
of coupling^
condenser Ce
d output capacitance
C, which are formed^
between different elements^ of^ the^ transistor
Reactance of^
condenser
varies with^ frequency^
of the^ applied
voltage and due^ to this
reason
voltage
gain
of the amplifier
varies with^ frequency
of the^ input voltage
as
represented
in the
graph.
The
graph
which represents
variation of the^ gain of
amplifier
with variation in (^) frequency
of the applied signal
is called
frequency
response curve.
Band
Width-The
frequencies
at which gain of the
amplifier
falls (^) to 70.7% or 1W
times of the maximum gain are known as the cut off frequencies denoted by fi & f
in the graph. The diference of upper cut off frequency f2 and lower cut off frequency fi is
called the band width.
Nature of the frequency response curve can be understood as below-
At mid frequency ranger- In this range, effect of coupling condenser Cc and output
condenser C, can be neglected. Reactance of C[X= 1/ 27t fCe ] is very small, since Ce
is connected in series ( low reactance in series) it may be neglected so the effect of Ce can
be neglected in this frequency range, The reactance of C% [ Xo=. 1/27 f C% ]
has a high value as Co is very small. But Co being connected in parallel may be neglected
in equivalent circuit (high reactance in parallel may be neglected).Hence the equivalent
circuit of RC coupled amplifier does not contain condensers in the mid frequency range
hence the voltage gain remains constant.
At low frequency range-In this range voltage gain of the amplifier decreases due to
following reasons-
(1) Reactance of coupling condenser Ce becomes very high and its effect can not be
neglected in the lower frequency range. Due to high reactance of Ce the output current
passing to RL decreases and as a result output voltage and consequently voltage gain of
the amplifier decreases.
(2) Reactance of condenser CE connected in the emitter circuit becomes very high and
can not shunt the^
emitter
resistance
properly.
The
voltage drop
across the emitter
resistance makes^ the^ emitter^ less (^) forward
biased so
the collector current or the
output
voltage reduces hence the voltage gain also reduces.
At high frequeney range- In this range effect of couplinig condenser Ce is neglected
while effect^ of (^) condenser
can (^) not be
neglected as^ its^ reactance
Xo= 1/2n^
f Co
becomes very small and small reactance in parallel can not be neglected. Due to this
loading
effect
voltage gain
of
the
amplifier reduces^ at
high frequencies.
ie Band oidth
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Yege-
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Veloge
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