1
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What
is reason behind the name RC coupled amplifier?
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RC
coupled amplifier
- The coupling between two
stages in the RC coupled amplifier is done by coupling capacitor followed by
shunt resistor connection so it is called as RC coupled amplifier.
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2
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Describe the function of coupling
capacitor in the RC coupled amplifier.
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Function
of coupling capacitor
- It allows AC signal to
pass but blocks DC signal. To prevent DC interference between amplifier stages
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3
|
Describe the main reason for low
voltage and low power gain of the multistage RC coupled amplifier.
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Gain of
the multistage RC coupled amplifier
- The total gain of the multistage RC coupled
amplifier is less than the product of the gains of the individual stage or
amplifiers due to fact that when effective load of the first stage is reduced
because of shunting effect of the input resistance of the second stage
(amplifier).
- The gain of the first three stages is decreased due to shunting
effect of the 4th stage or amplifier in the four stage amplifier. However the gain of the 4th
stage does not affected.
Gain of
the stage : 4th
stage gain > 3rd stage gain > 2nd stage gain >
1st stage gain
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4
|
Why the voltage gain of the RC coupled
amplifier falls for lower and higher range of frequencies?
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Voltage gain of the RC coupled amplifier
- The voltage gain of the RC coupled amplifier is
constant over the range of frequencies from 50 Hz to 20 kHz whereas its value
drop for frequency less than 50 Hz and higher than 20 kHz.
At low range of frequencies, the voltage gain drops
due to
- The reactance of coupling capacitor becomes very
high resulting only small signal will pass through one stage to the next
stage.
- The shunt effect is not done effectively between emitter resistance
and emitter bypass capacitor due to higher reactance.
At very high range of frequencies, the voltage gain
drops due to
- The coupling capacitor acts as a short circuit due
to very low reactance therefore it will increase the loading of the next
stage resulting gain of the amplifier is reduced.
- The base current increases
due to reactance of the base-emitter become low at high range of frequencies.
- This will reduce the current amplification factor β ( IC = βIB
).
- Therefore the voltage gain is also reduced.
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5
|
What are the disadvantages of the RC
coupled transistor amplifier?
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Disadvantages
of the RC coupled transistor amplifier
- The output of the RC coupled amplifier is several
hundred ohms whereas the input of the speaker is few ohms therefore impedance
matching is very poor.
- Low voltage and power gain due loading effect of the
next stage.
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6
|
State the applications of the RC
coupled amplifier
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Application of RC coupled amplifier
- The RC coupled amplifiers are widely used as voltage
amplifiers.
- They are generally used in the initial stage of
audio system but due to poor impedance matching property it is rarely used in
the final stages of the amplification.
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7
|
State the applications of the Direct
coupling amplifiers
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Application of Direct coupled amplifier
- The direct coupled amplifiers are used for
amplifying extremely low frequencies.
- They are used for amplifying photo
electric current, thermo-coupled current etc.
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8
|
Give reason : The capacitors and
transformers are not used in the Direct coupled Amplifiers.
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Direct coupled amplifiers
- The direct coupled amplifiers are used particularly
at very low frequency ( < 10 Hz ).
- The size of the capacitor and transformer
becomes very large at low frequency therefore these types of coupling devices
are not used in the direct coupled transistor amplifiers.
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9
|
Why two complementary transistor ( PNP
and NPN ) are used in the Direct coupled amplifier?
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Direct coupled amplifier
- There are two complementary transistor NPN and PNP
are used in the direct coupled transistor in order to make the amplifier
circuit independent of the temperature changes.
- The collector of the NPN
transistor is connected to the base of the PNP transistor and so on...
- As the temperature increases, the collector current
of the NPN transistor increases resulting current amplification factor β also
increases. The above effect is compensated by PNP transistor.
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10
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What are the applications of the
transformer coupled amplifier?
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Impedance
matching
- The main application of the transformer coupled
amplifier is impedance matching.
- The impedance of the output of the amplifier
is very large whereas the load impedance consists of few ohms.
- A step down transformer of proper turns ratio is
used in order to transfer maximum power at the last stage of the amplifier. (
The impedance of the primary of the transformer is such that it is equal to
the output of the amplifier whereas the impedance of the secondary of the transformer
is equal to the load resistance. )
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11
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Why the frequency response of the
transformer coupled amplifier is poor?
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- The voltage gain of the transformer coupled
amplifier is constant only for small range of frequencies.
- This is due to
fact that the transformer primary winding is connected in series with the
collected circuit therefore the output voltage is given by product of
transformer primary reactance and collector current.
- The output voltage and
hence gain drops due to low reactance of the primary winding particularly at
low frequency.
- The capacitance between winding turns acts as a bypass
capacitor which will reduce the output voltage and hence gain particularly at
high frequency.
- Therefore the frequency response of the transformer coupled
amplifier is poor.
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12
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Describe the disadvantages of the
transformer coupled amplifiers.
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Disadvantages of the Transformer coupled amplifier
- The transformer is bulky and also expensive. Poor
frequency response over wide range of frequencies.
- Higher frequency
distortion
- The transformer coupling produces magnetic hum or noise.
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13
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Which amplifier is mostly used for the
power amplification?
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Transformer coupled amplifier
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14
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Which amplifier is mostly used for the
voltage amplification?
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RC coupled amplifier
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