Power amplifiers
It is used to deliver a large amount of power to the load it contain bulky component .A power transistor of large surface area and metal case is suitable for power amplifier .A power amplifier is used as the large stage of a communication system .It is widely used in audio component radios,TV receivers etc
Efficiency /Conversion efficiency/collector efficiency
η=\frac{P_{oac}}{P_{odc}}
=\frac{V_{rms}I_{rms}}{V_{cc}I_c}
=\frac{\frac{V_m}{\sqrt{2}}\frac{I_m}{\sqrt{2}}}{V_{cc}I_c}
=\frac{\frac{V_{pp}}{2\sqrt{2}}\frac{I_{pp}}{2\sqrt{2}}}{V_{cc}I_c}
=\frac{V_{pp}I_{pp}}{8V_{cc}I_c}
Class A Power amplifiers(series fed)
An amplifier is of type class A if its output remain in the active region during a complete cycle of sine wave input signal .It is an amplifier under normal condition i.e the output never saturates or cut off.If the input is 360°,then output is also 360° ,i.e distortion is very low
Efficiency
V_{pp}=V_{cc}
I_{pp}=\frac{V_{cc}}{R_c}
I_c=\frac{V_{cc}}{2R_c}
η =\frac{V_{pp}I_{pp}}{8V_{cc}I_c}
=\frac{V_{cc}\frac{V_{cc}}{R_c}}{8V_{cc}\frac{V_{cc}}{2R_c}}
=\frac{1}{4}=25%
η is low .So class A is never used as a power amplifier
Transformer coupled(Class A power amplifier)
Instead of resistive coupling transformer coupling is used
R^{,}_{L}=(\frac{N_1}{N_2})^2R_L
N_2=2N_1
\frac{N_1}{N_2}=\frac{V_1}{V_2} =>
V_2=2V_1
R^{,}_{L} is the resistance reflected to
1° winding.It acts like
R_c in series fed class A
V_{pp}=2V_{cc}
I_{pp}=\frac{2V_{cc}}{R^{,}_{L}}
I_c=\frac{V_{cc}}{R^{,}_{L}}
η =\frac{V_{pp}I_{pp}}{8V_{cc}I_c}
=\frac{2V_{cc}\frac{2V_{cc}}{R^{,}_{L}}}{8V_{cc}\frac{V_{cc}}{R^{,}_{L}}}=\frac{1}{2}=50%
Harmonic Distortion/Non-linear /amplitude distortion
The presence of unwanted frequency components in the output which are harmonics of the input frequency is called harmonic distortion .When a sinusoidal signal is applied to a transistor ,non-linearity occurs.Some portion of the signal is amplified more than the other portion
I_c=K_1I_b(linear circuit)
with harmonic distortion
I_c=K_1I_b+K_2I_B^2+K_3I_B^3....
if
I_b is sinusoidal
I_b=I_bcosωt
I_c=K_1I_bcosωt+K_2I_B^2I_bcos^2ωt+K_3I_B^3cos^3ωt....
=K_1I_bcosωt+K_2I_B^2[\frac{1+cos2ωt}{2}]....
=K_1I_bcosωt+\frac{1}{2}K_2I_B^2+\frac{1}{2}K_2I_B^2[cos2ωt]....
=B_1cosωt+B_0+B_2cos2ωt....
D_2=\frac{B_2}{B_1}(2^{nd}) D_3=\frac{B_3}{B_1}(3^{rd}) D_4=\frac{B_4}{B_1}(4^{th})
Total harmonic distortion=
\sqrt{D^2_2+D^2_3+D^2_4....}
Class A Push-pull power amplifier
During positive half cycle
Q_1 conducts,So
I_{c1} flows
During negative half cycle
Q_2 conducts,So
I_{c2} flows
R^{,}_{L}=\left( \frac{\frac{N_1}{2}}{N_2}\right)^2R_L
=\left( \frac{\frac{N_1}{2}}{N_1}\right)^2R_L=\frac{R_L}{4}
V_{cc} is center tapped to
N_1 in first half cycle ,only
Q_1 conduct.So effective primary winding is
\frac{N_1}{2} same for
Q_2
Output current
I_{c1}=B_0+B_1cosωt+B_2cos2ωt+.... I_{c2}=B_0+B_1cos(ωt+180°)+B_2cos2(ωt+180°)+.... I_{c2}=B_0-B_1cos(ωt)+B_2cos(2ωt)+.... Total current
I_{c}=k[I_{c1}-I_{c2}] =2kB_1cos(ωt)+2B_3cos(3ωt)+....
Thus even harmonics are eliminated
Class B push-pull (Transformer coupled)
In normal class B amplifier output current flows only for one half cycle i.e conduction angle is 180° to ensure 360° operation class B push-pull is used
Q_1 conducts during positive half cycle of input ,so
I_{c1} flows
During negative half cycle of input ,
Q_2 conduct
I_{c2} flows.these 2 current are combined at output
Eficiency
V_{pp}=2V_{cc} I_{pp}=\frac{2V_{cc}}{R^{,}_{L}} This circuit resembles action of a fullwave rectifier therfore
I_c=\frac{2I_{m}}{Π}=\frac{2V_{cc}}{ΠR^{,}_{L}}η =\frac{V_{pp}I_{pp}}{8V_{cc}I_c}=\frac{2V_{cc}\frac{2V_{cc}}{R^{,}_{L}}}{8V_{cc}\frac{2V_{cc}}{ΠR^{,}_{L}}}=\frac{Π}{4}=78.5%
Complementary Class B push-pull power amplifier
or
transformerless power amplifier
Class B push-pull amplifier uses 2 transfomer which make it bulky and costly .To avoid using transformer complementary symmetry class B is used
It consist of NPN and PNP transistor >During positive half cycle of input ,
Q_1 conducts and
I_{c1} flows from
V_{cc1} through
R_L .During negative half cycle of input
Q_2 conducts and
I_{c2} flows from
V_{cc2} through
R_L .Thus we get a complete amplified waveform of input across
R_L
Cross over distortion
Since the transistors are biased at cutt-off no current flows through the load till the amplitude of input signal exceeds cut in voltage .It means that the amplifier cannot reproduce or amplify the input signal at zero crossing point ,This type of distortion is called cross over distortion
Class AB push-pull(Circuit same as Class A push-pull)
Class AB is a compromise between class A and class B .Class B amplifiers are highly efficient but their output waveform is distorted due to cross over,This occur in class B because of the absence of current at zero crossing point of input signal .This limitation can be overcome by biasing the transistor just at cut in (0.7V for Si).This resulting configuration is Class AB. To reduce harmonic distortion also class A is used the value of
R_2 is chosen in such a way that only 0.7v is allowed to drop across
R_2 .Performance of class AB is between class A and class B ,less efficient than class B and more than class A(0.5 & 0.785) .Distortion in Class AB is less than class B
Large signal tuned amplifier
Class C power amplifier
Class C amplifier is tuned amplifier which can amplify only a narrow band of frequencies around the center frequency.The conduction angle is less than 180°.The output contain lots of harmonics and these are eliminated by using tuned circuit as load
C_{c},R and Base emitter diode acts as a clamping circuits which clamps the input signal towards negative .Hence the transistor conduct only for a short duration during each positive peak of input signal .the output current
I_{c} appears in the form of pulses .But the CE voltage will sinosuidal because of tank circuit
V_{CE} will be 180° out of phase with
V_{BE}
Power Dissipation is less
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