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Power Amplifiers
Unit – 4.1
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Classification of Power Amplifiers Power amplifiers are classified based on the Q
point If the operating point is chosen at the middle of the
load line, it is called Class A amplifier If the operating point is chosen at the cut-off point
it is called Class B amplifier If the operating point is chosen beyond the cut-off
point it is called Class C amplifier It conducts for 3600
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Class A amplifier
The Q point is chosen at the middle of load line This will give equal swing on either direction Both halves of the input comes at the output Hence Class A will give (amplitude) distortionless
output It can handle only small signals Its efficiency is less
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Ic
Vce
Ib = 60μA
Ib = 50μA
Ib = 20μA
Ib = 30μA
Ib = 40μA
10mA
2mA
4mA
6mA
8mA
24 V0 V
Q
A
B10mA
6mA
4mA
Class A
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Class B amplifier The Q point is chosen at the cut-off point This will give swing only on one direction Only one half of the input comes at the output Hence Class B will give (amplitude) distorted
output It can handle large signals Its efficiency is high It conducts for 1800
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Ic
Vce
Ib = 60μA
Ib = 50μA
Ib = 20μA
Ib = 30μA
Ib = 40μA
10mA
2mA
4mA
6mA
8mA
24 V0 V
Q
10mA
0mA
Class B
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Class C amplifier The Q point is chosen at the beyond the cut-off
point This will give only a partial swing in one direction Only a portion of the input comes at the output Hence Class C will give (amplitude) severely
distorted output It can handle large signals It conducts for less than 1800
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Ic
Vce
Ib = 60μA
Ib = 50μA
Ib = 20μA
Ib = 30μA
Ib = 40μA
10mA
2mA
4mA
6mA
8mA
24 V0 V
Q`
10mA
0mA
Class C
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Class A
Class B
Class C
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Distortionless amplifier
Out of the 3 amplifiers, Class C is unsuitable as the distortion is very heavy
Class A is the best, as it gives distortionless output But Class A cannot handle large signals as
required by the Power Amplifier Though Class B gives heavy distortion, it gives out
one half of the signal perfectly And Class B can handle large signals
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Class A Audio Amplifier
As we have seen out of the 3 classifications, Class A is the best, as it does not give any distortion
Among the configurations, we know that CE is the best as it gives maximum power gain
A CE amplifier will have high output impedance Unfortunately for an audio amplifier, the output
device is the speaker which has a low impedance
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Impedance Matching The speaker impedance is typically about 4 Ω Hence there is a mismatch between the high Zo of
the amplifier and the low impedance of the speaker This will result in loss of gain This can be avoided by connecting a transformer at
the output stage The primary winding will match the high Zo of the
amplifier while the secondary will match the low impedance of the speaker
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Class A Audio Amplifier
270 K 5.6 K
Re
Rb1
Rb2 Ce
Rc
Vcc
270 K
Re
Rb1
Rb2 Ce
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Drawback The drawback of this circuit is that it cannot
handle large signals In a Class A amplifier, the operating point is
chosen around the middle of the load line If the signal exceeds the cut-off point, the output
current stops and any signal with a lower amplitude will not come at the output
Similarly, if the signal exceeds the saturation point, the output current cannot increase any further, even if the input signal increases
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Ic
Vce
Ib = 60μA
Ib = 50μA
Ib = 20μA
Ib = 30μA
Ib = 40μA
10mA
2mA
4mA
6mA
8mA
24 V0 V
Q
A
B Class A
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Class B Push-Pull Amplifier
To avoid this we can use Class B which has a greater signal handling capacity
But Class B will give only one half of the signal Hence we can use 2 Class B amplifiers One for one half and one for the other half This type of amplifier is called Push-Pull
Amplifier
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Class B Push-Pull
Vcc
T1
T2
T3
TR2
TR1
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Push-Pull Circuit TR1 and TR2 are output transistors connected
back to back, with their emitters grounded The output transformer TR1 couples the push-pull
output to the speaker In the Push-Pull arrangement T1 conducts for one
half of the signal & T2 conducts for the other half Both are biased in Class B and each gives one half
of the signal & the combined output is coupled to the speaker
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Push-Pull Circuit
The Driver Transformer TR2 gives 2 out of phase signals
During one half, the +ve half forward biases T1 while the –ve half reverse biases T2
Thus when T1 conducts, T2 is cut-off & vice-versa
This way both the transistors conduct alternately to give the full signal output
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Class D Amplifier During the +ve half cycle Q1 gets Forward Bias and it
conducts During the -ve half cycle Q2 gets Forward Bias and it
conducts Thus both the transistors conduct alternately The amplifier works for 3600
No distortion 100% efficiency
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During the first half T1 conducts
Ic flows from the centre-tapping through T1 to ground
This half is coupled to the speaker through TR1
Working of Push-Pull CircuitVcc
T1
T2
T3
TR2
TR1
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During the second half T2 conducts
Ic flows from the centre-tapping through T2 to ground
This half is coupled to the speaker through TR1
Working of Push-Pull CircuitVcc
T1
T2
T3
TR2
TR1
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Drawbacks Though this circuit functions well it has a few
drawbacks Transformer coupling affects the quality of
output Phase shifting circuit is a must Both these drawbacks can be avoided if we use
one pair of PNP and NPN transistors at the output
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T1
T2
Vcc Complementary Symmetry Amplifier
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Complementary Symmetry Amplifier
This circuit uses one NPN transistor & one PNP transistor at the output stage
During the +ve half, T1(NPN) base gets forward bias & it conducts while T2 (PNP) gets reverse biased and does not conduct
This gives one half of the signal at the speaker coupled to the emitter
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Complementary Symmetry Amplifier
During the other half, T2 gets forward bias and conducts while T1 gets reverse biased and does not conduct
Thus T1 & T2 conduct alternately giving a distortionless output
This circuit does not require a phase shifter
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Cross – over distortion Class B Push-Pull amplifier has one limitation As the phase of the signal changes from +ve to –ve
(or vice-versa) one transistor stops conducting while the other begins conducting
But the transistor cannot conduct instantaneously as it requires a minimum Vbe before it starts conducting
Thus as the signal crosses over zero, a distortion occurs
This is called Cross over distortion
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Cross – over distortion
Vbe
-Vbe
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Class AB amplifier This circuit overcomes cross-over distortion Biasing is done such that even if there is no input
signal, a small current keeps the output transistor conducting
This circuit uses 2 diodes whose characteristics matches with that of the BE junction of the output transistors
Biasing resistors R1 & R2 are also identical values
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T1
T2
Vcc Class AB amplifier
R1
R2
D1
D2
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Symmetrical components Since R1 & D1 are identical to R2 & D2, the diode
junction as well as the output point will be at half the supply voltage
Because of symmetry both T1 & T2 will conduct equally
Even when there is no input signal, there will be a current Icq = (I/2 Vcc – 0.6) / R1
This will keep the output transistors conducting
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Elimination of cross-over distortion Normally, during cross-over there will not be any
output till the non-conducting transistor gets the minimum Vbe
This causes distortion This has been eliminated here, since the 0.6 V
across the diodes keep the transistors on and gives a continuous output signal without producing cross-over distortion
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Thermal stability In addition, the two diodes also provide thermal
stability They prevent the output transistors going to
Thermal Run Away When the output current is high, heat dissipation is
more The increase in temperature produces more charge
carrier in the BE junction of T1 & T2
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This increases Ib & hence Ic This in turn increases the power dissipation &
hence the heat This chain goes on till too much current flows and
destroys the transistors This is called Thermal Run Away This is arrested by the diodes in the output circuit
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When the charge carriers increase in the B-E junction of T1 & T2, a similar increase takes place in D1 & D2, due to matching characteristics
This increase in the diode current, produces more drop across R1 & R2 and brings down the forward bias at the base of T1 & T2
Thus the 2 diodes prevent cross-over distortion as well as provide thermal stability
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End of Unit – 4.1