Download Small signal amplifiers

Power Amplifiers
Unit – 4.1
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
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
Ic
B
Ib = 60μA
10mA
Ib = 50μA
8mA
Q
6mA
Ib = 40μA
Ib = 30μA
4mA
Ib = 20μA
2mA
A
0V
24 V
Vce
Class A
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
Ic
Ib = 60μA
10mA
Ib = 50μA
8mA
Ib = 40μA
6mA
Ib = 30μA
4mA
Ib = 20μA
2mA
Q
0V
Class B
24 V
Vce
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
Ic
Ib = 60μA
10mA
Ib = 50μA
8mA
Ib = 40μA
6mA
Ib = 30μA
4mA
Ib = 20μA
2mA
Q`
0V
Class C
24 V
Vce
Class A
Class B
Class C
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
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
Impedance Matching
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
 The
Class A Audio Amplifier
Vcc
Rb1
270 K
Rb2
Rb1
Rc
5.6 K
Re
270 K
Ce
Rb2
Re
Ce
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
Ic
Class A
B
Ib = 60μA
10mA
Ib = 50μA
8mA
Q
6mA
Ib = 40μA
Ib = 30μA
4mA
Ib = 20μA
2mA
A
0V
24 V
Vce
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
Vcc
T1
TR1
TR2
T3
T2
Class B Push-Pull
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
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 & viceversa
 This way both the transistors conduct alternately
to give the full signal output
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

Working of Push-Pull Circuit
Vcc
 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
T1
TR2
T
3
T2
TR1
Working of Push-Pull Circuit
Vcc
 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
T1
TR2
T
3
T2
TR1
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
Vcc
Complementary Symmetry Amplifier
T1
T2
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
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
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
Cross – over distortion
Vbe
-Vbe
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
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
 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
 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