l09 power amplifier (class b and c)
TRANSCRIPT
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Lecture 9Power Amplifiers
-Class B
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Class-B Amplifier :-
vin
vout
Av
Class-B operation :-
Common-collector class-B amplifier :-
+VCC
v
0
+0.7V
Transistorconducts
outvin
Transistor off
Class-B amplifier provides an output signal varying over one-halfthe input
signal cycle + zero phase shift.
?? where is the Q-point on the load line ???
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The dc bias point for class-B amplifier is therefore at 0 volt.
i.e. biased at cutoff :-
ICQ
= 0 and
V =VCEQ CE (off )
The advantage of a class-B amplifier is that the collector current is zero
when the input signal to the amplifier is zero.
Therefore the transistor dissipate no power in the quiescent condition,
i.e. more efficient !!
class-B amplifier was developed to improve on the low efficiencyrating of the class-A amplifier.
Obviously, the output is not a faithful reproduction of the input if only onehalf-cycle is present.
Therefore, a two-transistor configuration, is necessary to get a sufficiently
good reproduction of the input waveform.
This amplifier configuration is known as push-pull emitter follower (push-
pull amplifier)orcomplementary-symmetry amplifier.
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4Push-Pull Operation :-
Class-B push-pull amplifier circuit :-
+VCC
Q1= on
RL
Q2= on
-VCC
Q1
Q2v
in
The circuit configuration feature is the use of complementary transistors,
i.e. one of the transistors is a npnand the other is a pnp.
The term push-pull comes from the fact that two transistors in a class-B
amplifier conduct on alternating half-cyclesof the input.
The combined half-cycles then provide an output for a full 3600of operation.
Note :- Need dual-polarity power supplies.
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No Input :-
When the transistor is in its quiescent state (no input), bothtransistors are
biased at cutoff.
Positive Input :-
During the positive half-cycleof the input signal, Q1is biased above cutoff,
and conduction results through the transistorRL.
During this time, Q2is still biased at cutoff.
provide output on the positive-output half-cycle.
Negative Input :-
During the negative half-cycle of the input signal, Q1 is returned to the
cutoff state, and Q2is biased above cutoff.
As a result, conduction of Q2start to built while Q1 remains off.
provide output on the negative-output half-cycle.
The combined half-cycles then provide an output for a full 3600of operation.
It is important that the two transistors in a push-pull configuration be
carefully matched.
?? WHY ??
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6Crossover Distortion :-
Among the disadvantagesof a class-B amplifier is that the nonlinear cut-
offregion is included in the operation range.
Because of the biasing arrangement, class-B amplifiers are subject to a type
of distortion.
When VB= 0, the input signal voltage must exceed VBEbefore a transistor
conduct.
Therefore, there is a time interval between the positive and negative
alternations when neither transistor is conduction.The resulting distortion in the output waveform is quite common and is
called crossover distortion.
To prevent crossover distortion, both transistors will normally be biased at a
level that is slightly abovecutoff.
Biasing both transistors slightly above cut-off will allow the amplifier to
provide a linear output that contains no distortion.
Class-AB Amplifier
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7Class-AB Amplifier :-
To eliminate crossover distortion, both transistors in the push-pull
arrangement must be biased slightly above cut-off when there is no signal.
This can be done with, for example, a voltage-divider arrangement.
This variation of the class B push-pull amplifier is designated as class-AB.
(1) Voltage-Divider Bias :-
+VCC
RL
R1
R2
R3
R4
Q1
Q2
Q1= on
Q2= on
vin
Voltage-divider bias class-AB amplifier.
Note RLis capacitively coupled
(dual-polarity power supplies single-polarity power supply)
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However, difficult to maintain a stable bias point with this circuit due to
changes in VBEover temperature changes. (i.e. temp Q-point )
A more stable arrangement
(2) Diode Biasing Circuit:-
R1
D1
D2
+VCC
Q1
RL
Q2
R2
When the diode characteristics of D1 and D2 are closely matched to the
transconductance characteristics of the transistors, a stable bias can be
maintained over temperature.
This can be also be accomplished by using the base-emitter junction of two
additional transistors instead of D1and D2.
Although technically incorrect, class-AB amplifiers are often referred to
as class-B in common practice.
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9DC Operating Characteristics :-
+VCC
+
D1
D2
R2
Q1
Q2
R1
VCC
2
-+ +
VCC VCC
2 2
- -
DC equivalent circuit.
Assume :- (i) R1
=R2,
(ii) transconductance characteristic of the diodes and the
transistors are identical.
Q-point :-
VBE1Q VBE2Q , VCE1Q VCE2Q , IC1Q IC2Q , VCC 2VCEQ
Because both transistors are biased near cutoff :- ICQ
= 0
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10AC Operating Characteristics :-
VCE
VCC
VCEQ
IC(sat)
IC(mA)
aclo
ad
line
0V
Q1= on
Q2= on
VCEQ
=V
CC
2
vce
ic
RL
Q1
Q2
21 RR
AC equivalent circuit. AC load line.
Under maximum conditions :-
both transistors Q1 and Q2 are alternately driven from near cutoff to near
saturation
V VCC CCQ V Q 0
1 CC 2
2 2
and :-
v Vce (peak ) CEQ
VCCV
CEQ
2
vce (peak )
VCEQ VCCI = = =
C ( sat )
R R 2RL L L
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11In ac operation -
+VCC
RL
R1
R2
Q1
Q2
Q1= on
Q2= on
vin
D1
D2
vo
2CC
V
0
vL
0
CC
vin= +ve
When input vin is positiveand Q1is conducting, current is drawn from the
power supply and flows through Q1to the load.
vin= -ve
When Q1is cut-off by a negative input, nocurrent can flow from the supply.At those times, Q2 is conducting and capacitor CCdischarges through that
transistor.
Thus, current flows from the load, through CC, and through Q2 to ground
whenever the input is negative.
The RLCC time constant must be much great than the period of the lowest
signal frequency.
The lower cut-off frequency due to CC is given by f =CL
R C2
1
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12Power Calculations :-
(1) DC Input Power :-
The total (dc) input powercomes from the VCCsource :-
P (dc) = V Ii CC CC
I =I +ICC C ( ave ) 1
I I (I >>I )CC C ( ave ) C ( ave ) 1
P (dc) = V Ii CC C ( ave )
The total current drawn from the supply is the sum of the average Q1collector current and the current through the amplifier base circuit. The
average value of the current through the collector of Q1is given as -
I
1 IC ( sat )I =
T
I dt =C ( ave ) 0 C
t T
Ic(sat)
TT/2
Ic(ave)
c
i.e. just a standardIaveequation for the half-wave rectifier.
IC ( sat )Pi(dc) = V
CC
V ICC C ( sat )Pi (dc) =
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13(2) Maximum AC Output Power :-
IC(mA)
VCE
VCC
VCEQ
IC(sat)
acloadlin
e
0V
Q1= on
Q2= on
VCEQ
=V
CC
2
vce
ic
RL
Q1
Q2
21 RR
The class-B amplifier has the same (ac)output power characteristics as the
class-A amplifier :-
v2
o ( rms )P (ac) = i v =o c ( rms ) o ( rms )
RL
The maximum load poweris:
P (ac) = i vo max c (max)( rms ) o (max)( rms )
I VC ( sat ) CEQ
P (ac) =o max
2 2
IC ( sat ) VCCP (ac) =o max 2 2 2
I VC ( sat ) CC
P (ac) =o max
4
I V
PC ( sat ) CC
o(ac ) 4 = 100% = 100% = 100% = 79%
P( V I 4i dc ) CC C ( sat )
max 79%
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Power Amplifiers
-Class C
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Class-C Amplifier :-
V in VoutAv
Class-C amplifier operation (inverting).
Basic Operation :-
The transistor is biased with a negative VBE. Thus it will conduct only whenthe input signal is above a specified positive value.
i.e. transistor ON when Vin> VBB+ VBE
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A class-C amplifier load line, where VBEQis set to a negative value.
The power dissipation of the transistor in a class-C amplifier is low because
it is on for only a small percentage of the input cycle.
For example :-
If a sinusoid forms the input to a class-C amplifier, the output consists of
blipsat the frequency of the input.
Since this is a periodic signal, it contains a fundamental frequency
component plus higher-frequency harmonics.
If this signal is passed through an inductor-capacitor (LC) circuits tuned to
be resonant at the fundamental frequency, the output is approximately a
sinusoidal signal at the same frequency as the input.
This approach is often used if the signal to be amplified is either a pure
sinusoid or a moregeneral signal with a limited range of frequencies.
Class-C amplifiers are capable of providing large amounts of power,
They are often used for transmitter power stages, such as radio or
communications, where a tuned circuit is included to eliminate the higherharmonics in the output signal.
max> 98%
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17Tuned Class-C Amplifier :-
Because the collector voltage (output) is not a replica of the inputs, the
resistively loaded class-C amplifier is of no value in linear applications.
It is therefore necessary to use a class-C amplifier with a parallel resonant
circuit, as shown in Figure (a).
The resonant frequency of the tuned circuit is determined by the formula :-
1f =
2 LC
The tuned circuit in the output will provide a full cycleof output signal for
the fundamental or resonant frequencyof the tuned circuit of the output.
This type of operation is therefore limitedto use at one fixed frequency, as
occurs in a communications circuit, for example.
Operation of a class-C circuit is not intendedprimarily for large-signal or
power amplifiers.
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18Power Transistor Heat Sinking :-
Power transistor can dissipate many watts.
For example :- 2N3055, an inexpensive power transistor of great popularity,can dissipate as much as 115 wattsif properly mounted.
All power devices are packaged in cases that permit contact between a metal
surface and an external heat sink.
In most cases that metal surface of device is electrically connected to one
terminal (e.g. for power transistors the case is always connected to the
collector).
Insulator :- insulated from heat sink, as is usually necessary, especially if
several transistors are mounted on the same sink.
Chassis or heat sink :- provides additional surface area to conduct heat
away from the transistors more quickly to prevent overheating.
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The whole point of heat sinking is to keep the transistor junction (or the
junction of some other device) below some maximum specified operating
temperature.
For silicon transistors in metal packages the maximum junction temperature
is usually 2000C, whereas for transistors in plastic packages it is usually
1500C.
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2020
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21Summary:-
Power Amplifiers
Amplifier Efficiency
Amplifier Classification
Class-A Amplifier
Basic Operation Principle
DC Operating Characteristics
AC Operating Characteristics
AC Load Line
Amplifier CompliancePower Calculations
Maximum Efficiency
Class-B Amplifier
Basic Operation Principle
Push-Pull Emitter Follower
Crossover Distortion
Class-AB Amplifier
Voltage-Divider ConfigurationDiode Bias Configuration
DC Operating Characteristics
AC Operating Characteristics
Power Calculations
Maximum Efficiency
Class-C Amplifier
Basic Operation Principle
Tuned Class-C AmplifierBasic Operation Principle
Power Transistor Heat Sinking