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   Lecture 9
Power Amplifiers
   -Class B
                                        2

Class-B Amplifier :-



              v in              Av                        v out




Class-B operation :-

Common-collector class-B amplifier :-


                                                  +V CC



                        +0.7V

                                Transistor
                                conducts

       v in                                               v out


                                                     0
                                 Transistor off




Class-B amplifier provides an output signal varying over one-half the input
signal cycle + zero phase shift.


              ?? where is the Q-point on the load line ???
                                        3



The dc bias point for class-B amplifier is therefore at 0 volt.

i.e. biased at cutoff :-

                                      I CQ = 0            and

                                     VCEQ = VCE (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 efficiency
      rating of the class-A amplifier.



Obviously, the output is not a faithful reproduction of the input if only one
half-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) or complementary-symmetry amplifier.
                                      4
Push-Pull Operation :-

Class-B push-pull amplifier circuit :-

                                                     +V CC




                                                Q1




      Q 1 = on
                                                                 RL


                     v in                       Q2

      Q 2 = on




                                                     -V CC




The circuit configuration feature is the use of complementary transistors,

     → i.e. one of the transistors is a npn and 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-cycles of the input.


The combined half-cycles then provide an output for a full 3600 of operation.


Note :- Need dual-polarity power supplies.
                                      5



No Input :-
When the transistor is in its quiescent state (no input), both transistors are
biased at cutoff.


Positive Input :-
During the positive half-cycle of the input signal, Q1 is biased above cutoff,
and conduction results through the transistor RL .
During this time, Q2 is 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 Q2 is biased above cutoff.
As a result, conduction of Q2 start 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 3600 of operation.


It is important that the two transistors in a push-pull configuration be
carefully matched.

                               ?? WHY ??
                                      6
Crossover Distortion :-
Among the disadvantages of a class-B amplifier is that the nonlinear cut-
off region 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 VBE before 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 above cutoff.
Biasing both transistors slightly above cut-off will allow the amplifier to
provide a linear output that contains no distortion.

                        → Class-AB Amplifier
                                        7
Class-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 :-

                                                              +V CC


                                   R1



                                                 Q1

                                   R2



                                   R3                                 RL
  Q 1 = on
                                                 Q2

                   v in
  Q 2 = on                         R4




                 Voltage-divider bias class-AB amplifier.


Note RL is capacitively coupled
     (dual-polarity power supplies → single-polarity power supply)
                                     8



However, difficult to maintain a stable bias point with this circuit due to
changes in VBE over temperature changes. (i.e. Δtemp → ΔQ-point )

A more stable arrangement →

(2) Diode Biasing Circuit :-

                                                +V CC


                               R1



                                         Q1

                               D1



                               D2                       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 D1 and D2.

Although technically incorrect, class-AB amplifiers are often referred to
as class-B in common practice.
                                       9

DC Operating Characteristics :-
                                            +V CC



                                                      +


                              R1
                                                     VCC
                                                      2
                                           Q1

                              D1
                                                      -
                      +                               +
                              D2

                     VCC                   Q2
                                                     VCC
                      2                               2

                              R2


                      -                               -

                           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 :-         I CQ = 0
                                       10


AC Operating Characteristics :-
                                                                I C (mA)
                                               I C(sat)




                                                                           ac l
                                                          ic




                                                                                oad
                                                                                lin e
                                                                                                         V CC
                                                                                              V CEQ =
                                                                                                          2
                     Q2    Q1
                                      RL
                                                                                                                V CE
         R1    R2                                                               vce           Q 2 = on




                                                                       Q 1 = on

                                                                  0V                  V CEQ              V CC




     AC equivalent circuit.                                       AC load line.

Under maximum conditions :-

both transistors Q1 and Q2 are alternately driven from near cutoff to near
saturation

                     VCC                                                   VCC
              Q1 →       ⇔ VCC                             Q2 → 0 ⇔
                      2                                                     2

and :-
                                 v ce ( peak ) ≈ VCEQ

                                                   VCC
                                    VCEQ ≈
                                                    2

                                                   v ce ( peak ) VCEQ VCC
                                   I C ( sat ) =                =    =
                                                      RL          RL   2 RL
                                     11
In ac operation -
                                  +VCC



                                                         vo

                             R1                                         vL
                                               VCC
                                                2
                                          Q1
                                                     0                  0
                             D1


                                                 CC
                             D2                               RL
    Q1 = on
                                          Q2

                 v in
    Q2 = on                  R2




vin = +ve
When input vin is positive and Q1 is conducting, current is drawn from the
power supply and flows through Q1 to the load.


vin = -ve
When Q1 is cut-off by a negative input, no current can flow from the supply.
At those times, Q2 is conducting and capacitor CC discharges 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.

                                                                      1
The lower cut-off frequency due to CC is given by –           f =
                                                                    2πRLCC
                                                 12
Power Calculations :-

(1) DC Input Power :-

The total (dc) input power comes from the VCC source :-

                                  Pi ( dc ) = VCC I CC

                                 I CC = I C ( ave ) + I 1

                                 I CC ≈ I C ( ave )                           ( I C ( ave ) >> I 1 )

                                 Pi ( dc ) = VCC I C ( ave )

The total current drawn from the supply is the sum of the average Q1
collector current and the current through the amplifier base circuit. The
average value of the current through the collector of Q1 is given as -

               Ic
   Ic(sat)
                                                                     1 T          I C ( sat )
   Ic(ave)                                            I C ( ave )   = ∫0 I C dt =
                                             t                       T              π
                    T/2    T

             i.e. just a standard Iave equation for the half-wave rectifier.


                                               ⎛ I C ( sat ) ⎞
                                 Pi (dc) = VCC ⎜             ⎟
                                               ⎝ π ⎠

                                                         VCC I C ( sat )
                                 Pi ( dc ) =
                                                                π
                                                 13
(2) Maximum AC Output Power :-
                                                                                  IC(mA)
                                                       IC(sat)




                                                                                           ac l
                                                                 ic




                                                                                               o ad
                                                                                                line
                                                                                                                       VCC
                                                                                                             VCEQ =
                                                                                                                        2
                      Q2            Q1
                                               RL
                                                                                                                             VCE
       R1    R2                                                                                              Q2 = on
                                                                                                vce




                                                                                         Q1 = on

                                                                                   0V                 VCEQ             VCC



The class-B amplifier has the same (ac) output power characteristics as the
class-A amplifier :-
                                                             vo2( rms )
                         Po ( ac ) = ic ( rms ) vo ( rms ) =
                                                               RL
The maximum load power is:

                                           Po ( ac ) max = ic (max)( rms ) v o (max)( rms )

                                                                      I C ( sat ) VCEQ
                                           Po ( ac) max =
                                                                          2         2
                                                               I C ( sat ) VCC
                                           Po ( ac) max      =
                                                                     2 2 2
                                                                 I C ( sat )VCC
                                           Po ( ac ) max =
                                                                        4
                                       I C ( sat )VCC
                  Po ( ac )                                    π
            η=                × 100% =        4        × 100% = × 100% = 79%
                  Pi ( dc )            VCC I C ( sat )         4
                                           π

                                         → ηmax ≈ 79%
       14




Power Amplifiers
   -Class C
                                      15
Class-C Amplifier :-



        V in                     Av                         V out


                 Class-C amplifier operation (inverting).



Basic Operation :-




The transistor is biased with a negative VBE . Thus it will conduct only when
the input signal is above a specified positive value.

                i.e. transistor ‘ON’ when Vin > VBB + VBE
                                     16




A class-C amplifier load line, where VBEQ is 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
“blips” at 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 more general 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 higher
harmonics in the output signal.
                                ηmax > 98%
                                      17


Tuned 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 :-

                                      1
                              f =
                                    2π LC


The tuned circuit in the output will provide a full cycle of output signal for
the fundamental or resonant frequency of the tuned circuit of the output.

This type of operation is therefore limited to use at one fixed frequency, as
occurs in a communications circuit, for example.

Operation of a class-C circuit is not intended primarily for large-signal or
power amplifiers.
                                     18
Power 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 watts if 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.
                                    19




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.
20
                                   21


Summary:-

Power Amplifiers
Amplifier Efficiency
Amplifier Classification
Class-A Amplifier
   Basic Operation Principle
   DC Operating Characteristics
   AC Operating Characteristics
   AC Load Line
   Amplifier Compliance
   Power Calculations
   Maximum Efficiency
Class-B Amplifier
   Basic Operation Principle
   Push-Pull Emitter Follower
   Crossover Distortion
Class-AB Amplifier
   Voltage-Divider Configuration
   Diode Bias Configuration
   DC Operating Characteristics
   AC Operating Characteristics
   Power Calculations
   Maximum Efficiency
Class-C Amplifier
   Basic Operation Principle
Tuned Class-C Amplifier
   Basic Operation Principle
Power Transistor Heat Sinking

				
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posted:8/13/2011
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