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Power Amplifiers

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Chapter 11 Power Amplifiers Amplifier Efficiency The ideal amplifier would be 100% efficient. The lower the Q-point position on the dc load line, the higher its ... – PowerPoint PPT presentation

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Title: Power Amplifiers


1
Chapter 11
  • Power Amplifiers

2
Amplifier Efficiency
  • The ideal amplifier would be 100 efficient.
  • The lower the Q-point position on the dc load
    line, the higher its efficiency. Class A is the
    least efficient. Class C is the most efficient.

3
The AC Load Line
  • AC load line A graph of all possible
    combinations of ic and vce for a given amplifier.
  • The ac load line for a given amplifier does not
    normally match its dc load line.

4
AC Load Line Endpoints
  • The ac values of collector saturation current and
    collector-emitter cutoff voltage are calculated
    using Q-point circuit values.

5
Compliance (PP)
  • Compliance The limit that the output circuit of
    an amplifier places on its peak-to-peak output
    voltage.
  • A measure of a circuits ability to comply with
    the demands of its load.

6
Clipping
  • Saturation clipping A type of distortion caused
    by driving a transistor into saturation.
  • Cutoff clipping A type of distortion caused by
    driving a transistor into cutoff.

7
Class A Amplifier DC Power
  • Total DC power equals the product of supply
    voltage and current.

8
AC Load Power
  • Using rms load voltage
  • Using peak load voltage
  • Using peak-to-peak load voltage
  • Maximum load power

9
Transformer-Coupled Class A Amplifier
  • This type of amplifier uses a transformer to
    couple the output signal to its load.

10
AC Operating Characteristics
  • The value of vce(off) for a transformer-coupled
    class A amplifier is nearly twice the value of
    VCC.
  • The maximum theoretical efficiency for this type
    of amplifier is 50.

11
Class B Amplifiers
  • Contain two transistors. These transistors are
    matched or complementary, depending on the
    circuit configuration.
  • Complementary transistors are npn and pnp
    transistors with identical characteristics.
  • Have a maximum theoretical efficiency of nearly
    78.5.

12
Complementary-Symmetry Class B Amplifier
  • Also referred to as a push-pull emitter follower.
  • The circuit contains complementary transistors
    that are connected as shown.

13
Class B Operation Overview
  • The transistors conduct during alternating
    half-cycles of the input signal.

14
Crossover Distortion
  • Crossover distortion Distortion caused by class
    B transistor biasing. It occurs during the time
    neither of the transistors is fully conducting.

15
DC Biasing Characteristics
  • The class B amplifier has a vertical dc load
    line.
  • The transistors are biased in cutoff.
  • The transistors have equal values of VCEQ.

16
AC Load Line
17
Circuit Gain and Impedances
  • The complementary-symmetry amplifier is an
    emitter follower, so the gain and impedance
    relationships are similar to those for a class A
    emitter follower.

18
Power Calculations
19
Diode Bias
  • Diode Bias A biasing circuit that uses two
    diodes in place of the resistor between the bases
    of the two transistors.
  • Diode bias prevents crossover distortion and
    thermal runaway.

20
Diode Bias DC Characteristics
  • The transistors are biased just above cutoff.
  • VCEQ for each transistor is half the value of
    VCC.
  • The voltage across the diode pair is
    approximately 1.4 V.
  • The resistor currents (I1 and I2) are equal.

21
Circuit Currents
22
Class AB Operation
  • Each transistor in a class AB amplifier conducts
    for more than 180 and less than 360 of the
    input signal cycle.
  • Class AB operation prevents crossover distortion.

23
Preventing Thermal Runaway
  • The diodes in the class AB amplifier prevent
    thermal runaway when they are in thermal contact
    with the transistors.
  • Thermal contact Two or more components placed
    in physical contact with each other (or a common
    surface) so that their operating temperatures are
    equal.
  • Thermal contact can be achieved by attaching a
    diode to
  • The heat-sink tab on a power transistor.
  • The heat sink on which a transistor is mounted.

24
Single-Capacitor Input
25
Transistor-Driver Input
26
Darlington Class AB Amplifier
27
Transistor Biasing
28
Dual-Polarity Class AB Amplifier
29
Maximum Transistor Power
  • The maximum power dissipated by the transistor in
    a class A amplifier is found using
  • The maximum power dissipated by each transistor
    in a class B (or class AB) amplifier is found
    using

30
Heat Sinks
  • Heat sink A large metallic object that helps to
    cool components by increasing their effective
    surface area.
  • Heat sink compound is a compound used to aid in
    the transfer of heat from a component to a heat
    sink.
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