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.