Teacher: Dr. LUU THE VINH
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The chapter outline is shown below.
The chapter outline is shown below.
Basic Stages
Emitter Follower
Push−Pull Stage
and Improved
Variants
Basic Stages
Emitter Follower
Push−Pull Stage
and Improved
Variants
Large-Signal
Considerations
Omission of
PNP Transistor
High-Fidelity
Design
Large-Signal
Considerations
Omission of
PNP Transistor
High-Fidelity
Design
Heat
Dissipation
Power Ratings
Thermal Runaway
Heat
Dissipation
Power Ratings
Thermal Runaway
Efficiency and
PA Classes
Efficiency of PAs
Classes of PAs
Efficiency and
PA Classes
Efficiency of PAs
Classes of PAs
PA - Power Amplifier
13.1 General Considerations
•The reader may wonder why the amplifier stages studied
in previous chapters are not suited to high-power
applications. Suppose we wish to deliver 1 W to an 8Ω-
speaker. Approximating the signal with a sinusoid of peak
amplitude VP, we express the power absorbed by the
speaker as:
•The reader may wonder why the amplifier stages studied
in previous chapters are not suited to high-power
applications. Suppose we wish to deliver 1 W to an 8Ω-
speaker. Approximating the signal with a sinusoid of peak
amplitude VP, we express the power absorbed by the
speaker as:
Where VP = p2 denotes the root mean square (rms) value
of the sinusoid and RL represents the speaker impedance.
For RL = 8Ω and Pout = 1 W,
Where VP = p2 denotes the root mean square (rms) value
of the sinusoid and RL represents the speaker impedance.
For RL = 8Ω and Pout = 1 W,
Also, the peak current flowing through the speaker is given by
Also, the peak current flowing through the speaker is given by
VP = 4V
(13.1)
Important observations
1) The resistance that must be driven by the amplifier is
much lower than the typical values (hundreds to
thousands of ohms) seen in previous chapters.
2) The current levels involved in this example are much
greater than the typical currents (milliamperes)
encountered in previous circuits.
3) The voltage swings delivered by the amplifier can hardly
be viewed as “small” signals, requiring a good
understanding of the large-signal behavior of the circuit.
4) The power drawn from the supply voltage, at least 1W,
is much higher than our typical values.
5) A transistor carrying such high currents and sustaining
several volts (e.g., between collector and emitter)
dissipates a high power and, as a result, heats up. High-
power transistors must therefore handle high currents
and high temperature.
1) The resistance that must be driven by the amplifier is
much lower than the typical values (hundreds to
thousands of ohms) seen in previous chapters.
2) The current levels involved in this example are much
greater than the typical currents (milliamperes)
encountered in previous circuits.
3) The voltage swings delivered by the amplifier can hardly
be viewed as “small” signals, requiring a good
understanding of the large-signal behavior of the circuit.
4) The power drawn from the supply voltage, at least 1W,
is much higher than our typical values.
5) A transistor carrying such high currents and sustaining
several volts (e.g., between collector and emitter)
dissipates a high power and, as a result, heats up. High-
power transistors must therefore handle high currents
and high temperature.