Download Digital Representation of Audio Information

EE599-020
Audio Signals and Systems
Amplifiers
Kevin D. Donohue
Electrical and Computer Engineering
University of Kentucky
Related Web Sites
The understanding and design of amplifiers are
primarily the domain of electrical engineers and
technicians. Web sites below related to the design
and characterization of amplifiers:
http://www.pha.inecnet.cz/macura/audiopage.html
http://www.elexp.com/t_audio.htm
http://users.pandora.be/educypedia/electronics/amplifierclasses.htm
http://links.epanorama.net/links/audiocircuits.html#amp
Amplifier Usage in Audio
Preamplifier – Amplify small signals to intermediate
levels while maintaining good SNR and fidelity.
Sometimes performing compression (nonlinear /
logarithmic transfer characteristic to prevent hard
clipping).
Intermediate Stage Amplifier – Amplify and process
intermediate level signals with good SNR for mixing,
equalization, compression, expansion, and other signal
processing effects.
Power Amplifier – Amplifies high SNR intermediate
level signals to high power levels for driving electroacoustic transducers.
Amplifier Performance
Efficiency – Output power over Input power (including
that of the power supply).
Distortion – Total harmonic distortion (THD). For
sinusoidal signal inputs, THD is the ratio of power at
harmonic frequencies (excluding the fundamental) of the
output signal over the total output signal power.
Fidelity – Flatness of frequency response characterized
by frequency range and transfer function variation in that
range.
Amplifier Classes
Class A - Low distortion, bad efficiency. Output stage
with single transistor requires DC biased output (10-20%
efficiency).
Class B - Crossover distortion, good efficiency. Output
stage has 2 transistors so bias current is zero (~80%
efficient).
Class AB – Reduced crossover distortion, good
efficiency. Output stage has 2 transistors with biasing to
push signal out of crossover distortion range.
Class D – Moderate distortion, high efficiency, operates
in switch mode. Good for battery driven applications.
Example
Given the transfer characteristic for a class B amplifier
below, compute the THD for a 3 volt input sinusoid.
Vout
7v
-3v
-0.6v
Vin
0.6v
-7v
3v
Example
Given the transfer characteristic for a class AB amplifier
below, compute the THD for a 3 volt input sinusoid.
7v
Vout
-3v
-1.75v
Vin
1.75v
3v
-7v
Compressors
Compression: In order to improve the dynamic range of a
system without overdriving the amplifiers (or other
elements in the system) resulting in hard clipping, a softer
clip can be applied to more gradually compress the signal
amplitude as it approaches the saturation limits of the
system.
 1

 A x(t )
x(t )
y (t )   X T

 Ax(t )
for x(t)  X T
for x(t)  X T
where  is between 0 (hard clipping) and 1 (no compression).
Expanders
Expansion: In order to improve the dynamic range of a
system by reducing low level signal inputs, a lesser gain can
be applied to the signal components below a certain
threshold and a normal linear gain can be applied to signal
levels above the threshold.
 1

 A x(t )
x(t )
y (t )   X T

 Ax(t )
for x(t)  X T
for x(t)  X T
where  is between 1 (no attenuation) and  (complete
attenuation - behaves as a gate).
Example
Plot the transfer characteristics for compressors and
expanders. Process some signals with these and listen for
type of distortion/enhancements they produce.