EE599-020Audio Signals and Systems

Amplifiers

Kevin D. DonohueElectrical and Computer Engineering

University of Kentucky

Related Web SitesThe 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

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 electro-acoustic transducers.

Amplifier Usage in Audio

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 Performance

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.

Amplifier Classes

Given the transfer characteristic for a class B amplifier below, compute the THD for a 3 volt input sinusoid.

Example

Vin

Vout

0.6v

-0.6v

7v

3v

-7v

-3v

Given the transfer characteristic for a class AB amplifier below, compute the THD for a 3 volt input sinusoid.

Example

Vin

Vout7v

3v

-7v

-3v

1.75v-1.75v

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.

Compressors

for

for 1

T

TT

Xx(t)tAx

Xx(t)txX

txAty

)(

)()(

)(

where is between 0 (hard clipping) and 1 (no compression).

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.

Expanders

where is between 1 (no attenuation) and (complete attenuation - behaves as a gate).

for

for 1

T

TT

Xx(t)tAx

Xx(t)txX

txAty

)(

)()(

)(

ExamplePlot the transfer characteristics for compressors and expanders. Process some signals with these and listen for type of distortion/enhancements they produce.