GB2410142A

GB2410142A - An audio power amplifier with automatic class selection

Info

Publication number: GB2410142A
Application number: GB0400614A
Authority: GB
Inventors: Gordon Leslie Scott
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-01-13
Filing date: 2004-01-13
Publication date: 2005-07-20
Also published as: GB0400614D0

Abstract

A Power Amplifier Arrangement for high quality audio systems whereby the operating classes of the amplifier are selected to best suit the conditions under which the amplifier is operating. High quality power amplifiers are subject to many conflicting needs and aims. Various operating classes have evolved to give optimal operation in various conditions but none work totally satisfactorily in all conditions. The invention uses intelligent monitoring of the conditions under which the amplifier operates and will change the operating class of the amplifier in order that the amplifier shall be in its most appropriate class of operation at all times.

Description

Section 2. Description 2410142

2.1. Background.

High quality audio amplifiers are subject to many conflicting needs and aims. Amplifiers are designed using several operating classes including those known as Class A, Class B. Class AB, Class G. Each of these classes addresses one or more particular aspects of desired amplifier behaviour but all of them have constraints or tradeoffs that are substantially unavoidable.

Class A Class A is characterized by the output power transistors having a quiescent or bias current sufficiently high that the transistors never stop passing current.

Class A is used where the lowest possible distortion is required because this class does not generate crossover distortion at the output mid-point.

The high quiescent current in the output transistors results in a high energy dissipation and low efficiency. This constraint means that class A amplifiers are normally of a much lower output power rating that other classes.

The high quiescent current means that it is easily overloaded by unusual and excessive output loads.

Unfortunately such loads are not so unusual in high fidelity loudspeaker systems.

Class B Class B is characterized by the output power transistors carrying just sufficient current in their quiescent or idle state that when signal is applied they each change between conducting and non-conducting with a closely controller symmetry. In and around this quiescent condition both transistors contribute to the output stage gain such that the sum of their gains in the idle region remains substantially similar to the gain of each transistor in isolation when fully conducting.

Class B is by far the most common class and performs quite well in relation to class A but is inferior because it generates crossover distortion as the output signal changes polarity.

Class B is considerably more efficient than class A because the quiescent current in class B is tiny in comparison with the delivered power.

Class AB Class AB is characterized by the output power transistors biassed such that they operate in class A at low signals but as the signal increases the non-active output transistor eventually switches off.

Unfortunately this class of operation suffers from most of the problems of both class A and class B whilst failing to gain many of the expected benefits.

The class continues to suffer with quite poor efficiency because of the still quite high quiescent current.

The class continues to suffer with a form of crossover distortion where the transistors switch off.

The class suffers from an uneven output gain distribution that is almost absent in both class A or class B and which causes an additional crossover-like distortion.

Class AB is only very occasionally used in high fidelity systems.

Sliding bias Systems Sliding bias Systems are characterized by an attempt to make the output transistors in class B stay "just conducting" and so avoid the crossover distortion.

The gain of both output power transistors combines at low signal levels, which causes an uneven gain distribution as with class AB and results in the same crossover-like distortion that afflicts class AB.

Class G Class G comprises combination of a conventional amplifier output stage with a class B or C "booster" to the power supply nodes of the conventional amplifier.

Class G is characterized by output power transistors typically operating in class B from a comparatively low voltage supply and an outer set of transistors that, in effect, raise the power supply voltages sufficiently for the inner transistors only when they need to handle high signal peaks.

The main benefit of this configuration is in its potentially substantial efficiency improvement over conventional amplifiers because the higher supply voltage is used only when needed.

The output signal quality is substantially defined by the output signal quality of the conventional amplifier part of the circuit, switching artifacts in the outer transistors being both containable and substantially isolated from the output stage proper.

2.2. The Invention.

The invention recognises the various characteristics and constraints of the output stage classes and attempts to match the operating class to the conditions under which the amplifier is operating such that the best attributes of the various classes are used as appropriate.

The invention analyses the signal, load and temperature conditions that apply and sets the operating class accordingly. The monitoring is such that changes between classes are kept fairly infrequent. The signals and conditions are considered in a fairly long term analysis to avoid excessive class changes.

When the output signal is at or very close to the noise floor the amplifier operates in class B. This allows an amplifier generating no significant output to take maximum advantage of the efficiency of class B. At levels where the signal is close to inaudible, all distortion artifacts will be well below the threshold of hearing.

When the signal rises to a modestly higher level and when automatic class management is in operation the amplifier changes to class A and maintains that class whilst the amplifier operates within the ideal parameters for the class.

When the signal or load rises above the ideal levels for class A, the amplifier changes to class B. At these higher levels the crossover distortion artifacts are such that they are less significant than the effects of overload in the class A stage.

When signal levels rise above the bias threshold of the outer transistors the latter begin to supply power for the output stage as is normal in class G. This means that short duration signal peaks can occur where the outer transistors operate, even when the output stage is nominally in class A. This further means that on occasions the power stage may operate for a few moments in class AB, however should this continue, the management function will soon cause a change to a class B and G combination. Note that the class G outer transistors are not part of the class management scheme.

A class A amplifier operating at these higher power levels would be a gross consumer or power.

Figures I and 2 illustrate the general arrangement of conventional pushpull output stages and are appropriate for any of class A, class B and class AB. Typically the Vbias component is comprised of a circuit commonly known as a Vbe multiplier but other configuration can be used, particularly in class A operation. Figure 4a illustrates a typical Vbe multiplier, this one with an additional enable/disable control.

Figure 5 illustrates output voltage and output current monitoring points and that class management is applied to the Vbias stage or stages. The class management defines the class in which the stage operates and does not directly control the actual bias voltage. The bias voltage is controlled within the Vbias circuit in the usual way.

Figures 4a, 4b 4nd 4c illustrate some Vbias generators and ways in which they are controlled. 4a and 4b are Vbe multipliers and are a method of modelling the biasing needs of the output stage. In the case of 4b, it is convenient to vary the bias however the other two classes have only an operating state and a non-operating state. Example 4c is a quiescent current feedback circuit rather than a model and in a high fidelity amplifier, 4c is normally suitable only for classes A and AB. This example is simplified and any competent electronics engineer should see ways to improve its performance and ways to make adjustable the amount of quiescent current it defines.

Figures 3 and 6 illustrate the outer transistors for the class G function. The Vbias levels for Q3 and Q4 remain conventional but the operating modes for the Vbias for Ql and Q2 and controlled by the class manager.

Claims

Section 4. Claims I Claim...

1. An audio amplifier circuit comprising, signal input means, a power amplifier connected to the said input signal means having complementary push-pull emitter follower output transistors, each of the said transistors comprising a base, a collector and an emitter, the emitters of the said transistors communicating with an output node, means of biasing the said output transistors. Figure 1 illustrate* a circuit of this configuration.

2. The circuit of Claim I wherein each of the said output transistors are replaced by a complementary feedback pair. A complementary feedback pair comprises two transistors of complementary types wherein the first transistor of the said pair operates as an emitter follower but with its collector connected to the base of the second transistor of the said pair, the emitter of the second said transistor connects to the power supply and the collector of the said second transistor connects to the emitter of the said first transistor. Figure 2 illustrates a circuit of this configuration.

3. The circuit of Claim I or Claim 2 wherein an additional active stage supplies higher operating voltages for the output devices during high signal peaks in class G operation. The additional amplifier stage comprises cascade transistors having their collectors connected to a higher voltage power supply, their emitters connected to the collectors of their respective said output transistors, means of biasing the said cascade transistors and a connection from the input signal means to the bases of the said cascade transistors. Figure 3 illustrates a circuit of this configuration.

4. The circuits of Claim I, Claim 2 or Claim 3 wherein any or all of the said transistors are replaced by MOSFET type transistors.

5. The circuits of Claim I, Claim 2 or Claim 3 wherein any or all of the said transistors are replaced by thermionic valves.

6. The circuits of any of the above claims wherein the bias of the output devices can be controlled or adjusted by means of a stimulus possibly external to the amplifier. Figure 4 illustrates example circuits that achieve this control or adjustment.

7. The circuits of the above claims wherein the operating conditions of the amplifier are monitored by a function possibly external to the amplifier.

8. The circuits of the above claims wherein the said external function can adjust the operating class of the said output devices by means of the said adjustable bias in response to the said monitored operating conditions. Figures 5 and 6 illustrate circuits of this configuration.