original publication

PREPRIN_ NO 1574 (B2)
DISTORTION
POWER
By
Dr
CORRECTION
IN AUDIO
AMPLIFIERS
M J
University
Colchester
Essex
Hawksford
of
Essex
Presentedat
the 65th Convention
_r_
1980 February25 through 28
London
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from the Journalof the Audio EngineeringSociety.
AN AUDIOENGINEERINGSOCIETY PREPRINT
Distortion
Correction
in Audio
Power
Amplifiers
Dr M J Hawksford
Audio
Research
Group
Department
of Electrical
University
of
Engineering
Science
Essex
Colchester
Abstract
An audio
power
property
of minimising
Class
amplifier
A and Class
A modified
suited
AB output
feedback
of the
has been
minimal
cmossover
The paper
f_om which
linear
stage°
Consequently
model
From this theoretical
illustrate
implement
amplifier
only modest
negative
to design
feedback
in
transfer
of modest
can
resistance
a power
yet attains
damping
feedforwax_/feedback
several circuit
circuits
the output
That can compensate
curTent
by describing
which
the
is pa_'ticularly
with an adequate
a generalised
the distozvtion co.action
The papem concludes
whereby
model,
that only
that
The technique
it is possible
overall
is derived
and non-linear
Stages.
and mfnimise
together
initially
a system
voltage
modest
distortion
presents
that has
that is generated
identified
gain
chamacteristic
that uses fairly
is presented
distortion
stages.
technique
the transfer
output
a_pllffeP
power
technique
to the design of nea_ unity
lineartse
which
design
the non-linear
factor.
structure
for both non-
characteristics.
examples
a_e presented
complexity
are needed
to
techn{que.
a design
is appropriate
overall negative
philosophy
for both bipolar
feedback
for an audio
and FET devices,
is necessary.
1.
Introducti_
This paper
in class
use
is addressed
A and class
of output
to the problems
AB audio power
voltage
derived negative
of the output
transistors
in an attempt
to achieve
acceptable
exhibit
transistors
are generally
distortion
There are
negative
(i)
within
the audio
Since crossover
together
voltage
distortion
thus
input
inte_odulation
inte_al
component
exhibits
non-linearity,
A technique
output
cumrent
is described
device
transfer.
Hence
loop
limits
amplifiers,
gain,
the deg_'ee
_put
pD0cess
audio
signal.
is non-zero
load
Hence,
(independent
is an
if the load
components
are again
fed
staze.
an amplifier
that can dramatically
philosophy
+ (iv)).
2
stages
disto_ion_
both with respect to voltage
reduce the problems outlined ((i)
is fed back
as the di_ortlon
stage
loop.
dis%ortton
stage
of the
then the loudspeaker
then
the
Transistors
especially
of the output
in this paper
characteristics
and of wide
tile pre-output
exceed that
in the feedback
back to the amplifier's
30dB are possible.
severely
plus the output
is impaired
feedback),
bandwidth
in nature
feedback
Consequently,
signal
resistance
of any overall
when using
behaviou/, is required
high frequency
by the output
can significantly
If the output
suppression
possible.
derived negative
input circuitry.
bandwidth
falling
loop delay,
that is generated
both tBe desired
output
amplifiers:
of only
is transient
suppression
the
successful
of limited
then loop &ains
with the resulting
(iii) In output
in power
are usually
distortion
of success
since all
that can be encountered
thus if non-dynamic
then the inevitable
of distortion
to the
band,
then
biassing
is limited.
distortion
transistors
the
de_ees
However,
into cut-off,
problems
distortion
and appropriate
and that in pamticular,
techniques
fT = 1 + 5MHz),
bandwidth,
(iv)
these
to minimise
Bipolar power
feedback
lineamity.
operated
several fundamental
feedback
(typical
(ii)
non-linearity
using
crossover
Traditionally,
has been applied with varying
transistors
of the
of minimising
amplifiers.
evolves
linearise
transfeF
and
that helps to
the
2.
The Theoretical
The principle
of the distortion
by considering
Figure 1.
Model
the generalised
In this network,
feedback
applied
general
case, the
system,
is defined
of N, thus
around
will deviate
distortion
there
as the difference
can be described
structure
is both ezTor
element,
input N is unspecified.
shown in
sensing
feedforward
N, where
The error signal used
between
However,
in
the
the input and the output
in all practical
thus the error
and
in the most
(i.e. N = 1), then the error signal
is applied.
from unity,
technique
feedback
the non-linear
if N is ideal
and no coz_sction
cancellation
error
signal
is zero
amplifiers,
represents
N
the exact
due to N.
Analysis
Let Vn and N(V n) be the input
examination
of the signals
and output
in Figure
of the N network,
thus
1 reveals:
Vou t = N(V n) + b{V n - N(Vn)}
Vn
eliminating
c
Vin + a{V n - N(Vn)}
Vn,
Your = N(V n) {(1 - b) - _/_}
+
ih
Vin ........ 1
If
(l-a) = h
........ 9
then
¥out= Vin
........
3
Thus providing
cancellation
The result
solutions
feedforward
stability
results
(equations
extending
is maintained
when equation
2 and
and Vn remains
3) indicates
fTom an ezTor
finite, then distortion
2 is enforced.
that there
feedback
system
is a continuum
through
cf
to an error
system.
It is interesting
to note that the
therefore
be derived
structure
providing
directly
stability
a = O, b = 1, then the classic
input of N is derived
input of N is unspecified,
is maintained.
feedforward
f_om the output
then the Quad 1'2 feedback
structure
Figure 1).
3
it may
from V n or indeed any other point within the
For example
system results,
by putting
where if the
of the error difference
results
(see dotted
amplifier,
connection
in
In this paper we consider
the input of N is equal
by Llewellyn
in 19_13
Cherry _ in 1978.
particularly
the opposite
to Vn.
to the design
with modest
is possible.
The theory
3.
Circuit
Power
current
amplifiers
loudspeaker
generally
If distortion
is
type output
in performance
of devices
Linearisation
output
Consequently
transistors
when
configuration,
correction
to the amplifier
feedback
then it is possible
results
technique
follower
improvement
load as seen by the base temminals
contributes
stage
Eain,
to show that linearisation
Sta_e
emitterfotlower
Thus when combined
and later by
that this feedback
dramatic
use bipolar
gain.
and therefor_
sensing,
circuitry,
for Output
current
in a complementary
is of the type first discussed
amplifiers
of unity
is extended
a = 1, b = O and
gain is also feasible.
Topologies
low, non-linear
to valve
It will now be shown
stages, where
with non-linear
This system
in relation
relevant
extreme where
with voltage error
that can be driven
are used
the transformed
is rendered
non-linear
distortion.
is configured
to compensate
that exhibit
such devices
to include input current
for changes
in current
sensing feedback,
gain.
a unity gain
from a stage with a finite
output
resistance.
In Figure
sensing
2, the schematic
circuitry
how a practical
Analysis
shows
circuit
(Figure
3) may
with both voltage
system
and current
is configured
to illustrate
be realised.
k1 = 1 + --2R1
R2
.......... 5
R1R 3 = R2R _
.......... 6
gain is unity
even when the base currents
finite and VBe/I e introduce
As a point
of design
resistance
of the driving
not required
the
when;
that the voltage
are
of a system
is shown, where
interest,
to have zero
of T1 and T 2
non-linearity.
the resistor
R1 includes
stage,
consequently
output
resistance.
the output
the driving
amplifier
is
Corollar_
Since
of the
the voltage gain
stage
is zero,
is unity,
then
it follows
even when the output
4
that the output
resistance
resistance
of the drivin E
stage is finite.
feedback
output
As a result,
system, does not
voltage derived
negative
speaker damping.
Also,
from the overall
feedback
loudspeaker
generated
of the power
output
gain
stage
2.
products
complementary
circuits
from reaching
are shown
the
louddecoupled
input circuitry
simplified
to show the modest
attractive
the output
amplifier
3, 4 and 5.
and is
have adequate
then current
circuits
circuit
sensing.
sensing
of Figures
requirements
The circuit
as the transistors
error difference
devices
transistors),
the much
only error voltage
in Figures
current sensing
if the output
or Dsrlington
As a result,
to realise
is particularly
4.
adequate
and it is this factor that prevents
However,
4 and 5 are illustrated
for biassing
an overall
load is then effectively
3 has both voltage and
(e.g. MOSFET
is unnecessary.
needed
uses this error correction
loop to achieve
the loudspeaker
distortion
of Figure
derived from Figure
current
feedback
loop,
that
have to rely upon
amplifier.
Three practical
The circuit
an amplifier
in principle
that are
of Figure
5
T3, T4 form both a
as well as 'amplified
diodes'
transistors.
Conclusions
This paper
has described
non-linear
distortion
by simple,
fast acting,
of linearity
an approach
generated
local
to power
amplifier
by the output
circuitry
that is appropriate
design
transistors
that can result
to class
where the
is compensated
in a high degree
A and AB follower
type output
stages.
The technique
low feedback
should
when distortion
output
stage
be aided
error
find favour amongst
school of design,
in the output
(N) is designed
by parallel
designers
as corrective
stage
who adhere
feedback
is generated.
If, therefore,
to be as linear as possible,
connection
of output
to the
is only applied
transistors,
the
a fact that can
then only minimal
signals result.
Since output
isolated
stage
from the
produce
modest
attempt
to produce
and loudspeaker
input
voltage gains,
In practical
as large
a linear amplifier.
and the loop bandwidth
well in excess
generated
stages, then these
can be high,
of the audio
amplifier
distortions
stages
loop gains
only to
are not required
Consequently
enabling
are in principle
are required
in an
the loop gain is low
a non-dynamic
loop behaviour
bandwidth.
design,
5
the
sensitivity
of adjustment
of the balance
conditions
output
is dependent
transistors,
extremely
low biassing.
adjustment
overall
largely
where
on the
critical
It has been
is non-critical,
also
prototype
proved
effective.
encountered
Darlington
circuits
have
In these
other than
transistors
merles
stage
cozTection
with
been
appear
circuitry
References
1.
P J Walker,
feedback
3.
P J Walkem,
levels,
by modest
F B Llewellyn,
technique
In practice,
enables
power
due
though
the bandwidth
fast correction
it is partly
suppression
of
In fact,
is minimal,
has
have been
to oscillation
was employed.
In fact,
the
the speed
of
of the
of distortion
compared
system.
'Current
of the AES,
'CurTent
81, No 1480,
17 June
_.
bias
problems
on layout.
instability
is high which
M P Albinson,
of 50th Convention
where
no stability
critical
loop that enables a greater
an overall
Vol
of the
under
is aided
investigated
the susceptibility
which
non-linearities.
5.
2.
only
for normal
sensitivity
amplifiers,
Zobel circuitry
of the correction
with
found that
that
to the low loop gain load dependent
output
bias current
results
feedback.
Several
standard
quiescent
adjustment
dumping audio
pp 560-562,
dumping amplifier',
Proceedings
1975.
power
amplifier',
Wireless
World,
Decen_ber 1975.
_Wave translation
systems I, US Patent
2, 245, 598,
19_1.
E M Cherry,
applications
'A new result
to audio
Vol 6, pp 265-288,
in negative
power
1978.
feedback
theory,
amplifiers t, Circuit
Theory
and
its
and Applicatlons,
to
4J
.cj
!
Z
t-
>
....
I
I
.o
c
c
o
u
'o
0
A1, A2non-hnear
gains
output devices.
of
R2
p_
R01, R02 nonlinear
bias
Ix
.-
,wvv,.
resistors.
:R3
VI I
"_k:VE
--O
V_."
R01
"3
"02lVo
R4
_O
R2
F__]g2
Current
and
voltage
error
sensing
feedback
!R3
R&
R1
o
I(
Fig
3
Circuit
sensing
schemotic
of
current
ou___._tputstgge__
end
vo[tclqe
error
o
I
I
2R1
$
2R1
¢t
Vc
T1,
T*'*7'TS'
,r_or
omplm_r
12'l-3,'amphfied
diode'
bias
T1
T5.T6, garhngton
_'7
trans_stors
T5
T.
R1
Vout
Yin
T3
T/.
2R1
Fig.
/,
Example
2R1
of
voltaqe
error
sensing
O/P
circuit
I
I
I
Tl, T2, 'amplified
I
Driving
Stage
1'3
diode'
biasing and error
amplifier
I
T3, T4, driving
transistors
J
J
T5, T6, Darlington O/P
transistors
I
J
Bias equation:
J
If k=
I
!_3
RI.
IRT-_"Zl
j
Balance equation:
j
thenR2=R1(1k)
-v_
I
I
F.__iig.
5 Voltage error
diodes'
as
sensing
error
circuit
amplifier.
using
'amplified