13
PIP modules like the IQ-PIP-USP3 can use it to make decisions and
contr
ol the amplifi
er
. With ODEP you get the maximum power with
the maximum pr
otection — the show goes on!
Grounded Br
idge
Cr
own’
s Gr
ounded Bridge topology makes the amplifi
er deliver
peak-to-peak voltages to the load that ar
e twice the voltage seen
by the output devices and twice the voltage generated by the
power supplies. In other wor
ds, the amplifi
er can pr
oduce louder
sound without str
essing the output transistors. The r
esults ar
e
higher effi
ciency
, lower distor
tion and superior r
eliability
.
Class AB+B
Cr
own invented the Class AB+B amplifi
er design, which pr
ovides
both high effi
ciency and low distor
tion. AB+B cir
cuitr
y draws
less AC power and wastes less heat than Class A, and has less
distor
tion than Class B or Class AB. T
o
explain how AB+B works,
fi
rst we need to explor
e other amplifi
er classes.
Audio power amplifi
ers ar
e classifi
ed primarily by the design of
the output stage (the transistors and r
elated cir
cuitr
y that send
signals to the loudspeakers). Classifi
cation is based on the
amount of time the output devices (power transistors) ar
e made
to operate during each cycle of the signal. Amplifi
er classes ar
e
also defi
ned in ter
ms of output bias curr
ent (the amount of curr
ent
fl
owing in the output devices with no signal pr
esent).
In Class A operation, both output transistors conduct continuously
for the entir
e cycle of signal swing, so the bias curr
ent fl
ows
in the output devices at all times. Both devices ar
e always on.
Class A amplifi
ers ar
e single-ended designs with either PNP or
NPN output devices. Class A is the most ineffi
cient of all power
Gr
ounded Bridg
e theor
y
fo
r electr
onics-sa
vvy reader
s
The power
-supply bridge r
ectifi
er is not gr
ound
refer
enced, and the transfor
mer secondar
y is not center
-
tapped. This allows the power supply to dVCC
and -VCC fr
om the same bridge r
ectifi
er and fi
lter as a
total dif
fer
ence in potential r
egar
dless of their voltages
with r
espect to gr
ound.
Composite output devices ar
e arranged to function as
gigantic NPN and PNP devices. Each output stage has
two composite NPN and two composite PNP devices.
The devices connected to the load ar
e r
eferr
ed to as
“high-side NPN and PNP” and the devices connected
to gr
ound ar
e r
eferr
ed to as “low-side NPN and PNP
.”
Positive curr
ent is deliver
ed to the load by incr
easing
conductance simultaneously in the high-side NPN and
low-side PNP stage, while decr
easing conductance of
the high-side PNP and low-side NPN in synchr
ony
.
Imagine a graph of curr
ent versus voltage (I versus V) in
an output stage of a power amplifi
er
. This graph has four
quadrants: +V and +I, -V and +I, –V and –I, +V and –I.
Gr
ounded Bridge is a four
-quadrant amplifi
er topology
.
Resistive loads only use the fi
rst and thir
d quadrants (+V
,
+I and -V
, –I). Reactive loads also use the second and
four
th quadrants (–V
, +I and +V
, –I).
amplifi
er designs, averaging
only 20%. Because of this,
Class A amplifi
ers ar
e large,
heavy and run ver
y hot.
That is because the amp
runs constantly at full power
.
On the other hand, Class A
designs have the least amount
of distor
tion.
Class B operation is the
opposite of Class A. Both
output devices ar
e never
allowed to be on at the same
time. The bias is set so that
curr
ent fl
ow in a specifi
c
output device is zer
o when
not stimulated with an input
signal. Each output device
is on for exactly one half of
a complete sinusoidal signal
cycle. Due to this operation,
Class B designs show high
effi
ciency
, but poor linearity
ar
ound the cr
ossover r
egion.
This is because it takes time
to tur
n one device of
f and the
other device on, causing extr
eme cr
ossover distor
tion. All of
this r
estricts Class B designs to applications with low power
consumption, such as batter
y operated two-way radios and
other communications equipment.
