can eliminate the trim pot and virtually eliminate crossover
distortion. The VCE voltage of QMULT (also called BIAS of the
output stage) can be set by following formula:
emitter. This mechanism helps to limit the current and coun-
teracts thermal runaway.
LAYOUT CONSIDERATION AND AVOIDING GROUND
LOOPS
VBIAS = VBE(1+RB2/RB1) (V)
(7)
A proper layout is virtually essential for a high performance
audio amplifier. It is very important to return the load ground,
supply grounds of output transistors, and the low level (feed-
back and input) grounds to the circuit board common ground
point through separate paths. When ground is routed in this
fashion, it is called a star ground or a single point ground. It
is advisable to keep the supply decoupling capacitors of
0.1μF close as possible to LME49810 to reduce the effects of
PCB trace resistance and inductance. Following the general
rules will optimize the PCB layout and avoid ground loops
problems:
When using a bipolar output stage with the LME49810 (as in
Figure 1), the designer must beware of thermal runaway.
Thermal runaway is a result of the temperature dependence
of VBE (an inherent property of the transistor). As temperature
increases, VBE decreases. In practice, current flowing through
a bipolar transistor heats up the transistor, which lowers the
VBE. This in turn increases the current gain, and the cycle re-
peats. If the system is not designed properly this positive
feedback mechanism can destroy the bipolar transistors used
in the output stage. One of the recommended methods of
preventing thermal runaway is to use the same heat sink on
the bipolar output stage transistor together with VBE multiplier
transistor. When the VBE multiplier transistor is mounted to the
same heat sink as the bipolar output stage transistors, it tem-
perature will track that of the output transistors. Its VBE is
dependent upon temperature as well, and so it will draw more
current as the output transistors heat up, reducing the bias
voltage to compensate. This will limit the base current into the
output transistors, which counteracts thermal runaway. An-
other widely popular method of preventing thermal runaway
is to use low value emitter degeneration resistors (RE1 and
RE2). As current increases, the voltage at the emitter also in-
creases, which decreases the voltage across the base and
a) Make use of symmetrical placement of components.
b) Make high current traces, such as output path traces, as
wide as possible to accomodate output stage current require-
ment.
c) To reduce the PCB trace resistance and inductance, same
ground returns paths should be as short as possible. If pos-
sible, make the output traces short and equal in length.
d) To reduce the PCB trace resistance and inductance,
ground returns paths should be as short as possible.
e) If possible, star ground or a single point ground should be
observed. Advanced planning before starting the PCB can
improve audio performance.
15
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