The maximum device dissipation is obtained from
Figure 2
for the heat sink and ambient temperature conditions under
which the device will be operating. With this maximum al-
lowed dissipation,
Figures 3a, b and c show the maximum
power supply allowed (to stay within dissipation limits) and
the output power delivered into 4, 8 or 16
X
loads. The three
percent total-harmonic distortion line is approximately the
on-set of clipping.
TL/H/7380 – 6
FIGURE 4. Total Harmonic Distortion vs Frequency
Figure 4 shows total harmonic distortion versus frequency
for various output levels, while
Figure 5 shows the power
bandwidth of the LM380.
TL/H/7380 – 7
FIGURE 5. Output Voltage Gain vs Frequency
Power supply decoupling is achieved through the AC divider
formed by R
1
(Figure 1) and an external bypass capacitor.
Resistor R
1
is split into two 25 k
X
halves providing a high
TL/H/7380 – 8
FIGURE 6. Supply Decoupling vs Frequency
source impedance for the integrator.
Figure 6 shows supply
decoupling versus frequency for various bypass capacitors.
BIASING
The simplified schematic of
Figure 1 shows that the LM380
is internally biased with the 150 k
X
resistance to ground.
This enables input transducers which are referenced to
ground to be direct-coupled to either the inverting or non-in-
verting inputs of the amplifier. The unused input may be
either: 1) left floating, 2) returned to ground through a resis-
tor or capacitor or 3) shorted to ground. In most applications
where the non-inverting input is used, the inverting input is
left floating. When the inverting input is used and the non-in-
verting input is left floating, the amplifier may be found to be
sensitive to board layout since stray coupling to the floating
input is positive feedback. This can be avoided by employ-
ing one of three alternatives: 1) AC grounding the unused
input with a small capacitor. This is preferred when using
high source impedance transducer. 2) Returning the unused
input to ground through a resistor. This is preferred when
using moderate to low DC source impedance transducers
and when output offset from half supply voltage is critical.
The resistor is made equal to the resistance of the input
transducer, thus maintaining balance in the input differential
amplifier and minimizing output offset. 3) Shorting the un-
used input to ground. This is used with low DC source im-
pedance transducers or when output offset voltage is non-
critical.
OSCILLATION
The normal power supply decoupling precautions should be
taken when installing the LM380. If V
S
is more than 2
×
to 3
×
from the power supply filter capacitor it should be decou-
pled with a 0.1
m
F disc ceramic capacitor at the V
S
terminal
of the IC.
The R
C
and C
C
shown as dotted line components on
Figure
7 and throughout this paper suppresses a 5 to 10 MHz
TL/H/7380 – 9
*
For Stability With High Current Loads
FIGURE 7. Minimum Component Configuration
small amplitude oscillation which can occur during the nega-
tive swing into a load which draws high current. The oscilla-
tion is of course at too high of a frequency to pass through a
speaker, but it should be guarded against when operating in
an RF sensitive environment.
3
