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LM1896 Datasheet(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM1896
Description  LM1896/LM2896 Dual Audio Power Amplifier
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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LM1896 Datasheet(HTML) 6 Page - National Semiconductor (TI)

 
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Application Hints
AM Radios
The LM1896LM2896 has been designed fo fill a wide
range of audio power applications A common problem with
IC audio power amplifiers has been poor signal-to-noise per-
formance when used in AM radio applications In a typical
radio application the loopstick antenna is in close proximity
to the audio amplifer Current flowing in the speaker and
power supply leads can cause electromagnetic coupling to
the loopstick resulting in system oscillation In addition
most audio power amplifiers are not optimized for lowest
noise because of compensation requirements If noise from
the audio amplifier radiates into the AM section the sensitiv-
ity and signal-to-noise ratio will be degraded
The LM1896 exhibits extremely low wideband noise due in
part to an external capacitor C5 which is used to tailor the
bandwidth The circuit shown in
Figure 2 is capable of a
signal-to-noise ratio in excess of 60 dB referred to 50 mW
Capacitor C5 not only limits the closed loop bandwidth it
also provides overall loop compensation Neglecting C2 in
Figure 2 the gain is
AV(S) e
S a AV 0o
S a 0o
where AV e
R2 a R5
R2
0o e
1
R5C5
A curve of b3dBBW(0o)vsAV is shown in the Typical
Performance Curves
Figure 3 shows a plot of recovered audio as a function of
field strength in mVM The receiver section in this example
is an LM3820 The power amplifier is located about two
inches from the loopstick antenna Speaker leads run paral-
lel to the loopstick and are 18 inch from it Referenced to a
20 dB SN ratio the improvement in noise performance
over conventional designs is about 10 dB This corresponds
to an increase in usable sensitivity of about 85 dB
Bridge Amplifiers
The LM1896LM2896 can be used in the bridge mode as a
monaural power amplifier In addition to much higher power
output the bridge configuration does not require output cou-
pling capacitors The load is connected directly between the
amplifier outputs as shown in
Figure 4
Amp 1 has a voltage gain set by 1 a R5R2 The output of
amp 1 drives amp 2 which is configured as an inverting
amplifier with unity gain Because of this phase inversion in
amp 2 there isa6dB increase in voltage gain referenced to
Vi The voltage gain in bridge is
Vo
Vi
e
2
1aR5R2 J
CB is used to prevent DC voltage on the output of amp 1
from causing offset in amp 2 Low frequency response is
influenced by
fL e
1
2q RBCB
Several precautions should be observed when using the
LM1896LM2896 in bridge configuration Because the am-
plifiers are driving the load out of phase an 8X speaker will
appear as a 4X load and a 4X speaker will appear as a 2X
load Power dissipation is twice as severe in this situation
For example if VS e 6V and RL e 8X bridged then the
maximum dissipation is
PD e
V
2
S
20 RL
c
2 e
62
20 c 4
c
2
PD e 09 Watts
This amount of dissipation is equivalent to driving two 4X
loads in the stereo configuration
When adjusting the frequency response in the bridge config-
uration R5C5 and R10C10 form a 2 pole cascade and the
b
3 dB bandwidth is actually shifted to a lower frequency
BW e
0707
2qRC
where R e feedback resistor
C e feedback capacitor
To measure the output voltage a floating or differential me-
ter should be used because a prolonged output short will
over dissipate the package
Figure 1 shows the complete
bridge amplifier
TLH7920 – 9
FIGURE 3 Improved AM Sensitivity over Conventional Design
6


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