Application Hints
AM Radios
The LM1896/LM2896 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:
A
V
(S)
e
S
a
A
V
0
o
S
a
0
o
where A
V
e
R2
a
R5
R2
,
0
o
e
1
R5C5
A curve of
b
3 dB BW (
0
o
) vs A
V
is shown in the Typical
Performance Curves.
Figure 3 shows a plot of recovered audio as a function of
field strength in
m
V/M. 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 1/8 inch from it. Referenced to a
20 dB S/N ratio, the improvement in noise performance
over conventional designs is about 10 dB. This corresponds
to an increase in usable sensitivity of about 8.5 dB.
Bridge Amplifiers
The LM1896/LM2896 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
R5/R2. 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 is a 6 dB increase in voltage gain referenced to
V
i
. The voltage gain in bridge is:
V
o
V
i
e
2
#
1
a
R5
R2
J
C
B
is used to prevent DC voltage on the output of amp 1
from causing offset in amp 2. Low frequency response is
influenced by:
f
L
e
1
2
q
R
B
C
B
Several precautions should be observed when using the
LM1896/LM2896 in bridge configuration. Because the am-
plifiers are driving the load out of phase, an 8
X
speaker will
appear as a 4
X
load, and a 4
X
speaker will appear as a 2
X
load. Power dissipation is twice as severe in this situation.
For example, if V
S
e
6V and R
L
e
8
X
bridged, then the
maximum dissipation is:
P
D
e
V
2
S
20 R
L
c
2
e
6
2
20
c
4
c
2
P
D
e
0.9 Watts
This amount of dissipation is equivalent to driving two 4
X
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
0.707
2
q
RC
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.
TL/H/7920 – 9
FIGURE 3. Improved AM Sensitivity over Conventional Design
6
Obsolete
