LM4871
Application Information
(Continued)
AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8Ω Audio Amplifier
Given:
Power Output
Load Impedance
Input Level
Input Impedance
1 Wrms
8Ω
1 Vrms
20 kΩ
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4871 is unity-gain stable which gives a designer
maximum system flexibility. The LM4871 should be used in
low gain configurations to minimize THD+N values, and
maximize the signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power. In-
put signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the sec-
tion,
Audio Power Amplifier Design,
for a more complete
explanation of proper gain selection.
Besides gain, one of the major considerations is the closed-
loop bandwidth of the amplifier. To a large extent, the band-
width is dictated by the choice of external components
shown in
Figure 1.
The input coupling capacitor, C
i
, forms a
first order high pass filter which limits low frequency re-
sponse. This value should be chosen based on needed fre-
quency response for a few distinct reasons.
Selection Of Input Capacitor Size
Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
needed to couple in low frequencies without severe attenua-
tion. But in many cases the speakers used in portable sys-
tems, whether internal or external, have little ability to repro-
duce signals below 100 Hz to 150 Hz. Thus, using a large
input capacitor may not increase actual system perfor-
mance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
C
i.
A larger input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally 1/2 V
DD
). This
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the ca-
pacitor size based on necessary low frequency response,
turn-on pops can be minimized.
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, C
B
, is the most critical component to minimize
turn-on pops since it determines how fast the LM4871 turns
on. The slower the LM4871’s outputs ramp to their quiescent
DC voltage (nominally 1/2 V
DD
), the smaller the turn-on pop.
Choosing C
B
equal to 1.0 µF along with a small value of C
i
(in the range of 0.1 µF to 0.39 µF), should produce a virtually
clickless and popless shutdown function. While the device
will function properly, (no oscillations or motorboating), with
C
B
equal to 0.1 µF, the device will be much more susceptible
to turn-on clicks and pops. Thus, a value of C
B
equal to
1.0 µF is recommended in all but the most cost sensitive de-
signs.
Bandwidth
100 Hz–20 kHz
±
0.25 dB
A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the
Typical Per-
formance Characteristics
section, the supply rail can be
easily found. A second way to determine the minimum sup-
ply rail is to calculate the required V
opeak
using Equation 3
and add the output voltage. Using this method, the minimum
supply voltage would be (V
opeak
+ (V
ODTOP
+ V
ODBOT
)), where
V
ODBOT
and V
ODTOP
are extrapolated from the Dropout Volt-
age vs Supply Voltage curve in the
Typical Performance
Characteristics
section.
(3)
Using the Output Power vs Supply Voltage graph for an 8Ω
load, the minimum supply rail is 4.6V. But since 5V is a stan-
dard voltage in most applications, it is chosen for the supply
rail. Extra supply voltage creates headroom that allows the
LM4871 to reproduce peaks in excess of 1W without produc-
ing audible distortion. At this time, the designer must make
sure that the power supply choice along with the output im-
pedance does not violate the conditions explained in the
Power Dissipation
section.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 4.
(4)
R
f
/R
i
= A
VD
/2
(5)
From Equation 4, the minimum A
VD
is 2.83; use A
VD
= 3.
Since the desired input impedance was 20 kΩ, and with a
A
VD
impedance of 2, a ratio of 1.5:1 of R
f
to R
i
results in an
allocation of R
i
= 20 kΩ and R
f
= 30 kΩ. The final design step
is to address the bandwidth requirements which must be
stated as a pair of −3 dB frequency points. Five times away
from a −3 dB point is 0.17 dB down from passband response
which is better than the required
±
0.25 dB specified.
f
L
= 100 Hz/5 = 20 Hz
f
H
= 20 kHz * 5 = 100 kHz
As stated in the
External Components
section, R
i
in con-
junction with C
i
create a highpass filter.
C
i
≥
1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF
The high frequency pole is determined by the product of the
desired frequency pole, f
H
, and the differential gain, A
VD
.
With a A
VD
= 3 and f
H
= 100 kHz, the resulting GBWP =
150 kHz which is much smaller than the LM4871 GBWP of
4 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4871 can still be used without running into bandwidth limi-
tations.
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