Application Information
HIGHER GAIN AUDIO AMPLIFIER
(Continued)
ity. While such an instability will not affect the waveform of
V
O1
, it is good design practice to load the second output.
AUDIO POWER AMPLIFIER DESIGN
Design a 500 mW/8Ω Audio Amplifier
Given:
Power Output: 500 mWrms
Load Impedance: 8Ω
Input Level: 1 Vrms(max)
Input Impedance: 20 kΩ
Bandwidth: 20 Hz-20 kHz
±
0.25 dB
A designer must first determine the needed supply rail to ob-
tain the specified output power. Calculating the required sup-
ply rail involves knowing two parameters, V
opeak
and also the
dropout voltage. The latter is typically 0.7V. V
opeak
can be
determined from equation 3.
The LM4860 is unity-gain stable and requires no external
components besides gain-setting resistors, an input coupling
capacitor, and proper supply bypassing in the typical appli-
cation. However if a closed-loop differential gain of greater
than 10 is required, then a feedback capacitor is needed, as
shown in
Figure 2,
to bandwidth limit the amplifier. The feed-
back capacitor creates a low pass filter that eliminates un-
wanted high frequency oscillations. Care should be taken
when calculating the −3 dB frequency in that an incorrect
combination of R
f
and C
f
will cause rolloff before 20 kHz. A
typical combination of feedback resistor and capacitor that
will not produce audio band high frequency rolloff is R
f
=
100 kΩ and C
f
= 5 pF. These components result in a −3 dB
point of approximately 320 kHz. Once the differential gain of
the amplifier has been calculated, a choice of R
f
will result,
and C
f
can then be calculated from the formula stated in the
External Components Description
section.
VOICE-BAND AUDIO AMPLIFIER
Many applications, such as telephony, only require a
voice-band frequency response. Such an application usually
requires a flat frequency response from 300 Hz to 3.5 kHz.
By adjusting the component values of
Figure 2,
this common
application requirement can be implemented. The combina-
tion of R
i
and C
i
form a highpass filter while R
f
and C
f
form a
lowpass filter. Using the typical voice-band frequency range,
with a passband differential gain of approximately 100, the
following values of R
i
, C
i
, R
f
, and C
f
follow from the equa-
tions stated in the
External Components Description
sec-
tion.
R
i
= 10 kΩ, R
f
= 510k, C
i
= 0.22 µF, and C
f
= 15 pF
Five times away from a −3 dB point is 0.17 dB down from the
flatband response. With this selection of components, the re-
sulting −3 dB points, f
L
and f
H
, are 72 Hz and 20 kHz, re-
spectively, resulting in a flatband frequency response of bet-
ter than
±
0.25 dB with a rolloff of 6 dB/octave outside of the
passband. If a steeper rolloff is required, other common
bandpass filtering techniques can be used to achieve higher
order filters.
SINGLE-ENDED AUDIO AMPLIFIER
Although the typical application for the LM4860 is a bridged
monoaural amp, it can also be used to drive a load
single-endedly in applications, such as PC cards, which re-
quire that one side of the load is tied to ground.
Figure 3
shows a common single-ended application, where V
O1
is
used to drive the speaker. This output is coupled through a
470 µF capacitor, which blocks the half-supply DC bias that
exists in all single-supply amplifier configurations. This ca-
pacitor, designated C
O
in
Figure 3,
in conjunction with R
L
,
forms a highpass filter. The −3 dB point of this highpass filter
is 1/(2πR
L
C
O
), so care should be taken to make sure that the
product of R
L
and C
O
is large enough to pass low frequen-
cies to the load. When driving an 8Ω load, and if a full audio
spectrum reproduction is required, C
O
should be at least
470 µF. V
O2
, the output that is not used, is connected
through a 0.1 µF capacitor to a 2 kΩ load to prevent instabil-
For 500 mW of output power into an 8Ω load, the required
V
opeak
is 2.83V. A minimum supply rail of 3.53V results from
adding V
opeak
and V
od
. But 3.53V is not a standard voltage
that exists in many applications and for this reason, a supply
rail of 5V is designated. Extra supply voltage creates dy-
namic headroom that allows the LM4860 to reproduce peaks
in excess of 500 mW without clipping the signal. At this time,
the designer must make sure that the power supply choice
along with the output impedance does not violate the condi-
tions 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.
A
vd
= 2
Since the desired input impedance was 20 kΩ, and with an
A
vd
of 2, a ratio of 1:1 of R
f
to R
i
results in an allocation of
R
i
= R
f
= 20 kΩ. Since the A
vd
was less than 10, a feedback
capacitor is not needed. 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 bet-
ter than the required
±
0.25 dB specified. This fact results in
a low and high frequency pole of 4 Hz and 100 kHz respec-
tively. As stated in the
External Components
section, R
i
in
conjunction with C
i
create a highpass filter.
C
i
≥
1/(2π
*
20 kΩ
*
4 Hz) = 1.98 µF; use 2.2 µF.
The high frequency pole is determined by the product of the
desired high frequency pole, f
H
, and the differential gain, A
vd
.
With a A
vd
= 2 and f
H
= 100 kHz, the resulting GBWP =
100 kHz which is much smaller than the LM4860 GBWP of
7 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4860 can still be used without running into bandwidth
problems.
From equation 4, the minimum A
vd
is:
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