Datasheet

o p e a k L O
V 2 R P
LM4860
SNAS096C –AUGUST 1994–REVISED MAY 2013 www.ti.com
HIGHER GAIN AUDIO AMPLIFIER
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 application. However if a closed-loop differential
gain of greater than 10 is required, then a feedback capacitor is needed, as shown in Figure 3, to bandwidth limit
the amplifier. The feedback capacitor creates a low pass filter that eliminates unwanted 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 External Components Description .
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 3, this common application requirement can be implemented. The combination 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
equations stated in External Components Description .
R
i
= 10 kΩ, R
f
= 510k, C
i
= 0.22 μF, and C
f
= 15 pF (4)
Five times away from a −3 dB point is 0.17 dB down from the flatband response. With this selection of
components, the resulting −3 dB points, f
L
and f
H
, are 72 Hz and 20 kHz, respectively, resulting in a flatband
frequency response of better 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 require that one side of the load is tied to ground.
Figure 4 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 capacitor, designated C
O
in Figure 4, 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 frequencies 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 instability. 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 obtain the specified output power. Calculating the
required supply 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 5.
(5)
12 Submit Documentation Feedback Copyright © 1994–2013, Texas Instruments Incorporated
Product Folder Links: LM4860