LM4860 datasheet(12/20 Pages) TI

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LM4860

SNAS096C – AUGUST 1994 – REVISED MAY 2013

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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 Rf

and Cf will cause rolloff before 20 kHz. A typical combination of feedback resistor and capacitor that will not

produce audio band high frequency rolloff is Rf = 100 kΩ and Cf = 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 Rf will

result, and Cf 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 Ri and Ci form a

highpass filter while Rf and Cf form a lowpass filter. Using the typical voice-band frequency range, with a

passband differential gain of approximately 100, the following values of Ri, Ci, Rf, and Cf follow from the

equations stated in External Components Description .

Ri = 10 kΩ, Rf = 510k, Ci = 0.22 μF, and Cf = 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, fL and fH, 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 VO1 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 CO in Figure 4, in conjunction with RL, forms a highpass filter. The

−3 dB point of this highpass filter is 1/(2πRLCO), so care should be taken to make sure that the product of RL and

CO is large enough to pass low frequencies to the load. When driving an 8Ω load, and if a full audio spectrum

reproduction is required, CO should be at least 470 μF. VO2, 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 VO1,

it is good design practice to load the second output.

AUDIO POWER AMPLIFIER DESIGN

Design a 500 mW/8

Ω Audio Amplifier

Given:

Power Output:

500 mWrms

Load Impedance:

Input Level:

1 Vrms(max)

Input Impedance:

20 k

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, Vopeak and also the dropout voltage. The latter is typically

0.7V. Vopeak can be determined from Equation 5.

(5)

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