In order to eliminate "clicks and pops", all capacitors must be

discharged before turn-on. Rapidly switching V

allow the capacitors to fully discharge, which may cause

"clicks and pops".

AUDIO POWER AMPLIFIER DESIGN

Audio Amplifier Design: Driving 1W into an 8

Ω Load

Given:

Power Output

1 Wrms

Load Impedance

8

Ω

Input Level

1 Vrms

Input Impedance

Ω

Bandwidth

The design begins by specifying the minimum supply voltage

necessary to obtain the specified output power. One way to

find the minimum supply voltage is to use the Output Power

vs Supply Voltage curve in the Typical Performance Charac-

teristics section. Another way, using Equation (8), is to cal-

culate the peak output voltage necessary to achieve the

desired output power for a given load impedance. To ac-

count for the amplifier’s dropout voltage, two additional volt-

ages, based on the Dropout Voltage vs Supply Voltage in the

Typical Performance Characteristics curves, must be added

to the result obtained by Equation (8). The result is Equation

(9).

(8)

V

(9)

The Output Power vs. Supply Voltage graph for an 8

Ω load

indicates a minimum supply voltage of 4.6V. The commonly

used 5V supply voltage easily meets this. The additional

voltage creates the benefit of headroom, allowing the

LM4914 to produce peak output power in excess of 1W

without clipping or other audible distortion. The choice of

supply voltage must also not create a situation that violates

of maximum power dissipation as explained above in the

Power Dissipation section.

After satisfying the LM4914’s power dissipation require-

ments, the minimum differential gain needed to achieve 1W

dissipation in an 8

Ω load is found using Equation (10).

(RESUME HERE- All that is left is to discuss the BTL low

frequency phase shift.)

(10)

Thus, a minimum gain of 2.83 allows the LM4914’s to reach

full output swing and maintain low noise and THD+N perfor-

mance. For this example, let A

overall gain is set using the input (Ri), the first stage internal

feedback resistor, and the second stage’s fixed gain of 1.25.

With the desired input impedance set at 20kW, the feedback

resistor is found using Equation (11).

R

Ω /A

(11)

The value of Ri is 44.2k

Ω. The nominal output power is

1.13W.

The last step in this design example is setting the amplifier’s

pass band magnitude variation limit, the low frequency re-

sponse must extend to at least one-fifth the lower bandwidth

limit and the high frequency response must extend to at least

five times the upper bandwidth limit. The gain variation for

desired limit. The results are an

f

(12)

and an

f

(13)

As mentioned in the SELECTING EXTERNAL COMPO-

NENTS section, Ri and C

the amplifier’s lower bandpass frequency limit. Find the cou-

pling capacitor’s value using Equation (14).

C

πR

(14)

The result is

1/2

π x 44.2kΩ x 20Hz = 0.180µF

(15)

Use a 180µF capacitor, the closest standard value.

The product of the desired high frequency cutoff (100kHz in

this example) and the differential gain A

the upper passband response limit. With A

= 100kHz, the closed-loop gain bandwidth product (GBWP)

is 300kHz. This is less than the LM4914’s 3.5MHz GBWP.

With this margin, the amplifier can be used in designs that

require more differential gain while avoiding performance

restricting bandwidth limitations.

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