C

B

T

ON

0.47µF

940ms

1.0µF

2S

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 EXAMPLE

The following are the desired operational

parameters:

Given:

Power Output

100mW

Load Impedance

16

Ω

Input Level

1Vrms (max)

Input Impedance

20k

Ω

Bandwidth

The design begins by specifying the minimum supply voltage

necessary to obtain the specified output power. To find this

minimum supply voltage, use the Output Power vs. Supply

Voltage graph in the Typical Performance Characteristics

section. From the graph for a 16

Ω load, (graphs are for 8Ω,

16

Ω, and 32Ω loads) the supply voltage for 100mW of output

power with 1% THD+N is approximately 3.15 volts.

Additional supply voltage creates the benefit of increased

headroom that allows the LM4818 to reproduce peaks in

excess of 100mW without output signal clipping or audible

distortion. The choice of supply voltage must also not create

a situation that violates maximum dissipation as explained

above in the Power Dissipation section. For example, if a

3.3V supply is chosen for extra headroom then according to

Equation (3) the maximum power dissipation point with a

16

Ω load is 138mW. Using Equation (4) the maximum am-

bient temperature is 126˚C for the M08A package.

After satisfying the LM4818’s power dissipation require-

ments, the minimum differential gain is found using Equation

(6).

(6)

Thus a minimum gain of 1.27 V/V allows the LM4818 to

reach full output swing and maintain low noise and THD+N

performance. For this example, let A

er’s overall gain is set using the input (R

resistors. With the desired input impedance set to 20k

Ω, the

feedback resistor is found using Equation (7).

R

(7)

The value of R

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:

f

f

As mentioned in the External Components section, R

C

band pass frequency limit. Find the coupling capacitor’s

value using Equation (8).

C

(8)

C

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

example) and the differential gain, A

per pass band response limit. With A

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

127kHz. This is less than the LM4818’s 900kHz 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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