Selection of Input and Output Capacitor Size

Besides gain, one of the major considerations is the closed

loop bandwidth of the amplifier. To a large extent, the band-

width is dicated by the choice of external components shown

in Figure 1. Both the input coupling capacitor, C

output coupling capacitor, C

filters which limit low frequency response. These values

should be based on the desired frequency response

weighed against the following:

Large value input and output capacitors are both expensive

and space consuming for portable designs. Clearly a certain

sized capacitor is needed to couple in low frequencies with-

out severe attenuation. But in many cases the speakers

used in portable systems, whether internal or external, have

little ability to reproduce signals below 150Hz. Thus large

input and output capacitors may not increase system perfor-

mance.

In addition to system cost and size, click and pop perfor-

mance is affected by the size of the input coupling capacitor,

C

reach its quiescent DC voltage (nominally 1/2 V

charge comes from the output via the feedback and is apt to

create pops upon device enable. Turn on pops can be

minimized by reducing C

frequency response.

Besides minimizing the input and output capacitor values,

careful consideration should be paid to the bypass capacitor

value. Bypass capacitor C

minimize turn on pops since it determines how fast the

LM4811 turns on. The slower the LM4811’s outputs ramp to

their quiescent DC voltage (nominally 1/2 V

the turn on pop. While the device will function properly, (no

oscillations or motorboating), with C

vice will be much more susceptible to turn on clicks and

pops. Thus, a value of C

mended in all but the most cost sensitive designs.

Also, careful consideration must be taken in selecting a

certain type of capacitor to be used in the system. Different

types of capacitors (tantalum, electrolytic, ceramic) have

unique performance characteristics and may affect overall

system performance.

AUDIO POWER AMPLIFIER DESIGN

Design a Dual 70mW/32

Ω Audio Amplifier

Given:

Power Output

70mW

Load Impedance

32

Ω

Input Level

1Vrms (max)

Input Impedance

33k

Ω (min)

Bandwidth

A designer must first determine the minimum supply rail to

obtain the specified output power. By extrapolating from the

Output Power vs Supply Voltage graphs in the Typical Per-

formance Characteristics section, the supply rail can be

easily found. A second way to determine the minimum sup-

ply rail is to calculate the required V

and add the dropout voltage. For a single-ended application,

the minimum supply voltage can be approximated by

(2V

are extrapolated from the Dropout Voltage vs Supply Voltage

curve in the Typical Performance Characteristics section.

(3)

Using the Output Power vs Supply Voltage graph for a 32

Ω

load, the minimum supply rail is 4.8V. Since 5V is a standard

supply voltage in most applications, it is chosen for the

supply rail. Extra supply voltage creates headroom that al-

lows the LM4811 to reproduce peaks in excess of 70mW

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 conditions explained

in the Power Dissipation section. Remember that the maxi-

mum power dissipation point from Equation 1 must be mul-

tiplied by two since there are two independent amplifiers

inside the package.

The final design step is to address the bandwidth require-

ments which must be stated as a pair of −3dB frequency

points. Five times away from a −3dB point is 0.17dB down

from passband response assuming a single pole roll-off. As

stated in the External Components section, C

create first order highpass filters. Thus to obtain the desired

must be taken into consideration. The combination of two

single order filters at the same frequency forms a second

order response. This results in a signal which is down

0.34dB at five times away from the single order filter −3dB

point. Thus, a frequency of 20Hz is used in the following

equations to ensure that the response is better than 0.5dB

down at 100Hz.

C

i

≥ 1/(2π *33kΩ * 20 Hz) = 0.241µF; use 0.39µF. (4)

C

o

≥ 1/(2π *32Ω * 20 Hz) = 249µF; use 330µF. (5)

The high frequency pole is determined by the product of the

desired high frequency pole, f

A

100kHz, the resulting GBWP = 398kHz which is much

smaller than the LM4811 GBWP of 1MHz. This figure dis-

plays that at the maximum gain setting of 3.98 or +12dB, the

LM4811 can be used without running into bandwidth

limitations.

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