To achieve a transient free power-up and power-down, the

voltage seen at the input terminals should be ideally the

same. Such a signal will be common-mode in nature, and

will be rejected by the LM4765. In

Figure 3, the resistor R

INP

serves to keep the inputs at the same potential by limiting the

voltage difference possible between the two nodes. This

should significantly reduce any type of turn-on pop, due to an

uneven charging of the amplifier inputs. This charging is

based on a specific application loading and thus, the system

designer may need to adjust these values for optimal perfor-

mance.

As shown in

Figure 3, the resistors labeled R

the LM4765 off the half-supply node at the emitter of the

2N3904. But due to the input and output coupling capacitors

in the circuit, along with the negative feedback, there are two

different values of R

sistors bring up the inputs at the same rate resulting in a pop-

less turn-on. Adjusting these resistors values slightly may re-

duce pops resulting from power supplies that ramp

extremely quick or exhibit overshoot during system turn-on.

AUDIO POWER AMPLlFIER DESIGN

Design a 15W/8

Ω Audio Amplifier

Given:

Power Output

15 Wrms

Load Impedance

8

Ω

Input Level

1 Vrms(max)

Input Impedance

47 k

Ω

Bandwidth

20 Hz−20 kHz

A designer must first determine the power supply require-

ments in terms of both voltage and current needed to obtain

the specified output power. V

Equation (4) and I

(4)

(5)

To determine the maximum supply voltage the following con-

ditions must be considered. Add the dropout voltage to the

peak output swing V

of I

loaded voltage which is usually about 15% higher. The sup-

ply voltage will also rise 10% during high line conditions.

Therefore the maximum supply voltage is obtained from the

following equation.

Max supplies

For 15W of output power into an 8

Ω load, the required

V

from adding V

Equation (5). It should be noted that for a dual 15W amplifier

into an 8

Ω load the I

1.94 Apk or 3.88 Apk. At this point it is a good idea to check

the Power Output vs Supply Voltage to ensure that the re-

quired output power is obtainable from the device while

maintaining low THD+N. In addition, the designer should

verify that with the required power supply voltage and load

impedance, that the required heatsink value

θ

given system cost and size constraints. Once the heatsink

issues have been addressed, the required gain can be deter-

mined from

Equation (6).

(6)

From

Equation (6), the minimum A

A

By selecting a gain of 21, and with a feedback resistor, R

20 k

Ω, the value of R

R

(7)

Thus with R

Since the desired input impedance was 47 k

Ω, a value of

47 k

Ω was selected for R

dress the bandwidth requirements which must be stated as a

pair of −3 dB frequency points. Five times away from a −3 dB

point is 0.17 dB down from passband response which is bet-

a low and high frequency pole of 4 Hz and 100 kHz respec-

tively. As stated in the External Components section, R

conjunction with C

C

use 39 µF.

The high frequency pole is determined by the product of the

desired high frequency pole, f

A

which is less than the guaranteed minimum GBWP of the

LM4765 of 5 MHz. This will ensure that the high frequency

response of the amplifier will be no worse than 0.17 dB down

at 20 kHz which is well within the bandwidth requirements of

the design.

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