Datasheet
LM4809, LM4809LQBD
SNAS126F –FEBRUARY 2001–REVISED APRIL 2013
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AUDIO POWER AMPLIFIER DESIGN
Design a Dual 70mW/32Ω Audio Amplifier
Given:
Power Output 70 mW
Load Impedance 32Ω
Input Level 1 Vrms (max)
Input Impedance 20kΩ
Bandwidth 100 Hz–20 kHz ± 0.50dB
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 Figure 28 in the Typical Performance Characteristics
section. Another way, using Equation 5, is to calculate the peak output voltage necessary to achieve the desired
output power for a given load impedance. To account for the amplifier's dropout voltage, two additional voltages,
based on Figure 31 in Typical Performance Characteristics , must be added to the result obtained by
Equation 5. For a single-ended application, the result is Equation 6.
(5)
V
DD
≥ (2V
OPEAK
+ (V
ODTOP
+ V
ODBOT
)) (6)
Figure 28 for a 32Ω load indicates a minimum supply voltage of 4.8V. This is easily met by the commonly used
5V supply voltage. The additional voltage creates the benefit of headroom, allowing the LM4809 to produce peak
output power in excess of 70mW without clipping or other audible distortion. The choice of supply voltage must
also not create a situation that violates maximum power dissipation as explained above in the Power
Dissipation section. Remember that the maximum power dissipation point from Equation 1 must be multiplied by
two since there are two independent amplifiers inside the package. Once the power dissipation equations have
been addressed, the required gain can be determined from Equation 7.
(7)
Thus, a minimum gain of 1.497 allows the LM4809 to reach full output swing and maintain low noise and THD+N
perfromance. For this example, let A
V
=1.5.
The amplifiers overall gain is set using the input (R
i
) and feedback (R
f
) resistors. With the desired input
impedance set at 20kΩ, the feedback resistor is found using Equation 8.
A
V
= R
f
/R
i
(8)
The value of R
f
is 30kΩ.
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