AA4838
Agamem Microelectronics Inc.
PRELIMINARY
AUDIO POWER AMPLIFIER
Input Impedance: 20 kΩ,ꢀBandwidth: 100 Hz−20 kHz ± 0.25 dB
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 Characteristics section. Another
way, using Equation (10), 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 the Dropout Voltage vs. Supply Voltage in the
Typical Performance Characteristics curves, must be added to the result obtained by
Equation (10). The result is Equation (11).
… (11),
VDD≧(VOUTPEAK+ (VODTOP+VODBOT))… (12)
The Output Power vs. Supply Voltage graph for an 8Ω load indicates a minimum supply
voltage of 4.6V. This is easily met by the commonly used 5V supply voltage. The additional
voltage creates the benefit of headroom, allowing the AA4838 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 AA4838’s power dissipation requirements,
the minimum differential gain needed to achieve 1W dissipation in an 8Ωload is found using
Equation (12).
… (13)
Thus, a minimum overall gain of 2.83 allow the AA4838’s to reach full output swing and
maintain low noise and THD+N performance. The last step in this design example is setting
the amplifier’s −6dB frequency bandwidth. To achieve the desired ±0.25dB pass band
magnitude variation limit, the low frequency response 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 both response limits is 0.17dB, well within the
±0.25dB desired limit. The results are an fL=100Hz/5=20Hz…(14) and an fH=20kHz x
5=100kHz…(15) As mentioned in the Selecting Proper External Components section, Ri
(Right & Left) and Ci (Right & Left) create a high pass filter that sets the amplifier’s lower
band pass frequency limit. Find the input coupling capacitor’s value using Equation (14).
Ci≧1/(2πRifL)…(16) The result is
1/(2π*20kΩ*20Hz)=0.397µF…(17)
Use a 0.39µF capacitor, the closest standard value.
The product of the desired high frequency cutoff (100 kHz in this example) and the differential
gain AVD, determines the upper pass band response limit. With AVD= 3 and fH =100 kHz,
the closed-loop gain bandwidth product (GBWP) is 300 kHz. This is less than the AA4838’s
18
©Copyright Agamem Microelectronics Inc.
www.agamem.com.tw
2008/8/26
AGAMEM MICROELECTRONICS INCOPERATION RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE
TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. AGAMEM DOES NOT ASSUME ANY
LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCTS OR CIRCUIT DESRIBED HEREIN; NEITHER
DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
AGAMEM [ AGAMEM MICROELECTRONIC INC. ]
