AA4838
Agamem Microelectronics Inc.
PRELIMINARY
AUDIO POWER AMPLIFIER
enables the headphone function, turns off both of the "+" output amplifiers, and mutes the
bridged speaker. The remaining single-ended amplifiers then drive the headphones, whose
impedance is in parallel with resistors R2 and R3. These resistors have negligible effect on
the AA4838’s output drive capability since the typical impedance of headphones is 32Ω.
Figure 3 also shows the suggested headphone jack electrical connections. The jack is
designed to mate with a three-wire plug. The plug’s tip and ring should each carry one of the
two stereo output signals, whereas the sleeve should carry the ground return. A headphone
jack with one control pin contact is sufficient to drive the HP-IN pin when connecting
headphones. A microprocessor or a switch can replace the headphone jack contact pin.
When a microprocessor or switch applies a voltage greater than 4V to the HP-IN pin, a
bridge-connected speaker is muted and the single ended output amplifiers 1A and 2A will
drive a pair of headphones.
• GAIN SELECT FUNCTION (Bass Boost)
The AA4838 features selectable gain, using either internal or external feedback resistors.
Either set of feedback resistors set the gain of the output amplifiers. The voltage applied to
the GAIN SELECT pin controls which gain is selected. Applying VDD to the GAIN SELECT
pin selects the external gain mode. Applying 0V to the GAIN SELECT pin selects the
internally set unity gain. At low, frequencies CLFE is a virtual open circuit and at high
frequencies, it’s nearly zero ohm impedance shorts RLFE. The result is increased
bridge-amplifier gain at low frequencies. The combination of RLFE and CLFE form a -6dB
corner frequency at fC=1/(2 π RLFECLFE)…(9) The bridged-amplifier low frequency
differential gain is: AVD= 2(RF+ RLFE) / R i…(10) Using the component values shown in
Figure 1 (RF=20kΩ, RLFE = 20kΩ, and CLFE = 0.068µF), a first-order, -6dB pole is created
at 120Hz. Assuming Ri=20kΩ, the low frequency differential gain is 4. The input (Ci) and
output (CO) capacitor values must be selected for a low frequency response that covers the
range of frequencies affected by the desired bass-boost operation. In some cases a designer
may want to improve the low frequency response of the bridged amplifier or incorporate a
bass boost feature. This bass boost can be useful in systems where speakers are housed in
small enclosures. A resistor, RLFE, and a capacitor, CLFE, in parallel, can be placed in series
with the feedback resistor of the bridged amplifier as seen in Figure 4.
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©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. ]
