AA4838
Agamem Microelectronics Inc.
PRELIMINARY
AUDIO POWER AMPLIFIER
capacitor, CB, between the BYPASS pin and ground improves the internal bias voltage’s
stability and the amplifier’s PSRR. The PSRR improvements increase as the BYPASS pin
capacitor value increases. Too large a capacitor, however, increases turn-on time and can
compromise the amplifier’s click and pop performance. The selection of bypass capacitor
values, especially CB, depends on desired PSRR requirements, click and pop performance
(as explained in the following section, Selecting Proper External Components), system cost,
and size constraints.
• SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the AA4838’s performance requires properly selecting external components.
Though the AA4838 operates well when using external components with wide tolerances,
best performance is achieved by optimizing component values. The AA4838 is unity-gain
stable, giving a designer maximum design flexibility. The gain should be set to no more than a
given application requires. This allows the amplifier to achieve minimum THD+N and
maximum signal-to-noise ratio. These parameters are compromised as the closed-loop gain
increases. However, low gain circuits demand input signals with greater voltage swings to
achieve maximum output power. Fortunately, many signal sources such as audio CODECs
have outputs of 1VRMS (2.83VP-P). Please refer to the Audio Power Amplifier Design section
for more information on selecting the proper gain.
• INPUT CAPACITOR VALUE SELECTION
Amplifying the lowest audio frequencies requires a high value input coupling capacitor
(0.33µF in Figure 2), but high value capacitors can be expensive and may compromise space
efficiency in portable designs. In many cases, however, the speakers used in portable
systems, whether internal or external, have little ability to reproduce signals below 150Hz.
Applications using speakers with this limited frequency response reap little improvement by
using a large input capacitor. Besides effecting system cost and size, the input coupling
capacitor has an affect on the AA4838’s click and pop performance. When the supply voltage
is first applied, a transient (pop) is created as the charge on the input capacitor changes from
zero to a quiescent state. The magnitude of the pop is directly proportional to the input
capacitor’s size. Higher value capacitors need more time to reach a quiescent DC voltage
(usually VDD/2) when charged with a fixed current. The amplifier’s output charges the input
capacitor through the feedback resistor, Rf. Thus, pops can be minimized by selecting an
input capacitor value that is no higher than necessary to meet the desired −6dB frequency.
As shown in Figure 2, the input resistor (RIR, RIL = 20k) (and the input capacitor (CIR, CIL=
0.33µF) produce a −6dB high pass filter cutoff frequency that is found using Equation (7).
… (7)
As an example when using a speaker with a low frequency limit of 150Hz, the input coupling
capacitor, using Equation (7), is 0.053µF. The 0.33µF input coupling capacitor shown in
Figure 2 allows the AA4838 to drive a high efficiency, full range speaker whose response
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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. ]
