AA4838
Agamem Microelectronics Inc.
PRELIMINARY
AUDIO POWER AMPLIFIER
negative (-) output serves as the second amplifier’s input. This results in both amplifiers
producing signals identical in magnitude, but 180˚ out of phase. Taking advantage of this
phase difference, a load is placed between −OUTA and +OUTA and driven differentially
(commonly referred to as “bridge mode”). This result in a differential gain of AVD=2*(Rf/Ri)…
(1).
Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected
between a single amplifier’s output and ground. For a given supply voltage, bridge mode has
a distinct advantage over the single-ended configuration: its differential output doubles the
voltage swing across the load. This produces four times the output power when compared to
a single-ended amplifier under the same conditions. This increase in attainable output power
assumes that the amplifier is not current limited or that the output signal is not clipped. To
ensure minimum output signal clipping when choosing an amplifier’s closed-loop gain, refer to
the Audio Power Amplifier Design section. Another advantage of the differential bridge output
is no net DC voltage across the load. This is accomplished by biasing channel A’s and
channel B’s outputs at half-supply. This eliminates the coupling capacitor that single supply,
single- ended amplifiers require. Eliminating an output coupling capacitor in a single-ended
configuration forces a single-supply amplifier’s half-supply bias voltage across the load. This
increases internal IC power dissipation and may permanently damage loads such as
speakers.
• POWER DISSIPATION
Power dissipation is a major concern when designing a successful single-ended or bridged
amplifier. Equation (2) states the maximum power dissipation point for a single- ended
amplifier operating at a given supply voltage and driving a specified output load.
PDMAX=VDD2/2π2RL Single-Ended…(2)
However, a direct consequence of the increased power delivered to the load by a bridge
amplifier is higher internal power dissipation for the same conditions. The AA4838 has two
operational amplifiers per channel. The maximum internal power dissipation per channel
operating in the bridge mode is four times that of a single-ended amplifier. From Equation (3),
assuming a 5V power supply and a 4Ω load, the maximum single channel power dissipation is
1.27W or 2.54W for stereo operation.
PDMAX=4*(VDD)2/2π2RL Bridge Mode…(3)
The AA4838’s power dissipation is twice that given by Equation (2) or Equation (3) when
operating in the single-ended mode or bridge mode, respectively. Twice the maximum power
dissipation point given by Equation (3) must not exceed the power dissipation given by
Equation (4):
PDMAX'=(TJMAX−TA)/θJA…(4)
The AA4838’s TJMAX=150˚C. In the LQ package soldered to a DAP pad that expands to a
copper area of 5in2 on a PCB, the AA4838’s θJA is 20˚C/W. In the MTE package soldered to
9
©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. ]
