APA2068
Application Descriptions
BTL Operation
toa SE amplifier under the same conditions. A BTL
configuration, such as the one used in APA2068, also
createsa second advantage over SE amplifiers. Since
the differential outputs, ROUT+, ROUT-, LOUT+, and
LOUT-, are biased at half-supply, no needDC voltage
exists acrossthe load. This eliminates the need for an
output coupling capacitor which is required in a single
supply, SE configuration.
The APA2068 output stage (power amplifier) has two
pairs of operational amplifiers internally, allowed for
different amplifier configurations.
OUT+
Volume Control
OP1
amplifier output
signal
Single-Ended Operation
RL
Consider the single-supply SE configuration shown
Application Circuit. Acoupling capacitor is required to
block the DC offset voltage from reaching the load.
These capacitors can be quite large (approximately
33mF to 1000mF) so they tend tobe expensive, occupy
valuable PCB area, and have the additional drawback
of limiting low-frequency performance of the system
(refer to the Output Coupling Capacitor).The rules
described still hold with the addition of the following
relationship:
OUT-
OP2
Vbias
Circuit
Figure 1: APA2068 internal configuration
(each channel)
The power amplifier’s OP1 gain is setting by internal
unity-gain and input audio signal is come from internal
volumecontrolamplifier,whilethesecondamplifierOP2
isinternallyfixedinaunity-gain,invertingconfiguration.
Figure 1 shows that the output of OP1 is connected
to the input to OP2, which results in the output signals
of with both amplifiers with identical in magnitude, but
out of phase 180°. Consequently, the differential gain
for each channel is 2 x (Gain of SE mode).
1
1
1
£
<<
(1)
Cbypass x 125kW
RiCi
RLCC
Output SE/BTL Operation
The ability of the APA2068 to easily switch between
BTL and SE modes is one of its most important costs
savingfeatures.Thisfeatureeliminatestherequirement
for an additional headphone amplifier in applications
whereinternal stereospeakersare driven inBTLmode
but external headphone or speakers must be
accommodated.
Bydrivingthe loaddifferentiallythroughoutputsOUT+
andOUT-,anamplifierconfigurationcommonlyreferred
to asbridged mode isestablished. BTLmodeoperation
isdifferent from the classical single-ended SE amplifier
configuration where one side of its load is connected
Internal to theAPA2068, two separateamplifiersdrive
OUT+ and OUT- (see Figure 1). The SE/BTL input
controlstheoperationofthefolloweramplifierthatdrives
LOUT-andROUT-.
to ground.
A BTL amplifier design has a few distinct advantages
overtheSEconfiguration,asitprovidesdifferentialdrive
to the load, thus doubling the output swing for
aspecified supply voltage.
· When SE/BTL is held low, the OP2 is turn on and
the APA2068 is in the BTL mode.
· When SE/BTL is held high, the OP2 is in a high
Four times the output power is possible as compared
Copyright ã ANPEC Electronics Corp.
17
www.anpec.com.tw
Rev. A.2 - Jul., 2005
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
